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Research Strategies for the U.S. Global Change Research Program (1990)

Chapter: 2 Integrated Modeling of the Earth System

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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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Suggested Citation:"2 Integrated Modeling of the Earth System." National Research Council. 1990. Research Strategies for the U.S. Global Change Research Program. Washington, DC: The National Academies Press. doi: 10.17226/1743.
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2 Integrated Modeling of the Earth System OVERVIEW The possibility of major changes in the global environment presents the scientific research community with a difficult task: to devise ways of ana- lyzing the causes of and projecting the course of these shifts as they are occurring. Purely observational approaches are inadequate for providing the needed predictive or anticipatory information because response times of many terrestrial ecosystems are slow and there is a great deal of variability from place to place. Furthermore, many important processes cannot be measured directly over large areas, such as those processes that occur in soils. We need models to express our understanding of the complex sub- systems of the earth and how they interact with and respond to and control changes in the physical-climate and biogeochemical systems. By the year 2000, a fully coupled, dynamical model of the earth system (Figure 2.1) could be a reality. Such models would significantly improve capabilities for projecting changes in the earth system on a decadal time scale. The focus of this chapter is on the efforts required to achieve this goal. For instance, it is necessary to begin now to develop models that are more completely coupled albeit still partial-than those that are currently available. Even though these prototypes may themselves not be successful, This chapter was prepared by the working groups on Integrated Earth System Models established under the Committee on Global Change. Members of the group on Terrestrial-Atmosphere Modeling were Berrien Moore III, University of New Hampshire, Chair; John Aber, University of New Hampshire; Guy Brasseur, Na- tional Center for Atmospheric Research; Robert Dickinson, National Center for At- mospheric Research; William Emanuel, Oak Ridge National Laboratory; Jerry Melillo, 16

INTEGRATED MODELING OF THE EARTH SYSTEM cn I ~ i_ \ Biogeochemical Cycles 17 ~ E ~ cat a, c On 0 ~0 ~ ._ cn ~ a, c' ~ ~It -it 1 ' 1~' ~ | Tropospheric Chemistry 4 Pollutants | Atmosphenc Physics/Dynamics := ;! a!; IT Ti 1 ~ _ Terrestrial Or ean Dynamics Energy/Moisture ~ Marine l l Terrestrial Biogeochemistry I I Ecosystems | Global Moisture 1 ( Soil . ~ ~ ' ~' ~- Land _ Use Human Activities FIGURE 2.1 Status of earth system science in the year 2000 (ESSC, 1988~. they will teach us what is needed to realize our goal of a fully coupled, dynamical earth system model with a multidecadal scale of analysis. How- ever, it should not be overlooked that much of the real science is in the simple models and empirical observations that guide our understanding and give us a framework for interpreting (and creating) the more complex mod- els that evolve later. The early linking of complex models and the subse- quent addition of existing approaches should be balanced by efforts to cre- ate new, insightful simple models. Such insights provide the basis for qualitative improvements in model structures. It should be recognized at the outset that the muliidecadal temporal scale places important constraints and demands upon the character of earth sys- tem models (Bolin et al., 1986; ESSC, 1988; NRC, 1988~. For instance, the Marine Biological Laboratory; David Schimel, Colorado State University; Piers Sellers, University of Maryland; and Herman Shugart, University of Virginia. Members of the group on Ocean-Atmosphere Modeling were Berrien Moore III, University of New Hampshire, Chair; Mark Abbott, Oregon State University; Curt Covey, Lawrence Livermore National Laboratory; Nick Graham, Scripps Institution of Oceanography; Dale Haidvogel, Johns Hopkins University; Eileen Hoffman, Old Dominion University; Christopher Mooers, University of New Hampshire; James O'Brien, Florida State University; Albert Semtner, Naval Postgraduate School; and Leonard Walstad, Oregon State University. Members of the group on Atmospheric Physics-Atmospheric Chemistry were Berrien Moore III, University of New Hampshire, Chair; Guy Brasseur and Robert Dickinson, National Center for Atmospheric Research; Bill Gross, NASA Langley Research Center; and Chris Morris, University of New Hampshire.

18 RESEARCH STRATEGIES FOR THE USGCRP temporal scale demands inclusion of the biosphere and coupling across critical interfaces: terrestrial ecosystems and the atmosphere, the chemistry of the atmosphere and the physics of the atmosphere, and the oceans and the atmosphere. Advances at these interfaces are essential for progress. Differences in characteristic rates of change and fundamental processes of different components of the system will impose subsystem-specific de- mands and requirements on component models (Rosswall et al., 1988~. Ecological systems will most likely rest upon functional groups rather than species; understanding biogeochemical fluxes will require process-level models, but initial implementation at global scales will certainly require extensive pa- rameterization. Similarly, the nonlinear chaotic dynamics of the fluid sub- systems the oceans and atmosphere will continue to require a careful, step-by-step buildup in complexity; the simplistic thinking that must go into all initial modeling advances will tend to be eventually superseded by computationally intensive three-dimensional approaches. This is, in fact, occurring in many of the geophysical and biological-biogeochemical sci ences. The most complex models to date are the atmospheric and oceanic gen- eral circulation models (GCMs). These have structures largely determined by the need to solve the Navier-Stokes fluid equations, but they are rich in other physical processes as well. The atmospheric models and their climate role are especially strongly governed by water processes; however, it is precisely these aspects, including questions of scale and parameterization, that are among the least satisfactory of the models. Resolution is a problem in that the spatial scales of many of the impor- tant atmospheric water structures are poorly resolved by existing models. For example, many of the cloud systems that are most important for atmo- spheric radiation have vertical scales of less than the thickness of the layers in most existing GCMs. The horizontal structure of precipitating systems suffers not only from inadequate resolution but also from severe difficulties with the currently available numerical schemes that were designed prima- rily for effectiveness (minimal computational demands) in treating the model hydrodynamics. One obvious defect of these schemes is the tendency of truncated spectral series to give negative mixing ratios for water in high latitudes, a consequence of the failure of the series to represent properly the fields in going from relatively large mixing ratios to relatively small ones. The same difficulty can be encountered for any model tracer. For example, it was difficult to get models to treat global smoke fields properly in nuclear winter computations. The hope is that the new semi-Lagrangian schemes will cure these numerical difficulties. Another question in the treatment of water vapor in various atmospheric GCMs is whether vertical transport in the models resembles the process in nature, again because much of the real vertical transport occurs on scales that are small in comparison with that of the model. The subgrid-scale moist

INTEGRATED MODELING OF THE EARTH SYSTEM 19 convection parameterizations in the models are still fairly crude and have not improved much in the last decade, although considerable effort is now going into them (Anthes, 1983~. Adding the important chemical constituents and the reactions to an atmo- spheric GCM causes the issues of scale and computational challenges to become daunting. Many of the important chemical reactions are concentra- tion dependent and hence grid-scale dependent, and important processes often occur in the boundary layer, which generally is not well enough resolved. Further, the addition of atmospheric chemistry to a GCM places greater demands upon the terrestrial and oceanic boundary conditions and dynamic simulations (Lenschow and Hicks, 1989; NBC, 1984; Schimel et al., 1989~. In considering coupling atmospheric GCMs to terrestrial models, where the coupling transfers not only energy and water but also important gases, such as carbon monoxide, methane, and carbon dioxide for the carbon cycle, temporal- and spatial-scale issues again emerge. The macrobalance of ter- restrial carbon stocks, which determine the net flux of carbon dioxide, are difficult to derive by integrating across the short time scales at which en- ergy, water, and carbon dioxide and oxygen are actually exchanged because of the high degree of variability that these processes exhibit. Longer time step integrations have generally been more successful. On the other hand, the flux of methane and other short-lived species cannot be treated by simple mass balance and crudely time-averaged responses. Ecological changes, such as successional sequences of tree species, are not well treated on time steps that are appropriate for considering photon input and: water exchange or even trace gas fluxes and require some intermediate parameterization or model. The relatively simple coupling issue of land hydrology and atmosphere remains elusive, and yet it is quite important. The exchange of many re- duced gases (e.g., methane) depends on soil moisture conditions, and en- ergy fluxes are influenced by water balances. Modeling sensitivity studies have shown that if evapotranspiration were turned off over continental-scale areas, summer precipitation would be severely reduced and temperatures would be as much as 10 K higher than with normal fluxes. They also show that over tall vegetation the integrated resistance to transpiration implied by the stomata will have a major effect on Bowen ratios over the diurnal cycle. Since the rates of sensible heat exchange over the diurnal cycle determine the height reached by the planetary boundary layer as well as diurnal variations of precipitation in tropical and summer conditions, it is evident that it is important to include the role of vegetation in simulations of the hydrologi- cal cycle. Better field data are helping to establish the parameters needed for linking plant physiology to surface evapotranspiration, but considerable further effort is needed before the appropriate submodels can be applied with confidence over a wide range of vegetation cover (e.g., Dickinson, 1984; Eagleson, 1986; Sellers et al., 1986~.

20 RESEARCH STRATEGIES FOR THE USGCRP The coupling between the ocean and the atmosphere is central to the question of climate change. Atmospheric GCMs with prescribed oceans, long the mainstay of three-dimensional climate modeling, are inherently incapable of simulating the actual time-evolving response of the climate system to increasing greenhouse gases because this response involves heat uptake by the oceans. This is particularly clear when one realizes that the heat capacity of the atmosphere is roughly equivalent to that of the upper 3 m of the ocean. While it is true that the ocean may, partially, act in a passive manner, studies of the E1 Nino/Southern Oscillation (ENSO) show that the ocean-atmosphere system responds in a coupled fashion on interannual time scales, and paleo-oceanographic investigations suggest that aspects of longer-term climate change are associated with changes in the ocean's ther- mohaline circulation. The capability to predict these changes in circulation and heat exchange is necessary to describe the future evolution of global climate (e.g., Bryan et al., 1982; Cess and Goldenberg, 1981; IPCC, 1990; Sarmiento et al., 1988~. Fortunately, exciting and encouraging progress is being made in cou- pling key aspects of the major subsystems. Results from linking atmo- spheric and oceanic GCMs have already been reported in the literature and have shown significantly different behavior from that of simulations in un- coupled modes. Similarly, interactive simulations between atmosphere and land vegetation have been reported, and these have also exhibited new dynamical characteristics. The inclusion of biology in oceanic GCMs has begun, al- though the models are still simplistic and do not yet include climatic feedback in a coupled system. Representations of terrestrial biology are also preliminary and again without critical biogeochemical feedbacks. Finally, progress is being made toward model structures and data sets that will allow implemen- tation of atmospheric-oceanic-terrestrial models that include key biological- biogeochemical feedbacks. For the near term, developments in modeling the earth system should continue to focus on linking previously unlinked components, adding spe- cific subsystems to existing models (e.g., coupling oceanic and atmospheric GCMs or adding a marine biospheric model to an oceanic GCM), or im- proving existing linked treatments. In this spirit, the committee has arranged the following discussion around three interface models: 1. Atmosphere-terrestrial subsystem. 2. Physical-chemical interactions in the atmosphere. 3. Atmosphere-ocean subsystem including interactions with the biosphere. These three subsystems obviously overlap and do not include all interfaces. Further consideration is required on the issue of the role of the cryosphere and its coupling on multidecadal time scales (see OIES, 1989~. In the following sections the committee presents a brief general discus

INTEGRATED MODELING OF THE EARTH SYSTEM 21 sion of the current status of models at these three interfaces, including for each a focused report on recommended initiatives and themes. The two final sections deal with the cross-cutting issues of model tests and infra- structure. In order to provide perspective on the remainder of the chapter, the following considerations for each of the three interface models are pro- vided: For models that couple the terrestrial ecosystems and the atmosphere: · The coupling must address questions such as how will a changing climate affect terrestrial carbon dioxide uptake and storage; how will · · . evapotransp~rat~on change; how will the distribution of vegetation and its seasonal pattern change; what are the effects on climate of changing pat- terns of vegetation, including large-scale deforestation; and what is the ef- fect of changing chemical conditions on terrestrial vegetation and trace gas exchange? · The primary research issue in understanding the role of terrestrial ecosystems in global change is that of analyzing how processes with vastly differing rates of change, from photosynthesis to community change, are coupled to each other and to the atmosphere. · Modeling these interactions requires coupling successional models to biogeochemical models to physiological models. Of these, only the physi- ological models can currently describe the exchange of water and energy between the vegetation and the atmosphere at fine time scales. . Terrestrial models should focus on linked models addressing plant community change, biogeochemistry, and physiology and~biophysics. Mod- els of the physics of the atmosphere couple directly to terrestrial physiology models; biogeochemical models serve as a bridge between physiology and community change as well as coupling to the chemistry of the atmosphere. · The coupling must address how changes in the global environment, including the effects of land use and chemical stress, affect terrestrial eco- systems and how ecosystem changes affect the global system. Formidable problems of scale and parameterization are raised in three- and four-dimensional simulations of biology and atmospheric chemistry be- cause of nonlinear concentration-dependent phenomena. For models that couple physics and chemistry in the atmosphere: · The coupling must address questions such as what is the spatial-tem- poral distribution of carbon monoxide, methane, and tropospheric ozone and how might it change; what is the effect of changing climatic or chemi- cal conditions on the aerosol-initiated stratospheric ozone depletion in the Arctic and the Antarctic; how might the exchange of water vapor between the troposphere and the stratosphere change in a changing climate; and what is the vertical transport of trace species by cloud convection and how might it change?

