10

Modeling

INTRODUCTION

The possibility of major changes in the global environment due to human influences presents a difficult challenge to the scientific research community: to relate causes and effects and to project the course of change on a global scale and for many decades. Approaches based purely on observations are inadequate for prediction. Such rapid externally forced changes have no precedent. Moreover, the response times of many parts of the Earth system are slow, and there is a great deal of variability from place to place. Scattered observations over short time periods are unlikely to reveal clear and useful trends. Furthermore, many important processes—such as those that occur in the soil and in the interior of the ocean—cannot be measured directly or adequately over large areas. We therefore need models—numerical representations of the Earth system—to express our understanding of the many components of the system, how they interact, how they respond to perturbations, and how they feed back to provide dynamical controls on overall system behavior. It is thus evident that the study of global environmental changes—their causes, their impacts, and strategies for mitigation—inescapably requires models that encompass the mutual interactions of the principal components of the Earth system.

There is, however, a fundamental difficulty in that many environmental issues require prediction on relatively long timescales and require integration over large spatial scales. Extrapolations from models over such long time spans are prone to error as small discrepancies from reality compound; moreover, there remain open and complex issues regarding downscaling. Hence, research-quality observational datasets that span significant temporal and spatial scales are needed so that models can be refined, validated, or perhaps rejected. Such data must be



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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade 10 Modeling INTRODUCTION The possibility of major changes in the global environment due to human influences presents a difficult challenge to the scientific research community: to relate causes and effects and to project the course of change on a global scale and for many decades. Approaches based purely on observations are inadequate for prediction. Such rapid externally forced changes have no precedent. Moreover, the response times of many parts of the Earth system are slow, and there is a great deal of variability from place to place. Scattered observations over short time periods are unlikely to reveal clear and useful trends. Furthermore, many important processes—such as those that occur in the soil and in the interior of the ocean—cannot be measured directly or adequately over large areas. We therefore need models—numerical representations of the Earth system—to express our understanding of the many components of the system, how they interact, how they respond to perturbations, and how they feed back to provide dynamical controls on overall system behavior. It is thus evident that the study of global environmental changes—their causes, their impacts, and strategies for mitigation—inescapably requires models that encompass the mutual interactions of the principal components of the Earth system. There is, however, a fundamental difficulty in that many environmental issues require prediction on relatively long timescales and require integration over large spatial scales. Extrapolations from models over such long time spans are prone to error as small discrepancies from reality compound; moreover, there remain open and complex issues regarding downscaling. Hence, research-quality observational datasets that span significant temporal and spatial scales are needed so that models can be refined, validated, or perhaps rejected. Such data must be

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade adequate in temporal and spatial coverage, in parameters measured, and in precision to permit meaningful validation or rejection of models. It is equally important that models be designed to permit confrontation with the real world through observations and that they be tested sufficiently and explored, including creating ensemble runs under differing conditions. Over the past decade there has been remarkable progress in modeling, not only in simulating the principal individual subsystems but also in treating key linkages such as those between the ocean and atmosphere. This record of progress within the U.S. Global Change Research Program (USGCRP) makes it reasonable to expect that within the next 10 years of the USGCRP the scientific community will develop fully coupled dynamical (prognostic) models of the full Earth system (see Figure 10.11) that can be used on multidecadal timescales and at spatial scales relevant to important policy formulation and impact assessment. Such models exist in rudimentary form today. Future models will advance in completeness, sophistication, and proven predictive capability. The key will be to demonstrate some degree of prognostic skill in these future coupled models of the Earth system. This development process will not be isolated from the needs of policy and decision making. Some of these Earth system models will be integrated into more encompassing models that link human and nonhuman processes or will be employed in various analytical or deliberative processes to inform decisions. Providing useful insights to inform decision making on global change will require dynamic representations of complex possible cause-effect-cause patterns linking human and nonhuman components of the Earth system. To develop and validate such models, observations of the Earth system must include data on human impacts from, and contributions and responses to, global change. At present, human influences generally are treated only through emission scenarios that provide external forcings to the Earth system. In future comprehensive models, human activities will interact with the dynamics of physical, chemical, and biological subsystems through a diverse set of contributing activities, impacts, feedbacks, and responses. The focus of this chapter is on the path for realizing and evaluating a suite of such Earth system models. It should be recognized at the outset that the multi-decadal timescale places important constraints and demands on the character of such models. The most important constraint is that models must confront the ever-expanding (though still inadequate) set of time series data, both in situ and remote. The canonical example of the extraordinary value of time series information is the Keeling Record, the daily measured atmospheric concentration of carbon dioxide from Mauna Loa (see Figure 2.10 in Chapter 2).a The importance a It is worthwhile to note that obtaining this unique record was threatened more than once by budget cuts and shortsighted federal managers.