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EXECUTIVE SUMMARY The climate of Earth is evolving, and understanding this change can help us to be prepared to deal with the consequences for water resources, agriculture, energy demand and supply, health, recreation, and ecosystems (IPCC, 2001b). Climate changes can be initiated by external factors forcing the climate system. These climate forcings include natural factors such as changes in energy flux from the Sun, variations in the Earth's orbit, and volcanic eruptions, as well as human activities, such as production of greenhouse gases and aerosols and modification of the land surface. Over the next century it is likely that forcing of the climate system by human activities will greatly exceed changes in forcing caused by natural events. Processes in the climate system that can either amplify or damp the system's response to changed forcings are known as feedbacks. According to estimates generated by current climate models, more than half of the warming expected in response to human activities will arise from feedback mechanisms internal to the climate system, and less than half will be a direct response to external factors that directly force changes in the climate system (NRC, 200 la). Moreover, a substantial part of the uncertainty in projections of future climates is attributed to inadequate understanding of feedback processes internal to the natural climate system (IPCC, 2001a). Therefore, it is of central importance to understand, model, and monitor climate feedback processes. At the request of the interagency U.S. Global Change Research Program, the Panel on Climate Change Feedbacks of the Climate Research Committee was given the following tasks: 1. Characterize the uncertainty associated with climate change feedbacks that are important for projecting the evolution of Earth's climate over the next 100 years, and 2. Define a research strategy to reduce the uncertainty associated with these feedbacks, particularly for those feedbacks that are likely to be
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2 UNDERSTANDING CLIAl4TE CHANGE FEEDBACKS important and for which there appears to be significant potential for scientific progress. The study looks at what is known and not known about climate change feedbacks and seeks to identify the climate feedback processes most in need of improved understanding. This report suggests an approach by which progress toward better understanding of climate feedback processes can be measured and accelerated. Such improvements will serve policy makers as they deliberate on climate-related decisions. THE NEED FOR CLIMATE FEEDBACKS RESEARCH In recent years the principal way scientists have sought to understand changes in climate has been to simulate the record of global mean surface temperature over the period of the instrumental temperature record from about 1860 to the present (e.g., Hansen et al., 1981; IPCC, 2001a). Such comparisons allow testing of our understanding of climate forcing, climate sensitivity, and heat storage in an integrated global sense, but they are imperfect. A second approach has been to make model-to-model comparisons of climate simulations, and this has revealed significant differences and similarities between models (Gates et al., 1998; Covey et al., in press). At this point in time this Panel believes that an effort to refine our understanding of the key climate feedback processes and improve their treatment in models used to project future climate scenarios is an effective way forward in the quest to better understand how climate may evolve in the future in response to natural and human-induced forcings. An appropriate strategy for accomplishing this is to make more vigorous comparisons of models with data and to focus particularly on observational tests of how well models simulate key feedback processes. A key finding of this report is that an enhanced research effort is needed to better observe, understand, and model key climate feedback processes. Key Observations Needed to Monitor and Understand Climate Feedbacks Previous reports by the National Research Council (NRC) have emphasized the need for stable, accurate, long-term measurements of climate variables (NRC, 1999a). Because of their important role in determining the magnitude of climate change, additional variables must be monitored to
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EXECUTIVE SUMMARY assess the role of feedback processes in climate change. Observation of feedback processes is needed to better understand these processes, to identify the causes contributing to observed climate changes as they occur, and to test and improve simulations of climate. As described in the body of the report, some variables key to feedback processes are not being adequately monitored on a long-term basis. To understand and monitor climate feedback processes requires good observations of the basic state of the climate system, plus some additional variables that monitor specific feedback processes. Recommendation: An integrated global climate monitoring system must include observation of key climate feedback processes. Stable, accurate, long- term measurements should be made of the variables that characterize climate feedback processes. To better understand and model climate feedback processes and to interpret the role of feedbacks in climate changes that may develop in the future, research efforts must monitor not only traditional climate variables like temperature and precipitation but also variables that define the feedback processes. Key long-term measurements that are needed to monitor and understand climate change feedbacks are: temperature, humidity, precipitation, and wind; radiation budget at the top of the atmosphere and at the surface global cloud and aerosol distributions and properties; temperature and salinity of the upper ocean and of other portions of the ocean that affect interannual to decadal climate change; · terrestrial vegetation, soil moisture, snow extent and its properties, and sea-ice distribution and thickness; and · atmospheric C02, 03, O2-N2 ratio, and ocean color. Several of these variables are being monitored for purposes of weather analysis and prediction, but none adequately for climate purposes. As recommended in several previous NRC reports there are advantages to collecting these observations in the context of an integrated global climate monitoring system (e.g., NRC, 1999a). Such a system is required for other aspects of climate change research and applications not addressed in this report, including for climate change attribution and detection, and for providing a broad range of climate services (NRC, 2001 e). The collection