22 RESEARCH STRATEGIES FOR THE USGCRP Progress in the modeling of the coupled chemical-physical atmospheric system requires a better knowledge of surface sources of trace gases and their dependence on climatic conditions; chemical processes and reaction channels, both in the gas and in the aqueous phase, and their dependence on atmospheric conditions; and transport processes by advection and convec- tion, including the development of high-resolution transport models coupled to atmospheric GCMs with detailed representation of physical processes including cloud formation and associated transport, boundary layer trans- port, and troposphere-s~atosphere exchange. This progress is dependent on the acquisition of global data sets for validation of these treatments. Future progress will be dependent both on available computational resources and on progress in developing our understanding of fundamental physical and chemical processes and the nature of their coupling. For models that couple the ocean and the atmosphere: · The coupling must address questions such as how will changing cli- mate affect oceanic carbon dioxide uptake and storage; how will oceanic heat storage and transport change; how will the amount and distribution of primary production change; how will the marine hydrological cycle change; and how will a changing ocean affect a changing climate? Critical issues include widely differing temporal and spatial scales, inclusion of biological and biogeochemical dynamics, and sparse data. Par- ticularly important and difficult tasks are the scaling of the biological-bio- geochemical components from local-regional domains to basin-global do- mains, formation of the upper mixed-layer physics, and inclusion of possible biological feedbacks on mixed-layer dynamics. · Progress in the development of coupled oceanic-atmospheric models including biological-biogeochemical dynamics is limited, in part, by an in- adequate theoretical or observational understanding of certain key processes and a corresponding and continuing uncertainty as to how best to incorpo- rate or parameterize them in oceanic GCMs. . The set of field programs (JGOFS, WOCE9 the Coupled Oceans At- mosphere Research Experiment (COARE) organized under TOGA, and the Global Ocean Ecosystem Dynamics (GLOBEC)) required to acquire the data needed to advance our knowledge of fundamental oceanic processes is already well defined. These programs also offer valuable opportunities for simultaneous observational efforts, and these should be encouraged. · The development of fully coupled models should be encouraged along two parallel paths: the first devoted to developing basin- and global-scale models with increasing levels of coupling, and the second leading to a series of regional fine-scale models that could provide boundary conditions and parameterization tests for the larger-scale models. Several overarching issues exist regarding approaches to and testing of models and the infrastructure necessary for their development:

INTEGRATED MODELING OF THE EARTH SYSTEM . 23 Validation is extremely difficult; models should be subjected to natu- rally occurring perturbation tests that exercise the coupling. In addition, large-scale phenomena offer a valuable opportunity for focusing model de- velopments and testing model dynamics. Studies of these large-scale pro- cesses will serve not only as diagnostic tests but also as prognostic tools. It is urgent that testing of models and model combinations begin as soon as possible. Experiments with global models will initially use simple representations, but the lessons learned and data bases developed will be critical to future improvements. Prototype global experiments will be espe- cially important to exploring feedbacks between the production of long- lived trace gas species and climate. Two important themes are important in early testing of partial earth system models: the global carbon cycle (carbon dioxide, methane, and carbon monoxide) and the transient response to a changing greenhouse forc- ing. The former exercises the chemistry and biology, whereas the latter stresses the physics and biology. The obvious next step is coupling these . . two themes. . The importance of experience gained through prototype modeling ex- periments, including failure, should not be underestimated. Careful analy- sis of failures can provide valuable information. · Earth system modeling should serve as a focus and catalyst for inter- disciplinary science. No one institution or group of investigators has more than a fraction of the interdisciplinary talent necessary for the development of an earth system model focused on multidecadal time scales. Thus sev- eral teams and talented individuals should be supported, who with some coordination could help perform the incremental steps toward the integrated earth system model. Some of these groups may act primarily as synthesiz- ers, their principal interest being in linking component pieces, while in other groups the interest would be in component development. Also needed in an overall modeling strategy are centralized facilities and associated staff to serve the common needs of the various teams and individuals and focus on issues of synthesis, continuity, documentation, and extensive numerical experiments. ATMOSPHERE-TERRESTRIAL SUBSYSTEM The primary research issue for coupling atmosphere-terrestrial models is understanding how processes with vastly differing rates of change, from photosynthesis to community change, are coupled. Representing this cou- pling in models is the central challenge to modeling the terrestrial biosphere as part of the earth system (e.g., Allen and Wyleto, 1984; Huston et al., 1988; King et al., 1990; Moore etal., 1989b, Smith et al., 1989~. Terrestrial ecosystems participate in climate and in the biogeochemical cycles on several temporal scales. The metabolic processes that are respon

24 RESEARCH STRATEGIES FOR THE USGCRP sible for plant growth and maintenance, and the microbial turnover associ- ated with dead organic matter decomposition, move carbon and water through rapid as well as intermediate time scale circuits in plants and soil. More- over, this cycle includes key controls over biogenic trace gas production. Some of the carbon fixed by photosynthesis is incorporated into plant tissue and is delayed from returning to the atmosphere until it is oxidized by decomposition or fire. This slower carbon loop through the terrestrial com- ponent of the carbon cycle, which is matched by cycles of nutrients required by plants and decomposers, affects the increasing Rend in atmospheric car- bon dioxide concentration and imposes a seasonal cycle on that trend (Fig- ure 2.2~. The structure of terrestrial ecosystems, which responds on even longer time scales, is the integrated response to the intermediate time scale carbon machinery. The loop is closed back to the climate system since it is the structure of ecosystems, including species composition, that sets the terrestrial boundary condition in the climate system from the standpoint of surface roughness, albedo, and, to a great extent, latent heat exchange. These separate temporal scales contain explicit feedback loops that may modify the system dynamics. Consider again the coupling of long-term climatic change with vegetation change. Climatic change will drive vegeta- tion dynamics, but as the vegetation changes in amount or structure, this 355 350 Q Q z o E z C) z o Cal 8 345 340 335 330 325 320 315 310 _ ~,.1., I ,l,,,,,l,,,,,l,,,,,l,,,,,l, l , lL,.,,l,, ,l l l l l l l , ,,,l,.,,,,,,,,,l,,,,,,, ~ 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 YEAR FIGURE 2.2 Concentration of atmospheric carbon dioxide in parts per million of dry air (ppm) versus time for the years 1958 to 1989 at Mauna Loa Observatory, Hawaii. The dots indicate monthly average concentration. (From C.D. Keeling et al. (1989). Copyright (3 1989 by the American Geophysical Union.)

INTEGRATED MODELING OF THE EARTH SYSTEM 25 will feed back to the atmosphere through changing water, energy, and gas exchange. Biogeochemical cycling will also change, altering the exchange of trace gas species. The long-term change in climate, driven by chemical forcing functions (carbon dioxide and methane) will drive long-term eco- system change. Modeling these interactions requires coupling successional models to biogeochemical models to physiological models that describe the exchange of water and energy between the vegetation and the atmosphere at fine time scales. There does not appear to be any obvious way to allow direct reciprocal coupling of GCM-type models of the atmosphere, which inherently run with fine time constants, to ecosystem or successional mod- els, which have coarse temporal resolution, without the interposition of a physiological model. This is equally true for biogeochemical models of the exchange of carbon dioxide and trace species. This cross-time-scale coupling is important and sets the focus for the modeling strategy. A Modeling Strategy: Prognosis for Progress Intuitively, we might develop a global model of terrestrial ecosystem dynamics by combining descriptions of each of the physical, chemical, and biological processes involved in the system. In such a scheme, longer-term vegetation changes would be derived by integrating the responses of rapidly responding parts of the model. But we cannot simply integrate models that describe the rapid processes of carbon dioxide diffusion, photosynthesis, fluid transport, respiration, and transpiration in cells and leaves in order to estimate productivity of whole plants, let alone entire ecosystems. The nature of the spatial averaging implied in the selection of parameters and processes to consider is difficult because of nonlinearities, which means that the choice of scale influences the calculation of averages (see Rosswall et al., 1988~. To progress in the development of terrestrial ecosystem models, we choose processes to treat in different models based on the phenomenological scales involved. As is common in physical models, terms in fundamental equa- tions can be included or ignored depending on the temporal and spatial scales of interest (e.g., ignoring gravitational effects in quantum physics and including Coriolis effects in large-scale fluid motion). Careful organi- zation of a suite of models, each describing processes that operate at differ- ent rates, is crucial to the practical development of terrestrial ecosystem models for use in earth system models of global change. Based on current model structures, atmosphere-biosphere interactions can be captured with simulations operating with three characteristic time con- stants (Figure 2.3~. The first level represents rapid (seconds to days) bio- physical interactions between the climate and the biosphere (Figures 2.4a and b). The dynamics at this level result from changes in water, radiation,

26 C L 1 M A T 1 C D R 1 V E R S RESEARCH STRATEGIES FOR THE USGCRP | TIME STEP 1 , SECONDS - DAYS TIME STEP DAYS - WEEKS TIME STEP 1 ANNUAL H2O EVAPOTRANS PI RATI ON ENERGY / WATER / CO2 LAI (SEASONAL) FOLIAR C / N (SEASONAL) HYDROLOGY / SOIL CHEMISTRY / TRACE GASES DECOMPOSITION / MINERALIZATION / UPTAKE LAl COTS) NPP TOTAL) DECOMPOSITION / MINERALIZATION / UPTAKE NET CARBON EXCHANGE / NET ECOSYSTEM PRODUCTION FIGURE 2.3 Three different time steps at which existing models of terrestrial ecosystems use climatic information to modify rates of ecosystem function. LEVEL 1 A 4) C H2O B HZO ~ ~ ~1! CO2 3- TOTAL PRIMARY r , PRODUCTION _ I TEMPERATURE - _ l | l _ WATER, LIGHT FAST TIME 1 _: 1 DECAY POOL FIGURE 2.4 Two diagrammatic representations of models converting short-time- step environmental data (minutes to hours) into balances of energy, water, and carbon. For these models, ecosystem structure, including leaf display and canopy structure, are fixed. Nutrient fluxes other than emission and consumption of trace gases are not dealt with.