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade FIGURE 10.1 Conceptual model of the Earth system. SOURCE: Adapted from NASA (1986). of this record flows from several aspects: (a) its scientific quality in terms of accuracy and precision, (b) its temporal quality in terms of resolution and duration, (c) the importance of the parameter measured (atmospheric CO2), and (d) the site (remote and well positioned for a global measure). The Keeling Record set a standard that subsequent measurements have sought to emulate. Moreover, it demonstrates the value of measurements taken to determine the state of a system rather than to test a specific hypothesis.b The long temporal scale also demands inclusion of the biosphere and other coupling across critical interfaces. Over timescales of decades and more, the biosphere may be expected to respond dynamically to changes in many compo- b This report emphasizes the latter, but the importance of the former must not be overlooked. See Chapter 8.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade nents of the Earth system. More broadly, if we are to understandboth the function of living ecosystems and their effects on the environment, we must have a better grasp of the controls and distribution of biological activity in the context of the overall Earth system, including the actions of humans. While necessary, observations will hardly be sufficient to understand the present and to predict the future role of ecosystems in this global context. Consequently, in this context, developing more realistic models that include successional dynamics and migration patterns of vegetation will be increasingly important in the coming decade. In sum, interactions among components over these longer timescales are likely to be as important as processes within each. Models must therefore deal with interactions between terrestrial ecosystems and the atmosphere, physical and dynamic interactions between the ocean and the atmosphere, the chemistry and physics of the atmosphere and ocean themselves, the land-ocean interface, and even the challenge of incorporating the human component. Each of these heterogeneous components and each of the diverse interfaces between them pose particular demands on research and model development. Models of the fluid subsystems, the atmosphere and the ocean, have been developed almost in parallel with the advance of computational capacity. On the other hand, models of the terrestrial and marine biosphere have been paced by a shortage of observations at adequate time- and space scales and by the slow development of a new community of scholars willing to confront the biological system at large spatial scales. Modeling the role of humans in the Earth system has been controversial from the outset. Computational consideration of the global role of humans dates back, in part, to the provocative early system dynamics studies sponsored by the Club of Rome.3 These models were criticized for their simplistic assumptions about complex human behavior; their inadequate treatment of market forces; and their lack of explicit treatment of physical, chemical, and biological processes. However, they did awaken many to the possibility for quantitative simulation of complex systems beyond econometrics, and they contributed toward convincing the policy community of the importance of taking a system view, including explicit consideration of feedback loops and environmental constraints. Looking more closely, we find that computer-based atmospheric models were first developed in the 1940s for weather forecasting—that is, to predict the near-term physical behavior of the atmosphere. In the subsequent development, there has been a natural branching on temporal scales: in parallel with the continuing refinement of weather forecasting models with increased skill, models that treat the longer-term dynamics inherent in climate studies have been advanced. In the process certain boundary conditions become incorporated as interactive components of the models; this is often the case where the increase in temporal scales logically forces “annexation” of what were initially external conditions (sea surface temperature is a good example). More recently, chemical processes are being included in transport codes that had their origin in weather and climate studies, so that today quite elaborate models are available to study the physical and chemical behavior of the atmosphere.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade BOX 10.1 In the fall of 1994, the interagency Subcommittee on Global Change Research arranged for the special Forum on Global Change Modeling to provide an indication of the state of current progress in improving understanding of global change and to provide direction for future research. This forum served as a means of bringing together a representative set of scientists to develop a consensus statement on the credibility of global model estimates of future climatic change. The charge to those attending the forum and to those who submitted written comments was to develop a brief statement on the credibility of projections of climate change provided by general circulation models (GCMs) as background for potential interpretation of model results in the context of developing and considering national policy options. The focus of the forum was specifically on the climate aspects of the entire global change issue—thus not on the emission scenarios, the consequences of change to ecosystems and natural resource systems, or the socioeconomic implications and potential for responses. Still the results of the forum are of significant value to this chapter. The forum identified a number of areas where sustained or intensified research efforts would bring important gains in understanding and predictive capabilities. As an overarching statement it was noted that “while progress is clear as a result of ongoing research efforts and important steps can be taken over the coming decade that will bring new insights, significant reductions of the uncertainties in projecting changes and trends in the climate will require sustained efforts that are very likely to require a decade or more.” “Progress will require significant effort because the problems are complex, because improvements in model parameterizations will require a sustained and long-term program of research and observations, and because the records of past changes and influences require careful reconstructions to make them more complete and more useful. Although progress may be modest, there are a number of processes and feedbacks on which research must be sustained because of the large leverage to be gained from improved understanding. These processes and feedbacks include: cloud-radiation-water vapor interactions, including treatment of solar and infrared radiation in clear and cloudy skies (also including resolution of uncertainties concerning anomalous solar absorption); ocean circulation and overturning; aerosol forcing, requiring information on aerosol character and extent; decadal to centennial variability; land-surface processes, including the climate-induced changes in the structure and functioning of ecological systems with resultant changes in global chemical cycles; short-term variability affecting the frequency and intensity of extreme and high impact events (e.g., monsoons, hurricanes, mesoscale storm systems, etc.); non-linear and threshold effects that create the potential for surprises; and interactions between chemistry and climate change and improved representation of atmospheric chemical interactions within climate models, thereby leading to improved understanding of the causes of trends in CH4, N2O, O3, CFCs, and aerosols.”2

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade Development of models for general circulation of the ocean started slightly later but has proceeded in a manner similar to that for the atmospheric models. Rather elaborate models that deal with the physics of the oceans are now available, and, as the preceding paragraph implied, ocean models have been linked to models of the atmospheric system. Within ocean models the inclusion of geochemical and biological interactions has begun, with a focus on the carbon cycle. Since the late 1960s, the geochemical aspects of the carbon cycle have been included in low-dimensional box models.4 More recently, including the carbon-alkalinity system in general circulation models has simply been a question of allocation of computing resources. Modeling of the biological system, however, has been more challenging, and it has only been of late that primitive ecosystem models have been incorporated into global general circulation ocean models.5 Even though progress has been significant, much remains to be done. Coupling difficulties remain between the ocean and the atmosphere (though the worrisome issue of flux correction is beginning to be resolved or at least better understoodc). Fully eddy-resolving models with chemistry and biology need to be tested and validated in a transient mode. Finally, the prognostic aspects of marine ecosystems, including nutrient dynamics, need greater attention at basin and global scales. Model development for the ocean and the atmosphere has had a fundamental theoretical advantage: it is based on the firmly established hydrodynamic equations. For example, the geostrophic constraint is particularly valuable. There is less constraint, however, on the dynamics of the global energy and water cycles, and at present there is far less theoretical basis for a “first principles” development of the dynamical behavior of the terrestrial system. We therefore need to develop a fundamental methodology to describe this very heterogeneous and complex system. For the moment it is necessary to rely quite heavily on parameterizations and empirical relationships. Such reliance is data intensive, and hence independent validation of terrestrial system models is problematical. Returning to the atmospheric models, which as noted are the most advanced dynamically, key processes like cloud formation remain too cloaked in parameterization. Despite the difficulties that face modelers of terrestrial ecosystems, a coordinated strategy has been developed over the past five years to improve estimates of terrestrial primary productivity and respiration by means of measurement and modeling (see Box 10.1 and Chapter 2).6 For terrestrial ecosystems at the global scale, there has been a focus on the carbon cycle. This reflects demands on the science and advances in the theoretical foundation of the biogeochemical dynamics of terrestrial systems (at least under current conditions), and in this setting the strategy has begun to yield dividends. Several independent global models at mesospatial scales (roughly 50- c This topic and others are discussed more fully in subsequent sections.