INTEGRATED MODELING OF THE EARTH SYSTEM 27 and wind and accompanying physiological responses of organisms. These dynamics occur rapidly in relation to plant growth and nutrient uptake-far more rapidly than species replacement can occur. Simulations at this level are required to provide information to climate models on the exchange of energy, water, and carbon dioxide. Tests of this level of model can be accomplished using experimental methods including leaf cuvettes, microme- teorological observations, and eddy correlation flux measurements (e.g., Farquhar and von Caemmer, 1982; Markar and Mein, 1987; Sellers et al., 1986~. The second level captures important biogeochemical interactions. This level captures weekly to seasonal dynamics of plant phonology, carbon ac- cumulation, and nutrient uptake and allocation (Figures 2.5a and b). Most extant models at this level use integrative measures of climate such as monthly statistics and degree-day sums. Changes in soil solution chemistry and microbial processes can be captured at this level for calculation of trace gas fluxes. Primary outputs from this level of model are carbon and nutri- ent fluxes, biomass, leaf area index, and canopy height or roughness. This level of model is usually tested in field studies with direct measurements of biomass, canopy attributes, and nutrient pools or fluxes (e.g., Melillo et al., 1982; Parton et al., 1988; Pastor and Post, 1985, 1986; Schimel et al., 1985). A third level of model represents annual changes in biomass and soil carbon (i.e., net ecosystem productivity and carbon storage) and in ecosys- tem structure and composition (Figures 2.6a and b). Inputs are calculated indices summarizing the effects of climatic conditions on biomass accumu- lation and decomposition. Outputs include ecosystem element storage, allo- cation of carbon and other elements between tissue types, and community structure. This type of model currently represents individual organisms or populations and is difficult to apply at large scales because of computa- tional and data requirements (e.g., Aber et al., 1982; Botkin et al., 1972a,b; King et al., 1990; Shugart, 1984; Shugart and West, 1980~. Considerable work will be required to develop large-area implementations. This type of model is validated using a combination of process studies, as described above, but integrated to derive annual fluxes, and comparative studies. The community composition and population dynamics aspects of these models are often validated using paleo-data. An approach to this coupling is highlighted in Figure 2.7. A level 3 model converts annualized indices of climatic conditions and the current ecosystem state into total leaf area and structure for the next year. Within these total values, the second level calculates the phonology of leaf production and loss, the rate of nutrient mobilization and uptake, and hence the seasonal pattern of ecosystem dynamics. Using these seasonal patterns, the first level converts climatic data into energy and water balances over very short

28 LEVEL 2 : A PAR H2O C ~ // CE ~ C , ~ ~ C/N C/N PIN WATER, LIGHT, CARBON / NUTRIENTS MEDIUM TIME RESEARCH STRATEGIES FOR THE USGCRP FIGURE 2.5 Two diagrammatic representations of models operating at the intermediate time step (days to weeks). These models capture seasonal phonology and use summary climate data to calculate water, carbon, and nugget balances at monthly time steps. Energy flux calculations are meaningless at this time step. B LIVE DRIVERS PRODUCTION BIOMASS TURNOVER ~ . | WATER 1: - 7ToTAL: E3: PRIMARY PRODUCTION NUTRIENTS' BIOTIC / 5<)URCE5 | NITROGEN LITTER HUMUS . ~ IMMOB. MINER. N N 1 2 3 ' '0{~ {I ,_~ ;~, NITROGEN AVAILABLE -L~[] _ NET ANNUAL MINERALIZATION OF ALL NUTRIENTS FLUX OF CO2

INTEGRATED MODELING OF THE EARTH SYSTEM LEVI A B _ _ ~ AET DE GD SPP SPP DBH DBH HT HT ... INITIAL TREE VECTOR UGHT INDEX / C ~///~i ~ Cal ~ ~0 _ _ _ _ x~ ~ _,,_ \ N WATER, LIGHT, CARBON / NUTRIENTS, SPECIES STRUCTURE SLOW nME LAND USE PRODUCn~ ~=~ ~ NEW INITIAL TREE VECTOR MORIARTY NEW COHORTS INmAL COHORTS TYPE WT SN SL ~ORTAUTY ~I no [I=FR FAT SPP SPP DBH DBH HT HT ... NEW STEMS STEIN ENTRANCE I _ A_ SPP ~ DBH DBH I HT HT | _ L CODIFIED TREE VECTOR NEW COHORTS TYPE _ WT ON AL ... _ , TYPE WT ON SL _ __ _ ~ DECOh4POSInON CODIFIED COHORTS TYPE WT SN AL TYPE WT SN SL l TYPE | l VET | l SN | l SL I ... I TYPE T 1 ~WTN l l SL I ... I _ NITROGEN I AVAILABILITY 1 - GROWTH UGHT _ ULTIPUERS AVAILABILITY ~l ~, \ TRANSFER l l \ NEW INmAL l I ~COHORTS ~ F · H ~ ~ ~1 _ ~I MINERAL SOIL | 29 1~4h40~LIZAnON ~, FIGURE 2.6 At the annual time step, models make use of the wealth of data available for growth and mortality of individual stems by species, or growth rate of entire stands of vegetation in relation to annual summations of climatic conditions. Fluxes of carbon and nutrients are calculated. Energy and water balances cannot be dealt with at this time step. NOTE: Acronyms and abbreviations used in this figure are as follows: AFT, actual evapotranspiration; DBH, tree diameter; DEGD, grow- ing degree days; F. forest floor; H. organic matter; HT, height; SPP, species; TYPE, litter type (e.g., leaves, wood, root); WT, weight; AN, percent nitrogen; Sol, percent lignin.

30 c L 1 M A T 1 C D R 1 V E R S Act_ M T C D R 1 V E R S RESEARCH STRATEGIES FOR THE USGCRP H2O EVAPOTRANSPIRATION ENERGY / WATER / CO2 TIME STEP SECONDS - DAYS TIME STEP 1 , DAYS - WEEKS ~ | CONST - I NTS LAI (SEASONAL) FOLIAR C / N (SEASONAL) HYDROLOGY / SOIL CHEMISTRY / TRACE GASES DECOMPOSITION / MINERALIZATION / UPTAKE ~ .... ~ ~ CONST~I NTS TIME STEP \ ~\ ANNUAL LAI ROTA L) NPP TOTAL) DECOMPOSITION / MINERALIZATION / UPTAKE NET CARBON EXCHANGE / NET ECOSYSTEM PRODUCTION MEDI UM SLOW it- -as} FIGURE 2.7 The use of longer-time-step models to constrain or bound the shorter- time-step efforts. Rather than attempting to run the minute-to-hour models for centuries, it is suggested that the annual-time-step models can be used to estimate changes in structure due to global change for some time into the future. These can then determine the gross ecosystem structure within which the moderate-time-step models can calculate phonological changes and gross water balances. These in turn determine fine-scale ecosystem structure required as inputs for the shortest-time- step models. The shorter-time-step models return indices of physiological stress to the longer-time-step models, which then alter the long-term course of ecosystem development. This approach should provide useful bounds for the shorter-time-step models and reduce computation time considerably.

INTEGRATED MODELING OF THE EARTH SYSTEM 31 time steps. Annual feedbacks serve as informational flows carrying back the integrated effect of fine- and medium-scale time steps. Decomposition calculations can be driven by level 2 vegetation modules and integrated to set nutrient availability in level 3 vegetation calculations. Inorganic soil chemistry routines can operate on almost any time scale, as they tend to be somewhat independent of temperature and linear with time but may be nonlinear with concentrations. Nutrient cycling and soil chem- istry modules can run under altered climatic drivers for some time, and then predictions can be made of the consequences of changes in ecosystem state for surface-atmosphere interactions and trace gas fluxes. This approach is consistent with those outlined in chapters 5 and 6, on water-energy-vegetation interactions and terrestrial trace gas and nutrient fluxes, respectively. Several issues must be resolved to develop and imple- ment such terrestrial modeling schemes in an earth system model of global change: 1. Calculation of indices of climatic effects on biological activity. There are several different ways to summarize the effects of climatic conditions on biological activity. These range from very simple calculations of esti- mated evapotranspiration, water deficits, and drought indices (Hanks, 1985; Vorosmarty et al., 1989) to physiologically sophisticated and computationally demanding models of water, energy, and carbon balances at the leaf and canopy level (Farquhar and von Caemmer, 1982; Pastor and Post, 1988; Running and Coughlan, 1988; Sellers et al., 1986; Shugart et al., 1986; Solomon, 1986; Solomon et al., 1984~. Which of these provides the most accurate depiction of climate-biotic interactions and which can be param- eterized most easily from existing or obtainable field data? 2. Spatial scale of species-functional class ecosystem models. Models of forest ecosystem dynamics are of two types: stem-oriented models, which enumerate all individuals within the modeled area, and aggregated models, which deal only with biomass compartments such as foliage, wood, and roots (e.g., see Figures 2.6a and b). Grassland models are generally of the species-aggregated type (e.g., Parton et al., 1988; Schimel et al., 1985~. The stem models are valuable for examining gap-phase dynamics within a landscape and can predict changes in species composition explicitly (e.g., Aber et al., 1982; Botkin et al., 1972a,b; Shugart, 1984; Shugart and West, 1980~. They are tied to a spatial scale at which canopy gaps occur. The aggregated models are independent of spatial scale and are computationally much simpler but do not capture the successional dynamics or species- specific characteristics of ecosystems (Emanuel et al., 1984; Moore et al., 1989b). At some cost, the stem models can be aggregated spatially through the use of subsampling and geographic information system (GIS) technol- ogy to reach GCM spatial scales. Alternatively, the aggregated models can be parameterized to include successional changes.

32 RESEARCH STRATEGIES FOR THE USGCRP 3. Linking processes across spatial scales. Models need to be organized for use at several levels of geographic detail and with the rapid and slow modules running in concert or separately. At the most detailed geographic level, underlying data are organized on a grid of land cells, and models are solved for each grid cell or with a sampling strategy. Both data and model solutions can be mapped and managed by a GIS. The data requirements and implementation logistics are very demanding at this level. In regional stud- ies, data and model results are tabulated against blame or ecosystem ex- tents, whereas in some applications it is useful to average or lump data and model results to global scale (see Figure 2.7~. An added complexity is that human activities affect a large fraction of the world's terrestrial ecosystems. These disturbances which range from total management, harvest, and land use change to subtle pollutant im- pacts cannot be ignored in any analysis of changes in the role of land systems (e.g., Emanuel et al., 1984; Houghton et al., 1983~. Two aspects of the land use perturbation and other human activities must be treated: (1) The rates and distributions of the disturbances per se must be described, perhaps by using a GIS. (2) The effects, particularly the redistribution of carbon and nutrients in the compartments as well as successional patterns, must be described, which is a more difficult issue. This effect of land use change is perhaps best initiated at level 3 and allowed to move upward through the constraint structure (see Figure 2.7~. Research Priorities As noted above, critical improvements in ecosystem modeling and its linkage to the earth system models will require the development of schemes for integrating processes with very different rates of change. This implies the continued development and validation of physiological, biogeochemical, and successional/population models that are capable of representing the range of processes and communities found in ecosystems worldwide. Ex- periments with coupling these three levels of models are required, as are tests of these models when run interactively with atmospheric models. The different levels of models have differing data requirements, and these must guide the collection and archiving of data. In addition to the coupled models, a parallel course needs to be pursued wherein detailed models should be developed that can allow analysis of ecosystem responses to forcing from scenarios of climatic change (i.e., those scenarios developed by exercising the first-generation earth system mod- els). These more detailed models will allow checking of the responses of the simple models included in the earth system models and can be used to develop improved parameterizations for the earth system model. The key issue

INTEGRATED MODELING OF THE EARTH SYSTEM 33 here is that the development of process models and parameterizations should proceed together and in a coordinated fashion. The above approach leads directly to a strategy for validation (see also the section "Critical Model Tests" below). Process models can often be tested directly in field and laboratory studies and should be so tested before being used to develop parameterizations for use in coupled models. Predic- tions of the earth system models can also often be tested by comparison to well-mixed, and hence well-sampled, attributes of the atmosphere. Ex- amples of such validation parameters include atmospheric concentrations and latitudinal distributions of atmospheric methane or carbon dioxide. Vali- dation of models at scales between those of field process studies and the globe is often more difficult, requiring intensive and extensive measurements. It is urgent that tests of models and model combinations begin as soon as possible. Exploration of the behavior of global and regional models with coupled atmosphere and terrestrial systems should begin in parallel. Ex- periments with global models will initially use simple representations, but the lessons learned and data bases developed will be critical to future im- provements. Early global experiments will be especially important to ex- ploring feedbacks between the production of long-lived trace gas species and climate. Global terrestrial ecosystem models will further our understanding of major phenomena operating within the earth system. Specific extended model studies will address coupled responses of climate and terrestrial eco- systems, perturbation of the global element cycles through the effects of chemical or climatic change on terrestrial ecosystems, and analyses of the effects of land use and terrestrial resource use patterns on the dynamics of climate and element cycling. Key modeling themes include the following: · Coupled responses of climate and terrestrial ecosystems. The ex- change of heat and moisture between the atmosphere and land systems is an integral part of the climate system that will be altered as terrestrial ecosys- tems respond to climatic change. Major land cover changes, such as shifts in the distributions of the major biomes, will be accompanied by companion responses in the moisture and heat regimes of the regions involved. Com- plex transient responses of climate and terrestrial ecosystems toward joint steady state conditions require scrutiny in terms of stability, resource and habitat maintenance, and sensitivity to further perturbation by human activi- ties. The joint responses of these systems will be complicated by broken correlations between environmental variables. For example, while tempera- ture at high latitudes may change, solar intensity and sun angle will not; both variables affect the structure and function of high-latitude vegetation in the northern hemisphere (see chapters 3 and 5~.