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade BOX 10.2 Global scales, The Global Analysis, Interpretation, and Modeling (GAIM) task force of the International Geosphere-Biosphere Program (IGBP) initiated an international model intercomparison, carried out through two workshops hosted in June 1994 and July 1995 at the Potsdam Institut für Klimatologie (PIK), in Potsdam, Germany. The purpose of the Potsdam workshops was to initiate and support a series of model intercomparisons by the various modeling teams that are currently modeling the terrestrial biosphere at the global scale. More than 15 models and modeling teams have participated in the intercomparison. One, but not the only, focus in the intercomparison was NPP, which is central to models of the global carbon cycle. There are significant differences in the calculation of NPP between current global biosphere models, and a particular focus of Potsdam ‘95 was to compare model parameters and outputs using standard input datasets to determine patterns and hopefully the causes of the variability. A fundamental problem in assessing the results of terrestrial ecosystem models, which are used to provide NPP intercomparisons, is a lack of good validation data.7 Continental scales. The Vegetation/Ecosystem Modeling and Analysis Project (VEMAP) is comparing models of vegetation distribution, biogeochemistry, and biogeography for the conterminous United States under current and GCM-simulated future climates. In addition to changes in climate, the models are tested in response to changes in the chemistry of the atmosphere, in particular, to changes in the CO2 concentration and to changes in nitrogen deposition. VEMAP is also conducting factorial experiments under different forcings and thereby setting the stage to tests under multiple stresses.8 km grids) now exist, and others are in various stages of development. With the one-half degree gridscale, it is now possible to investigate the magnitude and geographic distribution of primary productivity on a global scale by a combination of monitoring by remote sensing and modeling of the biogeochemical aspects of terrestrial ecosystems. These models range in complexity from fairly simple regressions between key climatic variables and biological production to quasi-mechanistic models that attempt to simulate the biophysical and ecophysiological processes occurring at the plant level (including their scaling to large areas). A fundamental difficulty remaining is the evaluation and perhaps validation of such models at the global scale. The interim step of model intercomparison has been taken (see Box 10.2). Other important model intercomparison projects currently under way include the Atmospheric Model Intercomparison Project, the Coupled Model Intercomparison Project, and the Paleoclimate Model Intercomparison Project.9 During the next decade, we need to expand our efforts in domain-specific

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade models. In the ocean we need to improve our understanding of the controls on thermohaline circulation, of the potential changes in biological productivity, and of the overall stability of the ocean circulation system. Within terrestrial systems the question of the carbon sink-source pattern (what it is and how it might change) is central. Connected to this question is the development of dynamic vegetation models, which treat competitive processes within terrestrial ecosystems and their response to multiple stresses. For the atmosphere, a central question has been, is, and likely will continue to be the role of clouds. Further, increased efforts will be needed to link terrestrial ecosystems with the atmosphere, the ocean with the atmosphere, the chemistry of the atmosphere with the physics of the atmosphere, the land to the ocean, and finally the human system to the physical and biogeochemical subsystems. In considering coupling atmospheric general circulation models (GCMs) to terrestrial models, where the coupling transfers not only energy and water but also important gases such as CO, CH4, and CO2, temporal and spatial scale issues again emerge. Energy, water, and CO2-O2 are actually exchanged across short timescales and exhibit a high degree of variability. Moreover, the gross fluxes are large in comparison with the net ecosystem fluxes, and hence the macro-balance of terrestrial carbon stocks, which determines the net flux of CO2, is difficult to derive by direct integration of the gross fluxes. Ecological changes, such as successional sequences of tree species, are not treated well on time steps that are appropriate for considering photon input, water exchange, or trace gas fluxes and require significant intermediate parameterizations or models. Longer time step integrations have generally been more successful for carbon dioxide. On the other hand, the flux of CH4 and other short-lived species cannot be treated by simple mass balance and crudely time-averaged responses. The relatively simple problem of coupling land hydrology to the atmosphere remains elusive and yet is quite important. Water balances influence the exchanges of energy and many reduced gases (e.g., CH4 depends on soil moisture conditions). Modeling sensitivity studies 10 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 degrees (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 and thus diurnal variations of precipitation in tropical and summer conditions, it is evident that the inclusion of the role of vegetation is important for simulations of the hydrological cycle. Better field data are helping to establish the parameters needed for linking plant physiology to surface evapotranspiration. Considerable further effort is needed before the appropriate submodels can be applied with confidence over a wide range of vegetation cover. The hydrological coupling between the land and the ocean has seen signifi-

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade cant advances in the past 10 years, but we are still challenged to develop a more complete biogeochemical coupling. The difficulties are several: (1) there is a lack of data about the loss of important chemical compounds, such as organic carbon and nitrogen compounds, from terrestrial systems and aquatic systems; (2) we are uncertain how these exchanges might change in the face of land use change or climate change—more generally under pressure of multiple stresses on terrestrial systems; and (3) there is inadequate process-level understanding and supporting data on how these organic compounds are processed within the wide range of river systems.d 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. Fortunately, the scientific community has recognized for some time that if we are to penetrate the transient behavior of climate change we must produce credible coupled ocean-atmosphere models. Significant progress has been made in treating this demanding challenge on timescales of decades to centuries (see Box 10.3). Moreover, we have now demonstrated potential predictive skill in modeling the El Niño-Southern Oscillation (ENSO),12 where the ocean-atmosphere system responds in a coupled fashion on interannual timescales. Finally, on very long timescales, we are probing the coupled ocean-atmosphere system for which paleo-oceanographic investigations suggest that aspects of longer-term climate change are associated with changes in the ocean's thermohaline circulation. As we seek to couple better the chemistry of the atmosphere with the physics of the atmosphere, for instance, by adding the important chemical constituents and reactions to an atmospheric GCM, the issues of scale and computational challenges become daunting for transient calculations. Many of the important chemical reactions depend on concentration and hence on grid scale. In addition, important processes often occur in the boundary layer, which generally is not adequately resolved. Adding atmospheric chemistry to a GCM thus places greater demands on the terrestrial and oceanic boundary conditions and dynamic simulations. As in most of the other areas, progress will depend in part on the availability of advanced computing facilities (and in the more distant future petaflop machines13). Finally, global climate and environmental changes often reflect the consequences of human actions superimposed on natural variability and change. It is d Certainly there are river systems that are well studied, and there is knowledge of general patterns of carbon and nutrient processing in rivers; however, there remain large gaps in both our observational records and in our understanding when we face the issue on continental to global scales.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade clear that humans can cause environmental change, even on a global scale. It is equally clear that environmental changes, whether human caused or not, can have impacts on humans. To understand these changes and to provide useful guidance to inform policy development and decision making will require increasingly integrated understanding of the diverse human and nonhuman components of the Earth system.e Environmental and climate change research must focus on predictions of key state variables such as rainfall, ecosystem productivity, and sea level that can be linked to estimates of economic and social impacts of possible environmental and climate change. Projections of emissions, land use, and other contributions must be related to underlying economic, technological, social, and political forces to understand linkages from causes to effects and back to causes. Uncertainties in the social side of the system, though of different character, are thus linked with the uncertainties of environmental and climate systems and are as important for understanding system behavior and informing decision making. Box 10.3 Effects of Anthropogenic Carbon Dioxide Emissions on the Atmosphere-Ocean System In 1967 Syukuro Manabe and Richard Wetherald published what is now regarded as one of the first credible calculations of the possible effect of increased carbon dioxide on climate. They calculated that a doubling of atmospheric carbon dioxide would warm the Earth's surface by about 2°C. This result laid the foundation for what has become an international multidisciplinary research effort on global warming. In a recent paper, published 26 years after Manabe's pioneering one-dimensional CO2 sensitivity study, he and Ronald Stouffer used a three-dimensional coupled ocean-atmosphere model to examine possible CO2-induced climate changes over several centuries (see Figure 10.2).11 Earlier studies had focused on shorter time horizons. In their scenario, CO2 quadruples over a period of 140 years, then no longer increases. This perturbation is enough to cause the ocean's global thermohaline circulation to almost disappear in the model (though in some experiments it reappears given sufficiently long integration times). This circulation is important because in the present climate it is responsible for a large portion of the heat transport from the tropics to higher latitudes. In addition, Manabe and Stouffer's study indicates that sea level continues rising steadily for centuries after the CO2 increase is halted. From this perspective, global climate change can no longer be viewed as just a problem of our own lifetimes but as a legacy—with uncertain consequences—now being passed forward to many future generations. e For instance, global climate change is the subject of policy debate in most nations and of negotiations at the Conference of Parties and the Framework Convention on Climate Change. This is an obvious area that centrally requires the human component of the Earth system.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade FIGURE 10.2 Impact of increasing CO2 on the Earth's climate as simulated in a Geographical Fluid Dynamics Laboratory coupled ocean-atmosphere climate model. Shown are time series of (a) prescribed CO2 concentration on a logarithmic scale in comparison to present levels; (b) global mean surface air temperature (°C); (c) global mean increase of sea level (cm) due to thermal expansion; and (d) intensity of the North Atlantic Ocean's meridional overturning circulation (10 6 m3/sec). The labels “S,” “2XC,” and “4XC” refer to separate experiments in which CO2 either remains constant (S) or increases at a rate of 1 percent per year (compounded) to double (2XC) or quadruple (4XC) the current concentration. Note that the sea level rise estimates do not include the effect of melted continental ice sheets. With this effect included, the total rise could be larger by a substantial factor. SOURCE: Manabe and Stouffer (1993). Courtesy of Macmillan Magazines Ltd.