34 RESEARCH STRATEGIES FOR THE USGCRP Responses of terrestrial element cycles to climatic change. Climatic change, caused by increasing greenhouse gas concentrations, will further perturb carbon cycles in terrestrial ecosystems. Significant additional re- leases of carbon dioxide and methane into the atmosphere may result, and the cycling of other major elements in addition to carbon will be affected as well. Time constants of major carbon pools on land and the phonology of seasonal variations will be altered. Changes in the geographic patterns in the amplitude and phase of the annual cycle of atmospheric carbon dioxide concentration may be an early indicator of terrestrial ecosystem responses to climatic change. Major model experiments will address these responses of the terrestrial components of the global element cycles to climatic change. The results will contribute to predictions of further greenhouse gas increases and interpretation of satellite data in terms of biogeochemical cycling and may suggest management and resource utilization schemes to minimize the impacts of global change (see chapters 3 and 5~. Atmospheric chemical forcing on terrestrial element cycling and eco- system structure and composition. There are already extreme cases of chemical- stress-induced chronic changes. The challenge is to understand and predict the effects of less extreme situations, which may, however, over time ex- hibit chronic conditions. In addition to considering the effect of chemical stress on terrestrial ecosystems, it will be important to couple the effect of changing climatic conditions as well (see chapter 6~. Modeling the effect of land use change on terrestrial ecosystems. Fu- ture patterns of land and resource use will continue to alter element cycles on land as climatic change occurs. Additional releases from terrestrial pools because of human activities will make a significant contribution to further increases in atmospheric greenhouse gas concentrations. Model studies integrated with satellite data on land cover and its state can monitor the effects of expanding human activity on the terrestrial components of the major element cycles. Finally, experiments using regional ecosystem mod- els and mesoscale climate simulations may begin once we have more so- phisticated models. These experiments will serve as test beds for the early inclusion of so- phisticated biology into atmospheric-biospheric models. Use of mesoscale ecosystem-atmospheric simulations will be important to understanding inter- actions between the biosphere and atmospheric phenomena such as precipita- tion, boundary layer dynamics, and mesoscale circulation. Efforts in this arena should be coordinated with ongoing field programs such as the Atmo- sphere Boundary Layer Experiment (ABLE) and the First ISLSCP Field Experiment (FIFE) (see chapters 5 and 6~. These intensive field experi- ments provide the empirical and theoretical framework for this type of mod- eling; this type of study should form the basis for research in this area. This level of model will also serve to address interactions between the

INTEGRATED MODELING OF THE EARTH SYSTEM 35 biogenic emissions of short-lived trace gas species and chemical climate. Coordination with the International Global Atmospheric Chemistry (IGAC) program and other ongoing activities could serve as a foundation for this type of activity. Summary The changes suggested by current, relatively simple GCMs make it im- perative that we seek to couple models of ecological and biogeochemical processes to climate models. Fortunately, the time is now ripe to make this coupling. Ecological and biogeochemical models consider processes at three char- acteristic rates of change, and these must be integrated in order to produce appropriate parameterizations for earth system models. Long-time-scale ecological models that emphasize carbon and element storage and popula- tion dynamics can be driven by integrations of climate models, but their primary feedback is through the carbon budget and surface roughness. Car- bon exchange, net ecosystem production, and trace gas biogeochemistry are best treated at an intermediate time scale and provide feedback to the atmo- sphere through trace gas exchange and water budgets. Physiological time scale models are required to handle surface exchange of water and energy but must be integrated with other exchanges such as nutrient availability and community type. Examples of these three types of models are now in use; the new challenge is their coupling to atmospheric models. In this process, it is likely that simulations will be both run together and linked only by exchange of parameterizations. At this stage, simple models (one dimensional, local scale, single pro- cess) play a critical role in the development and integration of new science into the dimensional models. It is also likely that development of coupled atmosphere-terrestrial ecosystem models will require some intermediate as- sembly and testing using regional atmospheric (mesoscale) models. While the multiscale approach appears complex, it provides critical opportunities for validation and testing using techniques that are now available. A1- though global ecosystem models do not have as much heritage as atmospheric and oceanic circulation models, the science is mature and ready to move into this arena. Finally, two questions should remain the focus for this atmosphere-ter- restrial modeling program: 1. How will global environmental change affect the ecosystems on which mankind depends? 2. How will changing ecosystem structure and function in turn influence the global system through feedbacks to the atmosphere and hydrosphere?

36 RESEARCH STRATEGIES FOR THE USGCRP These are both critical issues, and experience suggests that they drive rather different modeling efforts. Both the detailed modeling efforts suggested by the first question and the more heavily parameterized models that are re- quired to run in consort with atmospheric and oceanic models to address the second question are needed and should be pursued in an integrated fashion. PHYSICAL-CHEMICAL INTERACTIONS IN THE ATMOSPHERE Models of the physical processes in the atmosphere provide much of our current basis for understanding future climatic change. However, there are still considerable shortcomings in their formulation and implementation, and thus they provide very uncertain projections for the future. Present models have evolved through the combination of two separately developed approaches. On the one hand, they incorporate the contributions of atmo- spheric dynamics and adiabatic thermodynamics through an approach of computational fluid dynamics that was initially developed in the 1950s to provide an objective numerical approach to weather prediction. It is some- times forgotten that the initial developments of supercomputers at that time were motivated in large part by the need to solve this problem. The other approach focused primarily on energy balance and was the basis for climate models even in the "back of the envelope" mode. The thermal/fluid dynamics approach to the weather system has tended to focus on the application of the most efficient and accurate discrete repre- sentations of the Eularian, Navier-Stokes, and temperature equations for a compressible atmosphere on a rotating sphere. Meteorological observations are analyzed into initial fields consistent with the model dynamics and then the prognostic variables (e.g., the horizontal winds, temperatures, and sur- face pressure) are specified from these initial fields and integrated forward in time to generate future weather systems. In the 1960s, versions of these weather prediction models were developed to study the general circulation of the atmosphere, that is, the physical statistics of the weather systems. These models were designed to show how they satisfied global require- ments of conservation of momentum and energy. Such model experiments demonstrated that it was necessary to begin to include energy sources and sinks, in particular, exchanges with the surface, moist atmospheric pro- cesses (moist convective adjustments and precipitation), and the attendant latent heat release and radiative heat inputs. Until the recent emphasis on the climate problem, the treatment of these physical processes has always been oversimplified, and this is still generally true in current models. Perhaps the most progress has been made in the treatment of radiative processes, and this has been largely a result of the (initially) independent

INTEGRATED MODELING OF THE EARTH SYSTEM 37 research stream of radiative-convective energy balance models of the cli- mate system. The principle of global energy balance is a powerful concept and is the primary basis for our understanding of future global warming (Figure 2.8~. In simple terms, the absorption of solar radiation must over a long enough time be balanced by temperature-dependent emission of thermal infrared radiation. For a black body, this emission would be given by the Stefan- Boltzmann law. The point of simple energy balance models is to describe how the global average temperature actually responds to changes in radia- tive processes, using black body emission as a reference point. In including an atmosphere in these models, it is conventionally assumed that the tropo- sphere is in convective equilibrium, and thus there is only a single degree of freedom for determining temperature from energy balance, whereas the stratosphere and layers above are in radiative equilibrium, with each level adjusting its temperatures according to a local energy balance. The relative simplicity of these models has allowed both the development of conceptual descriptions of the various feedbacks that determine the response of global temperature to changes in radiative forcing and the development of more detailed and accurate radiative models for atmospheric gases and clouds. Hence the initial evaluation of the contributions of various trace gases to global warming has exclusively made use of such models. The effect of changing atmospheric composition, in simplest terms, is estimated by calcu- lating the change of radiative fluxes at the tropopause, assuming the tropo- sphere is unchanged, and then converting this to a temperature change according to our best estimates of climate feedback processes. In incorporating these concepts into the three-dimensional GCMs, it is imperative that the global energy conservation properties of these models be well understood and interpretable in terms of the simpler climate models. This is not a trivial requirement because the three-dimensional models have many complex energy exchange processes, and so it is easy to introduce spurious energy sources and sinks either through nonconservative numerical procedures or through physical approximations. This might be the case, for example, if a model uses a different treatment of latent heat release for precipitation than it does for surface melting and evapotranspiraiion. Be- cause of the large number of potential sources of error, it is probably im- possible to have a model that conserves energy perfectly, but models should be validated to conserve energy to better than 1 W/m2 and preferably have errors of less than 0.1 W/m2. The change of atmospheric radiation from doubling carbon dioxide is about 4 W/m2, a number that incidently comes from the one-dimensional model approaches referred to above. For an atmospheric model coupled to a surface with ocean temperatures prescribed from observations, the radiative imbalance at the top of the atmosphere should be considerably smaller than this to prevent spurious climatic change

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INTEGRATED MODELING OF THE EARTH SYSTEM 39 when coupled to an oceanic model. These concerns for conservation are related to the concern, discussed in the section "Atmosphere-Ocean Subsystem" (below), about numerical drift apparent in coupled atmospheric-oceanic GCMs. Future changes in the earth system will probably result from increasing emissions (and atmospheric concentrations) of greenhouse gases such as carbon dioxide, chlorofluorocarbons, methane, and nitrous oxide (see Logan et al., 1981; OIES, 1985; Schimel et al., 1989~. These substances have biological, industrial, and other anthropogenic sources. In addition to the direct radiative effect, some of these gases undergo atmospheric chemical and photochemical transformations that alter the natural balance of other atmospheric gases. For example, the changes in ozone levels in both the stratosphere and the troposphere are a source of serious global concern. Tropospheric ozone, which is toxic to plants and animals and also acts to enhance the greenhouse effect of the atmosphere, has strongly increased in recent decades. This trend is believed to result from increasing emissions of the oxides of nitrogen and volatile organic compounds as well as meth- ane and perhaps carbon monoxide. As a result, the "oxidation capacity" of the atmosphere has been modified, as has the atmospheric lifetime of green- house gases such as methane and certain halocarbons. Clearly, the chemis- try of the atmosphere plays a major role in the earth system, and models attempting to describe this system must include a chemical-photochemical component. Models that incorporate atmospheric chemical processes provide the ba- sis for much of our current understanding in such critical problem areas as acid rain and photochemical smog production in the troposphere and deple- tion of the ozone layer in the stratosphere. The very formidable nature of the problems requires that the models not only include chemical processes, but also dynamical and radiative processes, which through their mutual interactions determine the circulation, thermal structure, and distribution of constituents in the atmosphere (i.e., requires a coupling of the physics and chemistry of the atmosphere). Furthermore, the models must be applicable on a variety of spatial (regional-to-global) and temporal (days-to-decades) scales. The traditional tool in atmospheric chemistry studies for many years has been a one-dimensional model. Current versions of one-dimensional mod- els include a large number of chemical reactions (often exceeding 100) coupled with a radiative transfer model. Vertical transport is modeled through incorporation of an eddy diffusion term, not unlike the earlier ocean carbon one-dimensional models (e.g., Oeschger et al., 1975~. These models are often incorrectly interpreted as "globally averaged" models in some sense. This interpretation arises because the eddy diffusion coefficient is generally inferred from a well-mixed, long-lived tracer distribution. However, these

40 RESEARCH STRATEGIES FOR THE USGCRP models are generally most applicable to mid-latitude regions. The virtue of these models resides in their ability to provide long-term (decades) simula- tions with modest computational resources. The inability to model horizon- tal transport severely limits the utility of these models for future research. Progress in the development of two- and three-dimensional models and availability of more powerful computers have also negated some of the advantages of the one-dimensional model. However, these models will undoubtedly continue to be used for many years as a quick and convenient means of examining the effect of new reactions, changes in spectroscopic data, or reaction rates and alternate scenario studies. More recently, atmospheric chemistry studies have relied upon two-di- mensional models. These models solve the zonally averaged momentum, thermodynamic, and mass continuity equations, including detailed treatment of chemical and radiative processes. Because of the increased computa- tional requirements, many such models group related constituents into "families" to avoid explicit integration of a mass continuity equation for each indi- vidual chemical species (not unlike the grouping that occurs in ecosystem models). These models must include the effects of horizontal transport by zonally asymmetric motions (waves or eddies) by eddy diffusion terms analogous to the approach adopted for vertical transport in the one-dimensional models. As a consequence of these approximations of the actual physics, these mod- els do not correctly represent the interactive behavior of the chemical, ra- diative, and dynamical processes. Despite such shortcomings, these models have provided significant additional insight into atmospheric chemical pro- cesses through incorporation of horizontal motions. They will necessarily provide the basis for ozone assessment studies for at least several years until significant progress is made in developing three-dimensional models and acquiring more powerful and affordable computing resources (e.g., Bass, 1980; Demerjian, 1976~. Three-dimensional global transport models for chemically active species have been under development for the global atmosphere since the early 1970s. The first such models considered the fate of long-lived gases such as nitrous oxide or the CFCs and emphasized the improvement of the trans- port formulation associated with GCMs. More recently, attempts have been made to develop more chemically intensive three-dimensional global mod- els. Although efforts to include chemistry in a three-dimensional model date back at least two decades, progress has been relatively slow owing to the enormous computational requirements for treating the fluid dynamic equations alone. The additional burden imposed by incorporating detailed chemistry into a comprehensive GCM makes long-term simulations imprac- tical. Current three-dimensional atmospheric chemistry models seek a com- promise solution by some combination of the following: adopting a relatively coarse resolution (in both the vertical and the horizontal dimensions); incor