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade 68. For an excellent overview of the modern history and the issues confronting the modeling of ocean circulation, see Semtner (1995). 69. See Section 5.3.3 in Gates et al. (1996). For a particularly provocative article, see Broecker (1997). 70. Kattenberg et al. (1996, p. 346). 71. See http://www.gcrio.org/ipcc/cover.html. 72. http://topex-www.jpl.nasa.gov/index.html. 73. http://www.soc.soton.ac.uk/OTHERS/woceipo/ipo.html. 74. http://www.igbp.kva.se/. 75. http://ads.smr.uib.no/jgofs/jgofs.htm. 76. For a good discussion of these issues, see (particularly Section 10.3.2) Denman et al. (1996). 77. Sarmiento and Le Quéré (1996). 78. E.g., Najjar et al. (1992). 79. For instance see Fasham (1995) and Fasham et al. (1993). Also, Sarmiento et al. (1993) and Hurtt and Armstrong (1996), Flynn and Fasham (1997), Popova et al. (1997), Ryabchenko et al. (1997). 80. For a slightly broader set of modeling issues, see Section 10.4 in Denman et al. (1996). 81. Until recently this “nutrient-restoring” approach was limited to annual mean models (Najjar et al., 1992) due to the lack of seasonally resolved surface nutrient observations. An intensive data archeology program at the National Oceanographic Data Center has resulted in a much larger global database of nutrients (see Levitus et al., 1993, and Levitus and Boyer, 1994), so that seasonal analyses are now possible (e.g., Anderson and Sarmiento, 1994, 1995; Sarmiento and Le Quéré, 1996; Najjar and Keeling, 1997). 82. E.g., Gruber et al. (1996). 83. The role of the biological pump in the ocean carbon cycle, discussed only briefly in Houghton et al. (1996); Sections 1.3.3.3 and 1.4.3 are expanded and updated here. From Section 10.3.2.1 in Denman et al. (1996). 84. http://ads.smr.uib.no/jgofs/jgofs.htm. 85. See again Denman et al. (1996). 86. E.g., Broecker et al. (1985, 1995). 87. http://ingrid.ldgo.columbia.edu/sources/.geosecs. 88. See OCMIP discussion on http://www.ipsl.jussieu.fr/OCMIP. 89. E.g., Lefevre et al. (1996, 1998), Takahashi and Sutherland (1995), Takahashi et al. (1997). 90. E.g., Warner and Weiss (1992), England et al. (1994), and England (1993, 1995), Dixon et al. (1996), and Warner et al. (1996). See also Rhein (1994) and Smethie and Pickart (1993). 91. http://topex-www.jpl.nasa.gov/index.html. 92. E.g., Yoder et al. (1993) and Banse and English (1994). 93. For an important discussion of the radiative forcing implications associated with the chemistry of the atmosphere, see Schimel et al. (1996). 94. For instance, see Langner and Rodhe (1991), Kanakidou and Crutzen (1993), Chuang et al. (1994), Cooke et al. (1996), Klonecki and Levy (1997), Berntsen et al. (1996, 1997), and Berntsen and Isaksen (1997). 95. For an example array of atmospheric chemistry models see http://www-pcmdi.llnl.gov/, http://www.gfdl.gov/gfdl research.html, and http://www-as.harvard.edu/chemistry/trop/index.html. 96. See Sections 2.2.1 and 2.3.2 in Schimel et al. (1996). 97. E.g., Hough (1991) and Wang et al. (1998). 98. The importance of this associated effort can be seen in Ciais et al. (1997a, 1997b) and Bousquet et al. (1996). 99. We note that this is not an exhaustive review of chemistry-climate models and that the selection of the two models from the NCAR is simply for exposition. There are advanced chemistry-climate (or circulation) models in Europe (e.g., Max Planck) and in the United States