INTEGRATED MODELING OF THE EARTH SYSTEM 41 porating constituents by families (similar to the practice used in most two- dimensional models); omitting or simplifying parameterizations for tropo- spheric physical processes; and conducting "off-line" transport simulations in which previously calculated wind and temperature fields are used as known inputs to a series of mass continuity equations including chemical source-sink terms. This last approach renders the problem tractable and has provided much progress toward understanding the transport of chemically reacting species in the atmosphere. But this progress has come at the cost of neglecting the interactive feedback between the evolving species distri- butions and the atmospheric circulation, and so this approach is insufficient for predicting aspects of global change (see NRC, 1984~. A Modeling Strategy: Prognosis for Progress Considerable progress is still needed in the development of uncoupled atmospheric circulation and chemistry models, and it should be recognized that efforts to develop a fully coupled atmospheric GCM with linked chem- istry and physics are still in their infancy. The major hope for attaining this goal within the next decade lies in the concomitant developments in mas- sively parallel processing computers and our basic understanding of the complexities at the interface between the chemistry of the atmosphere and the physics of the atmosphere. The heterogeneous reactions that produce the ozone hole are but one reflection of these complexities. One of the major challenges is to incorporate heterogeneous reactions into models. Compelling evidence now exists that during antarctic spring- time in recent years reactions on the surface of polar stratospheric cloud particles have been instrumental in the destruction of polar ozone. Much more research in laboratory kinetic studies is required to determine the reaction rates to be used in model simulations. These reaction rates are functionally dependent on available particle surface area. Microphysical models that can describe the growth rate and size distribution of the cloud particles are thus an integral requirement for including heterogeneous reac- tions in atmospheric chemistry models. If particle growth is sufficient, sedimentation of particles can occur, resulting in denitrification and dehy- dration, which contributes further to the perturbed chemistry occurring in the polar vortex. Additional observations would be extremely useful to provide a basis for developing a parameterization for this process. The possibility of heterogeneous reactions occurring on sulfate aerosol particles at mid-latitudes has recently been suggested. Of particular con- cern are situations associated with enhancements of the stratospheric aero- sol burden by large volcanic injection events (e.g., E1 Chichon). Although a substantial aerosol data base (of both satellite and ground-based data) ex

42 RESEARCH STRATEGIES FOR THE USGCRP ists, global atmospheric chemistry models typically do not include aerosol effects. In addition to the issues directly related to chemistry, there are a number of shortcomings in current models that are related to dynamical processes and thereby affect the ability of the models to predict the distribution of chemically reacting species. For example, global models typically do not simulate those equatorial wave modes (Kelvin and Rossby gravity waves) that are thought to force the semiannual and quasi-biennial oscillations in the stratosphere. This inadequacy of the models is a result of either insuffi- cient resolution or failure to include properly tropospheric convective pro- cesses believed to be the source of the Kelvin and Rossby gravity waves. Some atmospheric chemistry models (notably two-dimensional models) have attempted to include these effects by ad hoc methods. Gravity waves that propagate into the mesosphere represent another ex- ample of an important atmospheric wave that global models fail to resolve. These waves deposit momentum as well as contribute to small-scale mixing of constituents in the mesosphere. Most global models that extend into the mesosphere incorporate only a simplified parameterization to represent gravity wave drag. Finally, current atmospheric models suffer from serious deficiencies that detract from their use to study atmospheric chemistry. A critical deficiency is the inadequate treatment of cloud processes and the hydrological cycle in general. This deficiency, which is discussed more fully below, is a result of both a shortage of computational resources and an incomplete understand- ing of the hydrological cycle, which results in poor simulations of the ob- served water distribution. In sum, there is an emerging consensus that three-dimensional models (and two-dimensional models as well) may require significantly higher resolution than is now commonly used. Furthermore, there is a need for long-term simulations (tens of years) to examine the interannual variability of both climate and chemical properties exhibited by the models and the degree to which that variability is consistent with observed statistics. These consider- ations pose resource demands in order to achieve progress. Additionally, there are many gaps in our fundamental understanding of chemical, physi- cal, and dynamical processes that inhibit progress in modeling climate and atmospheric chemistry and their coupling. Research Priorities Much progress has been made in recent years in the development of GCMs and the use of chemical models to understand key processes in the troposphere, stratosphere, and mesosphere. But more sophisticated physical parameterizations and chemical and transport codes are required as impor- tant components of earth system models and should be developed. In this

INTEGRATED MODELING OF THE EAR: TH SYSTEM 43 section the submodels or components needed to make further progress in simulating the chemistry of the atmosphere are briefly discussed. The focus is primarily on the chemistry, although some key issues regarding GCM development are presented. Because of the uneven distribution of emission sources at the surface of the earth and the role of meteorological processes at various scales, models of chemically active trace gases in the troposphere should be three-dimen- sional and resolve transport processes at the highest possible resolution. These models should be designed to simulate the chemistry and transport of atmospheric tracers on a global and regional scale, with accurate parameter- izations of important subscale processes. It is therefore necessary to have an ambitious long-term perspective to develop comprehensive models of the tropospheric system, including chemical, physical, and eventually biological components. The development of such models and their integration in even more complex earth system models will require stable long-term support for interdisciplinary research teams. A large effort will have to be devoted to studies of various individual processes, with an emphasis on those that affect atmospheric chemistry on the global and regional scales. Models of the critical components of the tropospheric system will pro- vide the basis for the three-dimensional chemistry-transport models. Impor- tant examples of such component models are the following: . Models for biological and surface sources. Three types of models of the bottom boundary condition are needed: (1) Global empirical models of surface emissions are necessary to extrapolate and interpolate individual measurements provided in different environments under different condi- tions. These models will be based on empirical relations accounting for the variation in emissions with climate parameters such as temperature, solar radiation, and soil moisture. (2) Models of detailed biological mechanisms associated with trace gas emissions from soil, vegetation, and oceans need to be developed. The processes to be considered will range from a leaf of a tree to an entire ecosystem. (3) Models of surface exchanges and transport in the boundary and surface layers need to be used to describe the transfer between the ocean or land surfaces and the atmosphere. These models will also simulate the physical mechanisms that are responsible for the deposi- tion of gases on different types of surfaces (see Bolin and Cook, 1983; IPCC, 1990; Lenschow and Hicks, 1989; NBC, 1984; Schimel et al., 1989~. · Models of long-range transport. Transport models driven at high tem- poral resolution by "weather systems" generated by GCMs are required to simulate how advection, turbulence, and convection affect the chemical composition of the atmosphere. Several numerical approaches can be used, including Eulerian and Lagrangian formulations. These models will be used (1) with minimum chemistry, to simulate the global distribution and variability of long-lived species including water vapor and emphasizing

44 RESEARCH STRATEGIES FOR THE USGCRP

INTEGRATED MODELING OF THE EARTH SYSTEM 45 are used in relation with the ozone budget. A large number of studies are based on two-dimensional models that include a relatively detailed chemi- cal scheme as well as a radiative code that takes into account the important coupling between radiative (temperature) and chemical processes. Three- dimensional models need to be developed and applied to the middle atmo- sphere. These models will be able to explicitly reproduce the propagation of planetary waves and simulate the effects of these waves on the meridional transport of the trace gases such as ozone. These models are also needed to study the dynamical and chemical mechanisms involved in the formation and dissipation of the ozone hole over Antarctica. Models of the strato- sphere should include the effects of aerosols and polar stratospheric clouds on chemical budgets. · Models of cloud processes. Models of cloud processes on the small scale to the mesoscale should be developed to parameterize the transport of trace species including water vapor and the feedback between clouds, their radiative properties, and large-scale circulation. Summary Further progress in the modeling of atmospheric chemistry requires a better knowledge of several factors: (1) Surface sources of trace gases, in particular emissions by the biosphere, will be better understood through systematic observations of emission fluxes by different ecosystems under variable meteorological conditions and by the development of ecosystem and surface models that will provide parameterizations of these emissions. (2) Chemical processes and reaction channels, both in the gas and in the aqueous phase, will be most usefully investigated in accelerated laboratory studies. (3) Transport processes by advection and convection will be illu- minated through the development of high-resolution transport models coupled to atmospheric GCMs with detailed representation of physical processes including cloud formation and boundary layer transport. Further progress in the modeling of atmospheric physical processes de- pends in part on carrying out simulations at higher resolution. This is achieved through the acquisition of more powerful computational resources through advancing technology and better funding. Some progress is also possible through improvements in numerical and programming procedures and the use of finer meshes over limited areas of the globe. Improvements in the treatments of model processes, especially those in- volving clouds and the hydrological cycle, surface energy exchanges, and interactions of these with radiation, are also crucial for further progress in modeling the atmosphere. These improvements must be paralleled by the acquisition of global data sets for validation of these treatments. Validation of models against global and regional requirements for conservation of en

46 RESEARCH STRATEGIES FOR THE USGCRP ergy is especially important in this regard. Feedbacks to and from the land require careful attention to the treatments of evapotranspiration, soil mois- ture storage, and runoff. All these occur on spatial scales fine in comparison with those of the model meshes, and so the question of scaling must be addressed. Other key developments are the treatment of subgrid-scale con- vection in the models including the planetary boundary layer with moist processes and the treatment of cloud and precipitation physics to provide radiatively important parameters such as cloud liquid water and drop size distributions. Future progress will depend not only on available computational resources but also on advances in understanding fundamental physical and chemical processes and the nature of their coupling. In more direct words, progress will be determined. in Dart. bv the scientific talent that is focused on the issues of global change. finally, -7 r ~ ~ ATMOSPHERE-OCEAN SUBSYSTEM The challenge is to develop sufficient understanding of the coupled at- mosphere-ocean system, including biogeochemical components, to allow well-reasoned prognostic statements about the dynamics of this system. Of particular interest is how the atmosphere-ocean system affects and is af- fected by changing global phenomena, such as climate, on interannual to decadal to centennial time scales. For example, in the context of a possibly changing climate under a given scenario of anthropogenic production of carbon dioxide what can be said about: the ocean)_ uptake and transport of carbon dioxide; the oceanic uptake and transport of heat; the distribution of new and total oceanic primary production; and, what will be the impacts on, and feedbacks from, the ocean as the result of a changing hydrological cycle? Answering such questions will require an expansion of the comprehen- siveness of the processes that current models incorporate explicitly or im- plicitly through parameterization. In addition, in some cases, parameterizations need improvement; in others, previously parameterized processes may need to be resolved or previously resolved processes may lend themselves to parameterization. There are also issues regarding the coupling of oceanic, atmospheric, and biogeochemical models, including the problems of pos- sible mismatches of time and space scales. The process for substantially improving current models as well as creat- ing new models should recognize the need for exploration, evolution, and close interaction with major field programs (see chapter 71. The develop

INTEGRATED MODELING OF THE EARTH SYSTEM 47 ment needs to be geared toward early tests and trial attempts. In fact, some "great failures" early in the effort would be very instructive in guiding model development not only in the atmosphere-ocean system but through- out the earth system modeling effort. Fortunately, the foundation the ex- isting models for this development is both broad and deep. These existing models include uncoupled and coupled models. Uncoupled Models Uncoupled models of oceanic circulation, atmospheric circulation, and biogeochemical-biological processes are being employed and improved. These three classes of models have varying degrees of maturity and sophistication. Perhaps the greatest degree of development has been reached by atmo- spheric GCMs, in which simulations with prescribed oceans (and other boundary conditions) replicate many of the observed features of weather and climate (see the section "Atmosphere-Terrestrial Subsystem" above). Similarly, although oceanic GCMs are somewhat less developed than their atmospheric counterparts, oceanic simulations in which air-sea exchanges are prescribed replicate many aspects of the large-scale characteristics of ocean tempera- ture, salinity, and circulation. Least developed are biogeochemical-biologi- cal models in which physical processes such as fluid flows and mixing are prescribed from either theory or observations and ecosystem dynamics de- velop under forcing and controls of sunlight, temperature, and nutrients. These models nevertheless reproduce the evolution in time (e.g., seasonal changes but not interannual trends) of biological populations and the corre- sponding rate of primary and secondary production. However, they tend to deal with lumped populations (phytoplankton, zooplankton, and so on) rather than biogeochemical state variables (such as total dissolved inorganic car- bon and alkalinity) necessary for issues such as the carbon cycle. Until recently, uncoupled oceanic GCMs have come in two distinct vari- eties: (1) basin-wide to global, coarse-resolution models that fail to resolve mesoscale eddies and (2) eddy-resolving models that are confined to limited domains. Now these two varieties of GCMs are converging. Advances in computational power and improvements in coding have allowed global oce- anic GCMs to achieve sufficiently fine spatial resolution to begin to resolve mesoscale eddies. This convergence between "coarse scale" and "eddy resolving" oceanic models is similar to that occurring between "weather prediction" and "cli- mate" models, as noted in the section "Physical-Chemical Interactions in the Atmosphere" (above). High-resolution models used in short-term weather forecasts now incorporate processes and components that are traditionally the purview of climatology (e.g., soil moisture and other parts of the global hydrological cycle) in an attempt to extend the useful range of forecasts;

48 BESEECH STRATEGIES FOR THE USGCRP coarse-resolution climate models are beginning to utilize enhanced spatial resolution and account for additional processes (e.g., topographic interac- tions with evapotranspiration). Uncoupled marine biological and biogeochemical models with prescribed atmospheres and oceans are in the beginning stages of development and again in the two typical forms: (1) localized, high-resolution and (2) lower- resolution extended-area versions. The former tend to incorporate more complex ecosystem representations, whereas the latter focus on simplified biogeochemistry in basin-wide or global domains. There are, however, some recent rather encouraging attempts (e.g., the joint work at Princeton University and the Geophysical Fluid Dynamics Laboratory (GFDL);. see Sarmiento et al., 1988; Toggweiler et al., 1989; and the work in Hamburg; see Maier-Reimer and Hasselman, 1987) at adding biological complexity to global treatments. There are several examples of high-resolution localized biological mod- els in which a specified flow field is used to advect and diffuse changing biological populations. Models of this type generally are formulated and implemented to study processes that occur over limited space and time scales in specific coastal regions and rely upon synoptic distributions of such properties as mixed-layer depth and phytoplankton distribution. At present, they are sufficient for investigating processes; they are not at the level of sophistication or resolution needed for attempting prognostic inves- tigation. Interestingly, the addition of simple (though increasingly complex) bio- logical-biogeochemical models into oceanic GCMs could allow for the study of possible biological changes under a forcing of changing climate and oceanic circulation fields. Of particular interest would be the species shifts and other ecological transitions that likely would occur in a changing cli- mate; these issues are not as yet considered in this complex modeling envi- ronment (within a GCM), and hence changes in the biogeochemical system that might result from ecological changes are not treated. Also in this context, although there is no true coupling (the physics drives the biology- there is no feedback from the biology such as effects on mixed-layer depth), the structure certainly has the potential to allow a biological feedback on the circulation. These biological effects are important, since simple box models indicate that changes in oceanic biogeochemistry can profoundly influence the atmo- sphere-ocean partitioning of carbon. Addressing future changes in oceanic biogeochemistry is essential to bracket adequately the oceanic uptake of carbon dioxide. This capability requires understanding the dependence of biological-biogeochemical processes, especially new production and decom- position and carbonate formation and dissolution, on current and future circu- lation patterns. Currently, we cannot even specify the sign of the feedback.

INTEGRATED MODELING OF THE EARTH SYSTEM Coupled Models 49 Coupled three-dimensional circulation models of both the ocean and the atmosphere have been under development for several years. Recently, some exploratory attempts have been made to study the response of the model coupled climate system to realistic time-evolving scenarios of the green- house forcing. However, coupling oceanic and atmospheric models intro- duces new problems of unrealistic long-term drift and instabilities that are not exhibited by the uncoupled models. Further, adding biogeochemical processes to coupled oceanic-atmospheric GCMs is a prerequisite for com- plete description of the effect of carbon dioxide removed from the atmosphere by the ocean. Adding biology and chemistry to GCMs is a nontrivial exer- cise. The complexity of the fine-scale response of uncoupled ecological models, if proven necessary, could potentially overwhelm available and foreseen computing resources. Coupled abiotic oceanic-atmospheric global GCMs are beginning to be used to project the delay in the climate's response to greenhouse forcing due to the thermal inertia of the oceans (e.g., Bryan et al., 1982; Cess and Goldenberg, 1981; Manabe and Wetherald, 1975~. However, the conclu- sions are still uncertain to within a factor of at least 2 (IPCC, 1990~. Fur- thermore, the pattern of oceanic response (e.g., which hemisphere warms faster) is also model dependent. The use of coupled oceanic-atmospheric general circulation models (O/A-GCMs) in short-term forecast and data assimilation models is expanding; this will probably put prognostic climate simulations on a firmer foundation. Coupled models of the oceans and biogeochemical systems are beginning to be explored. Particularly important in this exploration is the use of satellite-derived ocean color data (see chapter 7) and the consideration of the biological controls on mixed-layer depth and thermal distribution in the surface layer of the ocean. For example, vertical changes in the water column heating rate due to the vertical gradient of phytoplankton can poten- tially alter vertical mixing intensity and the depth of the mixed layer. In- clusion of such processes could be important in earth system models fo- cused on global change on decadal time scales since such processes provide a feedback from the biological system to the physical system. Also, changes in the intensity of solar irradiance constitute an important aspect of this physical-biological interaction, which in turn couples the oceanic physical- biological model to atmospheric processes. Most of this work, thus far, is in the form of either preliminary calculations using satellite data in order to determine the importance of the phenomena at global scales or generic mixed-layer models that are parameterized using oceanic data for specific regions (e.g., Bermuda), thereby producing simulations of biogeochemical cycles on both global and regional scales. Incorporating into the models

so RESEARCH STRATEGIES FOR THE USGCRP descriptions of the oceanic surface state (wave models) will improve the calculations of air-sea carbon dioxide flux calculations. Another theme in modeling a coupled ocean-biology system that merits further exploration is the use of physical and chemical tracers coupled with a set of independent constraints (e.g., geostrophy and poleward flux of heat) to infer, through a matrix inversion or linear (or quadratic) programming technique, the rates of oceanic motion and biological activity that are con- sistent with these tracer fields (e.g., Bolin et al., 1983; Moore et al., 1989a; Wunsch, 1978; see also Broecker and Peng, 1982; Riley, 1951~. The result- ing model is obviously not prognostic, and its coupling between biology and physics is but a simultaneous or perhaps statistical parameterization, but the approach has great value in that it yields a diagnostic tool for use in an active interplay with efforts to incorporate biological-biogeochemical models into oceanic GCMs. Finally, coupled models of the oceans, atmosphere, and the biology- biogeochemistry are in the beginning stages of development for at least local and regional domains. Such limited-area models, of course, are in- complete in the sense that boundary conditions at the sides of the domain must be externally specified. (This consideration also restricts the useful time span covered by such models to just one or two weeks.) Theoretically, such models could be extended to cover much larger-even global do- mains. It is an ambitious but not unrealistic goal to have a fully coupled, prognostic (highly simplified), three-dimensional oceanic-atmospheric GCM including biological-biogeochemical dynamics running by the year 2000. A Modeling Strategy: Prognosis for Progress There are traditionally two approaches to a modeling strategy; in jargon, they are called "bottom-up" and"top-down." The former seeks to improve models by improving specific processes through explicit process-specific experimentation or data recovery. The latter studies the macroresponse of models to major perturbations or phenomena and thereby seeks to evaluate and hence improve models. Bottom-Up: A Focus on Key Processes The realism or quality of today's generation of oceanic GCMs is limited by a variety of numerical and dynamical factors. Among these are (1) an inadequate theoretical or observational understanding of certain key pro- cesses fundamental to the earth's coupled climate and biogeochemical sub- systems and a corresponding and continuing uncertainty as to how best to incorporate or to parameterize them in oceanic GCMs and (2) a need for improved numerical methodologies (both prognostic and assimilative), for

INTEGRATED MODELING OF THE EARTH SYSTEM 51 the use of enhanced resolution within the models, and for the computational and observational resources necessary to run and to validate the models. Here, the committee attempts to identify those physical processes that, al- though important to the ocean's role in climate and biogeochemical sys- tems, are nonetheless poorly understood from either a theoretical or an observational perspective. In making this identification, the committee has tried to categorize crudely the processes according to their characteristic (horizontal) spatial scale; the three categories used here are (1) small scale (10 km or less in the horizon- tal or 10 m or less in the vertical), (2) mesoscale (from the grid scale of the oceanic GCM, assumed to be 10 km to a few hundred kilometers), and (3) basin and global scale. The resulting "small-scale" processes are by defini- tion therefore likely to be unresolved by any foreseeable GCM system, but linear effects on larger-scale phenomena will nevertheless need to be incor- porated via adequate parameterized terms in the oceanic equations of motion. The processes labeled "mesoscale" are those whose spatial extent is subbasin scale, resolvable by the numerical model grid, and known (or presumed) to be crucial to the form and strength of the global circulation. Likewise, processes operating on the largest spatial scales have been termed "basin and global scale," and discussion of these shifts to a "top-down" focus. Key Processes: Small Scales The small-, or subgrid-, scale processes, which need to be much more thoroughly understood and, ultimately, better param- eterized, can be broken into two categories: (1) those that have both physi- cal and biological consequences and (2) those that are strictly biological in their implications. In the former category are, for instance, horizontal and vertical diffusion, and small-scale convective processes; and in the latter are a wide variety of biological-biogeochemical interactions including graz- ing, photosynthesis, and microbial processes. Examples of questions in- clude the following: What processes determine the various temporal and spatial scales of tropical convection? How do the finest scales interact with the mixed layer? Does turbulence affect location and identification of prey by grazers? Is microscale patchiness in nutrients important in regulating nutrient incorporation by phytoplankton? Small-scale processes are important for one simple reason: their effects are biased. Thus the mean of the small scales over sufficient length and time scales is significant. A recent workshop on subgrid processes showed the dependence of vertical and horizontal fluxes on the nature of the small- scale parameterization. This bias may be taken advantage of when our interest lies in the longer space-time scale and we have sufficient data to parameterize the effect. The bias of turbulence is that in general small-scale processes are cas- cading to even smaller scales, which then are removed by friction. The

52 RESEARCH STRATEGIES FOR THE USGCRP small scale must be related to the large because the source of the small scale is the large scale and because scales will generally decrease. The reality is that coherent turbulent structures exist and at times small scales are combined to form larger scales. This implies, in effect, a negative eddy viscosity (diffusivity). The biological effect of small scales is seen in the nonlinear response of phytoplankton and zooplankton to light and nutrient concentrations. The result of such nonlinearity is that the average response of a system (biologi- cal communities) to a "patchy forcing" (by light or nutrient concentrations) is different from the response of the system (community) to the average forcing. In sum, bulb formulations seem adequate to a first approximation, but better parameterization of small-scale processes may be needed. By their very nature, all these processes are difficult to observe and model, being characterized by very small spatial (and sometimes temporal) scales. While the potential for small-scale effects has been identified, further progress is required in order to quantify the effects. The physical community is still working toward adequate parameterization of the eddy diffusivity, and the biological community is beginning to consider the effect of patchiness. Learning how to deal better with such processes, in the sense of providing adequate parameterizations or functional descriptions of them, will be a continuing concern; however, progress in these areas can be made by conducting care- fully designed oceanic and laboratory experiments to observe and quantify small-scale physical and biological processes. One example of the type of experiment necessary is the so-called Pacer release experiments, whose goal is to measure isopycnal and diapycnal mixing rates in the ocean. Limiting Processes: Mesoscales Over the past decade, much progress has been made in identifying and understanding several mesoscale processes that profoundly influence the oceanic circulation on either a regional or a global basis. The list of processes for which a preliminary, though far from complete, understanding is available includes mesoscale eddy dynamics, meandering jets, fronts, and upwelling. However, the relationships between mesoscale eddy dynamics and biological productivity are poorly understood. Despite this initial progress, however, other important phenomena on this scale remain incompletely explored. These include the formation and spreading of bottom and intermediate-depth water masses, and the effects of synoptic- scale atmospheric fluxes and forcing. Fortunately, on this scale, further understanding of processes including their respective roles in the climate and biogeochemical subsystems and their geographical and parametric dependencies-can be achieved as well by process-oriented numerical modeling as by direct observation. Both will be valuable and should be encouraged. For example, although much remains