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade (again for an example array of atmospheric chemistry models in the United States see http://www-pcmdi.llnl.gov/, http://www.gfdl.gov/gfdl_research.html, and http://www-as.harvard.edu/chemistry/trop/index.html.) Finally, in the context of climate modeling see also Section 6.7.3 in Kattenberg et al. (1996). 100. See http://www.esd.ornl.gov/programs/NIGEC/; see also http://cdiac.esd.ornl.gov/programs/NIGEC/fluxnet/index.html. 101. See http://www.unitus.it/eflux/euro.html. 102. See http://cdiac.esd.ornl.gov/programs/NIGEC/fluxnet/japan.txt. 103. An important summary of issues is provided by Bruce et al. (1996). 104. E.g., Claussen (1996) and Kutzbach et al. (1996). 105. E.g., Dickinson and Henderson-Sellers (1988), Salati (1986), Lean and Warrilow (1989), and Shukla et al. (1990). 106. A good introduction to this important literature can be found through the IGBP-IHNP Core Project: Land Use Cover Change (LUCC). See http://www.icc.es/lucc. 107. See again Bruce et al. (1996). 108. The important issue of scaling is addressed more fully in Gibson et al. (1998). See also http://www.uni-bonn.de/ihdp. 109. An important summary of progress on integrated assessment modeling can be found in Nakicenovic et al. (1994). See also the related publications Kaya et al. (1993) and Nakicenovic et al. (1994a, 1994b). 110. Houghton et al. (1996). REFERENCES AND BIBLIOGRAPHY Anderson, L., and J. Sarmiento. 1994. Redfield ratios of remineralization determined by nutrient data analysis . Global Biogeochemical Cycles 8:65-80. Anderson, L., and J. Sarmiento. 1995. Global ocean phosphate and oxygen simulations. Global Biogeochemical Cycles 9:621-636. Banse, K., and D.C. English. 1994. Seasonality of coastal zone color scanner phytoplankton pigment in the offshore oceans. Journal of Geophysical Research—Oceans 99:7323-7345. Berntsen, T., I.S.A. Isaksen, W.C. Wang, and X.Z. Liang. 1996. Impacts of increased anthropogenic emissions in Asia on tropospheric ozone and climate—a global 3-D model study. Tellus Series B—Chemical and Physical Meteorology 48(1):13-32. Berntsen, T.K., and I.S.A. Isaksen. 1997. A global three-dimensional chemical transport model for the troposphere model description and CO and ozone results. Journal of Geophysical Research—Atmospheres 102(ND17):21,239-21,280. Berntsen, T.K., I.S.A. Isaksen, G. Myhre, J.S. Fuglestvedt, F. Stordal, T.A. Larsen, R.S. Freckleton, and K.P. Shine. 1997. Effects of anthropogenic emissions on tropospheric ozone and its radiative forcing. Journal of Geophysical Research—Atmospheres 102(ND23):28,101-28,126. Billen, G., A. Eisenhauer, R.F. Spielhagen, M. Frank, and G. Hentzschel. 1994. BE-10 records of sediment cores from high northern latitudes—implications for environmental and climatic changes. Earth and Planetary Science Letters 124:171-184. Boer, G.J. 1998. A study of atmophere-ocean predictability on long time-scales. Submitted to Climate Dynamics. Bolin, B.B., ed. 1981. Carbon Cycle Modelling, SCOPE 16. Bolin, B. John Wiley & Sons Ltd., Chichester, U.K. Bolin, B.B., E.T. Deegens, S. Kempe, and P. Ketner. 1979. The Global Carbon Cycle, SCOPE 13. John Wiley & Sons, New York. Bolin, B.B., B.R. Doos, J. Jager, and R. Warrick. 1986. The Greenhouse Effect, Climate Change, and Ecosystems. John Wiley & Sons, Chichester, U.K.

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