INTEGRATED MODELING OF THE EARTH SYSTEM 53 to be learned about how best to model regional air-sea coupling and water mass formation processes, models do exist that are capable of preliminary, but sophisticated, exploration of such effects. Likewise, future observa- tional efforts (e.g., the Office of Naval Research's Oceanic Subduction Ex- periment, and TOGA/COARE; see chapter 7) will contribute the observa- tional basis for validating current mesoscale and basin-scale oceanic models. Mesoscale processes are particularly troublesome since they are too large to be safely ignored and yet the dynamics of biology and physics are not well understood at this scale. Further study employing both models and observations is needed to define which processes can be reasonably treated through parameterization and which must be modeled explicitly. Limiting Processes: Basin and Global Scales The behavior of the global climate and biogeochemical systems is clearly tied to the nature, structure, and properties of the global oceanic circulation and primary production. On its largest scales, the oceanic general circulation comprises several interact- ing elements, none of which is understood with the level of observational or theoretical understanding necessary, in the long term, to develop adequate global models. Of particular importance on the basin and global scales are intense boundary current systems (on both the eastern and the western bound- aries), basin-scale oceanic gyres, coastal/deep ocean interaction and exchange, ice dynamics, and deep abyssal circulation and its relationship to ocean topography. Perhaps the most important biogeochemical processes are those that regulate the "efficiency" of the biological pump in high-latitude waters: Why is surface nitrogen trioxide always so high in the Southern Ocean? Here again, a mutual interaction among process-oriented numerical experi- ments, global-scale observational programs (e.g., WOCE, JGOFS, and GLOBEC), and assimilative modeling is needed to fill out our understanding of global exchange processes to the level necessary for the testing and validation of coupled climate models. Top-Down: A Focus on Phenomena In the top-down approach the strategy is one of extracting information from large-scale natural phenomena. There are several phenomena that are crucial to the coupled atmosphere-ocean-biogeochemical system. These phenomena are large-scale events or processes that should be accommo- dated in future global change models and will act as specific tests of the model dynamics (see the section "Critical Model Tests" (below), where specific global change tests are discussed). Such model tests should also play a critical role in developing hypotheses and experimental designs for observational programs focusing on these specific phenomena. As an out- come of such programs, large-scale data assimilation may be feasible and

54 RESEARCH STRATEGIES FOR THE USGCRP may be particularly important as part of model evaluation. Furthermore, studies of these large-scale processes will serve not only as diagnostic tests but also as prognostic tools. In order to illustrate the range of these phenomena, a few of them are described here: · E1 Nino/Southern Oscillation (ENSO). The modeling activities sur- rounding this phenomenon could serve as a standard for other large-scale, phenomenon-oriented modeling efforts. Significant progress has been made in ENSO models, especially in terms of the coupling of atmospheric and oceanic dynamics (the TOGA program has been central in this develop- ment), but the coupling between physical and biogeochemical processes such as new primary production and carbon dioxide exchange has not been modeled, although work is in progress. Because of the very large changes in the atmosphere and ocean (both physically and biologically) that occur in conjunction with this phenomenon, it offers an ideal arena for validating physical and biological models. · Poleward heat flux. Poleward heat flux in the ocean and atmosphere is crucial in the earth's heat budget. Large-scale, "steady" transport is fairly well understood. However, the uncertainties are large, and the mag- nitude (as well as the sign) of the flux is extremely sensitive to the param- eterization of the eddy fluxes. Other processes such as the "eddy-Ekman" flux (the interaction between the zonal wind and ocean surface temperature variability) are not well understood. The challenge for modeling will be to resolve these processes as well as the time delay between oceanic heat uptake and release and changes in atmospheric circulation. · Spring bloom. One of the difficulties with incorporating biological processes in oceanic GCMs (aside from the scale differences) is that bio- logical patterns often do not evolve smoothly; rather, they behave like "switching processes," whereby the biological system and structures change rapidly from one state to another. The spring bloom in the North Atlantic is an example of this behavior as well as being important in overall primary production of the ocean and carbon dioxide exchange. A series of modeling efforts should be developed focusing on this region. · Cross-shelf exchange. Processes associated with continental margin systems are continuous with those of the open ocean; furthermore, the ex- changes between the margins and the open ocean (i.e., the cross-shelf ex- changes) of material, heat, and momentum are important. The implementa- tion of models (e.g., physical-biological) to estimate exchanges between the margins and the open ocean has associated with it a significant problem the space and time scales that characterize processes, as well as the processes themselves, can be quite different in coastal and open ocean environments. For example, low-frequency, large-scale events such as the ENSO and me- soscale eddies and large-scale permanent features such as boundary currents and shelf-slope fronts can result in significant high-frequency alterations of

INTEGRATED MODELING OF THE EAR TlI SYSTEM 55 coastal systems with possibly major effects on the biogeochemical systems. Thus a severe test of a coupled oceanic-atmospheric-biogeochemical model will be how it resolves scale mismatches between continental margin and open ocean systems. It is likely that the initial coupling between the high- resolution coastal models and the lower-resolution open ocean component will be through bulk parameterization of the processes. The concern, then, is the need to develop realistic approaches for such parameterizations. · Biogeochemistry of aeolian deposits. As the central gyres and the Southern Ocean have been hypothesized to be iron-limited, aeolian inputs of Asian dust and other land-derived aerosols may play a crucial role in ocean primary production. A coupled model including such a phenomenon would also need to include a component describing changes in vegetation cover and land-atmosphere interactions to provide realistic input of dust to the atmosphere and eventually to the ocean. Such a model is currently beyond our grasp, but tests of the effects of prescribed dust inputs on bio- geochemical cycling could be performed. Research Priorities Fundamental oceanic processes important in the determination of the global biogeochemical cycles and the world's climate are poorly under- stood. Proper inclusion of these processes in models will require field programs tailored to understanding individual processes. The objectives of these studies will be to understand the specific mechanisms involved and to develop useful parameterizations. The set of field programs required to acquire the data needed to advance our knowledge of fundamental oceanic processes is already well defined (e.g., see chapter 7~. These programs also offer valuable opportunities for simultaneous observational efforts, and these should be encouraged. When- ever possible, observations of the atmosphere-ocean-biosphere should be made in conjunction with specific studies. In particular, oceanographic cruises frequently neglect the atmospheric marine boundary layer, biology is neglected by physical oceanographers, and biologists may minimize the physical measurements. However, the earth system is an interactive system that requires measurements of all components. Fortunately, there has been already an encouraging spirit of cooperation between TOGA, WOCE, and JGOFS. Such cooperation should be further encouraged. From the perspective of testing atmospheric-oceanic-biological models by the use of large-scale events or phenomena, we must know the details of the phenomena even if the eventual "outcome" is to parameterize them. Characterizing the scale and magnitude of the events will constitute critical tests of model validity. The important scales are typically large. Therefore satellite measurements are important, although they are also limited. In situ

56 RESEARCH STRATEGIES FOR THE USGCRP measurements will be required to define the subsurface variability (satel- lites do not detect many key processes, for example, new production and vertical fluxes) and to provide a baseline for satellite measurements. Cur- rently, an important concern is whether or not there will be an ocean color satellite in orbit during the major field campaigns (i.e., WOCE and JGOFS). This represents a potential critical gap in being able to link biological and physical phenomena. Data assimilation is an emerging reality in physical oceanographic mod- eling and observational studies and may be used advantageously by biologi- cal and biogeochemical oceanographers. This methodology aids in the in- terpolation of physical observations by adding dynamical constraints. While the quantity of physical data required to describe oceanic phenomena may be reduced by the use of data assimilative models, it is more likely that field estimates will be improved as a consequence of data assimilation. The first attempts are now being made to develop the techniques neces- sary to assimilate ocean color measurements into regional physical-biologi- cal models. This is a promising direction for the development of models that ultimately will have predictive capability for biological distributions in the ocean. One aspect that makes data assimilation into physical-biological models challenging is that updating one ecosystem component (e.g., phytoplankton from ocean color) requires that all other ecosystem components be adjusted so that they are in equilibrium with the updated field. More specifically, assimilative models will require estimates of the error fields of both the assimilated data sets and the processes that are being parameterized. This will allow quantitative estimates of the confidence in the forecast (or hindcast) fields being produced. For example, in assimilating ocean color data into a multicomponent ecosystem model, one needs to have an estimate of the errors in the satellite data in time and space (i.e., particularly those associ- ated with gap filling) as well as an estimate on the effect of zooplankton grazing within the model. Such error estimation will require synergy be- tween the modeling effort and the field programs. In spite of these difficulties, the initial attempts at assimilating ocean color data into physical-biological models have shown that the accuracy of the model is improved, but that the improvement of the model diminishes after a short time. The implication that data are needed at frequent intervals for assimilation into physical-biological models is a potential area of re- search that could be an important aspect of developing models to address problems of carbon dioxide uptake by the ocean. Specifically, given the inherent nonlinearity of biological processes as well as their occasional "switching circuit" behavior, present data assimilation techniques are inad- equate. Research into techniques involving nonlinear and nondifferentiable forms is needed.

INTEGRATED MODELING OF THE EARTH SYSTEM 57 From a more classical approach, biological models in which the flow field is set as a boundary condition have been in use for about 15 years for specific regional studies. Consequently, the dynamics and limitations in- herent in these models are beginning to be understood. If physical-biologi- cal models are to be developed to address the larger question of carbon dioxide uptake by the ocean, then the question arises as to how to extend the knowledge gained from regional, physically forced biological modeling studies as well as from geographically restricted fully coupled models to models developed for basin or global domains. While, in principle, it seems straightforward to simply increase the model domain, in practice, this is not so. At least three issues must be addressed: (1) how to link the biological dynamics to biogeochemical changes important for global carbon studies, (2) how much of the complexity that characterizes coastal biological sys- tems needs to be transferred to larger-scale domains, and (3) how to match the space and time scale requirements of coastal processes with those of larger-scale systems. These three issues represent fundamental problems that must be addressed if coupled oceanic-atmospheric-biogeochemical models are to be developed to investigate carbon uptake by the world oceans. Biogeochemical models link biology and chemistry at the level of nutri- ents and carbon dioxide and are generally based on the forcing of nitrate or phosphate fields. The full effect of the biology on the chemistry is gener- ally not included, nor is the full effect of the chemistry on the biology. Yet it is likely in a changing climate system that these processes may be impor- tant. We need to better understand the sensitivity of the climate system to changing biogeochemical systems, and, if found to be relevant, biogeochemical systems must be included in our climate models. Including these relation- ships is expected to be computationally expensive, and yet ignoring the interaction of the physical-chemical-biological systems may lead to poor predictions of climatic change. Local to regional three-dimensional, coupled oceanic-atmospheric-bio- logical models that use circulation fields obtained from sophisticated re- gional primitive equation circulation models to produce "predictions" of biological distributions are currently under development. The development of this type of model should be given particular encouragement since this development is essential for the realization of fully coupled global carbon cycle models. In particular, these regional models can be used to test and validate parameterizations and generalizations that are used in larger-scale models. The development of realistic fine-scale regional models is also desirable in that the output from these models can be used to specify the boundary conditions for the basin- and global-scale models. Perhaps the appropriate direction for modeling is along two parallel paths: one to develop basin- and global-scale models with increasing levels of coupling and the second to develop a series of regional fine-scale models that could provide

58 RESEARCH STRATEGIES FOR THE USGCRP boundary conditions and parameterizations tests for the larger-scale models. Each regional model could, thereby, include the complexity and dynamics appropriate for simulating the processes in a specific region, and at the same time the necessity for maintaining reasonable and consistent interfaces with the basin- and global-scale models would give the entire modeling effort an overall framework. Finally, much can be learned from extant models despite their limita- tions. Careful analysis of the sensitivity of the systems, which are approxi- mations to the climate and biogeochemical systems, will indicate the emphasis needed in observational and modeling studies. The importance of experience gained through modeling experiments, including failure, should not be un- derestimated. Failure, when carefully analyzed in the refereed literature, can be valuable to the scientific community as a whole. Summary The strategy is to acquire data through field experiments (e.g., TOGA/ COARE, WOCE, JGOFS; see chapter 7) designed to develop an under- standing of processes and a description of phenomena. Models may aid in the design and execution of these experiments as well as in the analysis and interpretation of the measurements. New understanding of key processes will be used to improve models and reduce or improve parameterizations. These model enhancements are expected to lead to the ability to describe biogeochemical and physical phenomena. This enhanced modeling ability should lead to improved climate estimates, including error bounds from which rational decisions may be made. Hence the development of the fully coupled models should be encouraged along two parallel paths: one to develop basin- and global-scale models with increasing levels of coupling and the second to develop a series of regional fine-scale models that could provide boundary conditions and parameterization tests for the larger-scale models. Each regional model could include the complexity and dynamics appropriate for simulating the processes in a specific region, and the neces- sity for maintaining interfaces with the larger-scale models would provide an overall consistent structure for model development. In seeking to develop models of the coupled atmosphere-ocean-marine biosphere and biogeochemical system, it is important, as mentioned earlier, to recognize the value of "great failure." Linking atmospheric-oceanic- biospheric models, even though costly in terms of human and computer resources, should begin sooner rather than later. These early, relatively primitive attempts will shed light on the difficult issues of scale, both spatial and temporal, and the associated questions concerning the degree to which vari- ous process complexities or details are required. This clarification may be of particular importance as we scale the biological-biogeochemical compo

INTEGRATED MODELING OF THE EARTH SYSTEM 59 nents from local-regional domains to basin-global domains, or as we seek to better define and formulate the upper mixed-layer physics and possible bio- logical feedback, such as shading due to phytoplankton blooms. At the least, such attempts will encourage the creation of needed infra- structure and will provide a basis for assessing better the required resources. Part of the infrastructure enhancement would be the establishment of mod- eling teams. Obviously, several parallel efforts will be needed. Validation of these models is both difficult and critical. The first step is to ensure that they reproduce major climate phenomena (e.g., the spring bloom and E1 Nino). Testing (not validating) the "interfacing" models as well as the earth system models that link them can be addressed in the U.S. Global Change Research Program. CRITICAL MODEL TESTS The earth system modeling program should include three interface mod- els, as well as models of the fully coupled system. This approach allows for the rapid development of science and its inclusion into the less computationally demanding (although still challenging) interface models. Also, certain av- enues of validation are open to the interface models that will be difficult to use for a full earth system model. The required abilities of the models and the critical tests needed before they can be used with confidence are dis- cussed below. As concepts are developed and tested in the interface models, they should be included into an evolving earth system model that will form the basis for long-term prediction. The Challenge and Critical Tests All of the models described below must be able to simulate system re- sponse to the forcing induced, for example, by a carbon-dioxide-equivalent doubling in the atmosphere. That is, all of the models must be able to simulate the transient response of oceans, ecosystems, chemistry, or physi- cal atmosphere to a change in physical climate induced by a greenhouse gas ~ . forcing. Other drivers and critical feedbacks (e.g., land surface albedo, clouds, and oceanic heat transport) should be included when developing physical climate scenarios for use as forcing functions. The forcing functions given to the interface models will evolve as tested concepts from the interface models are incorporated into the earth system model, presumably modifying its predictions of whole-system response to changing greenhouse forcing. Thus a continual interplay of interface and earth system models is required, allowing for cyclic validation, failure, and modification.

60 RESEARCH STRATEGIES FOR THE USGCRP Finally, validation is impossible in the classical con~ol-experiment mode. We have no other earth system to serve as a control, not to mention the difficulties imposed by the multidecadal time frame and policy-relevant aspects of this science. As a step in appraising these models, a series of critical tests is described below that exploit various data sets, including satellite data (e.g., Earth System Science Committee, 1988) and paleo-records (see chapter 3~. These "tests" of these interface models provide an evalua- tion of the models' capabilities prior to either their use in a predictive mode or their inclusion in an earth system model. The Interface Models Atmosphere-Terrestrial Subsystem The challenge for an interface model of the atmosphere-land biosphere is to predict responses to changes in such phenomena as water and energy exchange (and more generally the hydrological cycle per se), trace gas biogeochemistry, · primary productivity and ecosystem carbon storage, and · vegetation composition and structure due to the changing macroclimatic forcing andlor atmospheric chemical composition. Critical tests of this model prior to its use in the predictive mode will be to · reproduce current patterns of biogenic trace gas and carbon exchange, using past and current climate as drivers; · reproduce key aspects of coupling between the paleoclimate and pa- leoecological records within regions of interest; · simulate contemporary spatial and seasonal patterns of vegetation properties, including primary productivity worldwide, using satellite indi- ces as validation data; · capture patterns of ecological change along anthropogenically induced chemical gradients in the land component of this interface model; and · simulate surface fluxes of radiation, especially solar, and including spectral surface albedos in the atmospheric component of this interface model. Adequate simulation of amounts and spatial and temporal distribu- tion of precipitation must also be addressed. Physical-Chemical Interactions in the Atmosphere For this interface model of the physical atmosphere and the chemical atmosphere, the challenge is to predict the change in chemical climate through

INTEGRATED MODELING OF THE EARTH SYSTEM 61 a macroclimatic transient forcing. This will, of course, require either simu- lation or forcing functions for the biospheric sources, which could be de- rived from the atmospheric-terrestrial-biospheric model. Critical tests will include simulation of contemporary variations in global trace gas fields, especially of"inte- grator species" such as methyl chloroform and carbon monoxide; · methane and carbon dioxide concentrations and isotope ratios; large-scale tropospheric ozone features such as are observed in the tropics; · high-latitude stratospheric ozone; and exchange of water vapor between troposphere and stratosphere. Atmosphere-Ocean Subsystem For the interface model of the atmosphere-ocean subsystem, the chal- lenge is to predict the responses of · water and energy exchange, · carbon dioxide exchange and carbon storage, · pattern of the spring bloom, and shifts in ecosystem composition and resultant shifts in oceanic mixed- layer chemistry (e.g., alkalinity). The critical tests of such models are whether they can capture the key aspects of such large-scale phenomena as · E1 Nino/Southern Oscillation (ENSO), · North Atlantic spring bloom, cross-shelf exchange, · poleward heat flux, and biogeochemistry of aeolian deposits. INFRASTRUCTURE . . . . .. . . . .. The global change modeling effort, particularly on these longer time scales, encompasses a class of scientific problems far broader than those cnaractenzed by the physical climate system alone. The biogeochemical system (see Figure 2.1) merits an equal emphasis. An overall strategy that favors diversity is required, in that no one institution or group of investiga- tors has more than a fraction of the interdisciplinary talent necessary for the complete task. Research teams in a range of sizes should be supported. Larger groups are needed for an overall integration role; smaller groups (5 to 10 people) would achieve the incremental steps (i.e., the linkages be- tween the interface models) toward integrated earth system models. Indi

62 i RESEARCH STRATEGIES FOR THE USGCRP vidual investigators will obviously also make important contributions along these same lines. Some of these groups may act primarily as synthesizers whose principal interest would be in linking component pieces; others would develop the components. The larger groups would most likely be associated with various central- zed facilities, which would also serve the common needs of the various teams for computational resources and linkages with large-scale models. Examples of such needs include the preparation and analysis of large-scale observational data sets (such as those that will be developed in preparation for the NASA Earth Observing System (EOS), not to mention the essential data set that EOS will provide following launch); operation of the large, computationally intensive GCMs; documentation and maintenance of baseline codes and protocols for information exchange used by the community; and diagnostics of model output. These larger facilities would provide the physical locations for the most capable supercomputers. One of the more intriguing advances in computer technology is in the area of massively parallel architectures. It appears that many of our current models may be recast to operate in a parallel mode. Centralized facilities could devote resources to this longer-term investment that would be diffi- cult for smaller modeling teams to provide. Given the rapid improvements in CPU power, especially in the worksta- tion class and with parallel architectures, the gap between hardware and software is increasing. Many of the model codes that we use are many years old, and it is difficult to find the funds or the researchers required to convert such codes to take advantage of new hardware. In addition, many of the codes are unwieldy and poorly documented. Again, this is an area to which larger, central teams could commit resources. Specifically, more effort needs to be placed on software development than simply applying existing codes in faster machines. To aid in this process, all modeling teams, large and small, should be encouraged to take advantage of various debugging and software develop- ment tools. For example, code profilers that aid in parallelizing or vector- ing codes should be used. Object-oriented methods that allow codes to be reused or reconfigured more easily should be incorporated into new models. One of the limitations of such efforts is that such tools are often cumber- some and difficult to learn. Thus it is essential that new partnerships be formed between the various hardware and software vendors and the scien- tific users so that appropriate tools can be developed. Lastly, the realm of visualization is becoming increasingly important in handling the volume and increasing dimensionality of the data sets. Visual- izat~on tools also need to be made more accessible. In addition to facilitat- ing the analysis of the model output, such tools can play an important role in testing and debugging by allowing the modeler to see every time step of

INTEGRATED MODELING OF THE EARTH SYSTEM 63 the model, rather than relying on summary data sets. Visualization also requires close coupling between He model and a data base system to Rack the model output. Clearly, these centers and the smaller modeling groups and individual investigators must be mutually supporting. Smaller groups must be pro- vided with the capability to run process experiments on full GCMs and full earth system models at larger centers; moreover, they must have on-site computer support, including workstations, advanced graphics, geographical information systems, and ma~nfrarnes and, most importantly, the technical staff to allow a full application of the on-site computer facilities as well as the off-site supercomputers. Correspondingly, the centers must be able to incorporate advances in subsystem and interface representations formulated by the smaller modeling groups. Such a strategy must necessarily involve multiagency support over many years. REFERENCES Aber, J.D., J.M. Melillo, and C.A. Federer. 1982. Predicting the effects of rotation length, harvest intensity, and fertilization on fiber yield from northern hardwood forests in New England. Forest Sci. 28~1~:31-45. Allen, T.F.H., and E.P. Wyleto. 1984. A hierarchical model for the complexity of plant communities. J. Theor. Biol. 101:529-540. Anthes, R.A. 1983. Regional models of the atmosphere in middle latitudes (a review). Mon. Weal Rev. 111: 1306-1335. Bass, A. 1980. Modeling long-range transport and diffusion. Pp. 193-215 in Conference Papers, Second Joint Conference on Applications of Air Pollution Meteorology, New Orleans, La. Bolin, B., and R.B. Cook (eds.~. 1983. SCOPE 21: The Major Biogeochemical Cycles and Their Interactions. John Wiley and Sons, Chichester, England. Bolin, B., A. Bjorkstrom, K. Holmen, and B. Moore. 1983. The simultaneous use of tracers for ocean circulation studies. Tellus 35B:206-236. Bolin, B., B.R. Doos, J. Jager, and R. Warrick (eds.~. 1986. SCOPE 29: The Greenhouse Effect, Climatic Change and Ecosystems. John Wiley and Sons, Chichester, England. Botkin, D.B., J.F. Janak, and J.R. Wallis. 1972a. Some ecological consequences of a computer model of forest growth. J. Ecol. 60:849-873. Botkin, D.B., J.F. Janak, and J.R. Wallis. 1972b. Rationale, limitations, and as- sumptions of a northeastern forest growth simulator. IBM J. Res. Develop. 16: 101-116. Broecker, W.S., and T.-H. Peng. 1982. Tracers in the Sea. Lamont-Doherty Geological Observatory, Columbia University, New York, N.Y. Bryan, K., F.G. Komro, S. Manabe, and M.J. Spelman. 1982. Transient climate response to increasing atmospheric carbon dioxide. Science 215:56-58. Cess, R.D., and S.D. Goldenberg. 1981. The effect of ocean heat capacity on global warming due to increasing carbon dioxide. J. Geophys. Res. 86:498-602.

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This book recommends research priorities and scientific approaches for global change research. It addresses the scientific approaches for documenting global change, developing integrated earth system models, and conducting focused studies to improve understanding of global change on topics such as earth system history and human sources of global change.

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