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4 UNDERSTANDING CLI~4TE CHANGE FEEDBACKS and validation of all these datasets will require international collaboration and cooperation among U.S. agencies. In addition to using the observations as climate data records (see NRC, 1999a and 2000b, for a description of the characteristics of climate data records), they should also be incorporated in 4-D data assimilation. Subject to important caveats, the resulting integrated datasets will be suitable for model initialization, model validation, and for multivariate diagnostic studies on climate time scales. Evaluating Progress in Understanding Climate Feedbacks To ensure focused research and to measure progress, we need observable climate metrics that define the feedbacks sufficiently both to understand the key processes and to test and improve the simulation of these processes in climate models. A climate feedback is a set of numbers that can be derived from both observations and model output, and that characterizes the nature of a climate feedback process. It is important that this characterization be useful for better understanding the feedback process and for assessing the accuracy of its simulation in climate models. Metrics can use observed past climate trends, but should also use the variability of climate on other time scales that are better observed and where forcing is larger, such as seasonal and diurnal time scales. Good metrics must be focused on objectives that will increase confidence in our ability to usefully model climate feedback processes, and must be defined in terms of variables that are well observed. They should evolve as our understanding and observations improve. Recommendation: Both global and regional metrics that focus on feedback processes responsible for climate sensitivity should be used to more rigorously test understanding of feedback processes and their simulation in climate models. A good set of diagnostic tests for climate feedback processes should capture the covariation or coupling between the system's components. If effectively employed, these metrics can be an essential tool to help organize and stratify diagnostic analyses, as well as to relate model simulations to the fundamental aspects of observed phenomena. Successful reproduction of these observed metrics by climate models will not guarantee that climate models will give reliable projections of future climates, but testing climate
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EXECUTIVE SUMAL1RY models against a large set of carefully considered metrics is an effective way forward. They can also be a useful tool for observing the evolution of the climate system and thus make important contributions to the field of climate change detection and attribution. The set of metrics will evolve with time as understanding and simulation of the climate system evolve and improve. A few examples of possible climate feedback metrics can be given. At the global or regional scale, the covariability of sea-surface temperature, clouds, upper-tropospheric water vapor, the vertical profile of atmospheric temperature, and other observations can be studied over a variety of time scales, including well-observed natural scales of variability, such as the diurnal, annual, and El Nino Southern Oscillation (ENSO) signals. These covariance metrics should then be applied to model simulations to pinpoint those aspects of the models that appear to represent nature accurately and those that require further work. A metric that might enable improvement of feedback processes over land would be observed diurnal and seasonal variations of temperature, clouds, precipitation, and soil moisture. Many other possible regional metrics for testing the simulation of climate system feedbacks can be envisioned, and some are discussed further in Chapters 2 through 8. A step toward developing widely accepted metrics to evaluate feedback processes could be for the relevant agencies to organize a workshop or series of workshops to define a set of observational and diagnostic metrics that can be used to test understanding and modeling of climate feedback processes. These workshops could include scientists engaged in observation, diagnosis, and modeling of climate and climate processes. Climate Modeling and Analysis for Climate Feedbacks Research To test understanding and modeling of climate feedback processes using a set of climate feedback metrics requires a substantial infrastructure and a proportionate intellectual effort. To undertake a rigorous program of testing the simulation of climate feedback processes in our most capable climate models requires that the observations and the expertise in applying them be brought together with the modeling capability. Previous NRC reports have stated the need for capable and effective climate modeling facilities (NRC 1998a, 2001 c), and have recommended the development of centralized operations for climate predictions and ozone assessments (NRC, 2001c). To advance understanding of climate change feedbacks and their role in climate sensitivity it is essential that U.S. climate modeling facilities also have the capability and mandate to test climate feedback processes and their
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6 UNDERSTANDING CLIMA TE CHANGE FEEDBACKS interactions using the most discriminating observational constraints. Within the context of climate feedback processes, this will also address the need for uniform criteria with which to judge climate models (NRC, 2001 c). Recommendation: Climate modeling facilities in the United States must be given the capability and mandate to test understanding and simulation of climate feedback processes and their interactions using the best observational constraints on climate feedback processes. Periodic assessment of the progress being made by major climate models should be conducted! to evaluate the ability of these modlels to simulate the processes underlying key climate system feedbacks. One interdisciplinary coordination challenge is to lessen the separation between U.S. observational and modeling research (NRC, 2001 c). Representation of processes related to climate feedbacks in global climate models is a complex and challenging undertaking, which often proceeds without adequate connection to the developing observational basis. It is also difficult for the observational community, which tends to focus on the technical aspects of data collection and analysis, to find the time and resources to assist in the development of Earth system models. While observations are used to test the climatological statistics derived from climate simulations, more attention needs to be given to using data to rigorously test the simulation of feedback processes in these models and their role in determining climate sensitivity. Another opportunity to encourage progress in climate feedbacks research is to reduce the separation between operational numerical weather prediction centers and climate research centers in the United States. Many climate feedback processes operate on time scales short enough to be tested effectively by comparing numerical weather forecasts with instantaneous measurements of cloud properties, humidity, or other variables that characterize the fast feedback processes in the climate system. Similar use can be made of seasonal forecasts, which bring slower feedback processes into play. Systematic biases in seasonal forecasts of climate often reflect problems with the treatment of climate feedback processes in the forecast models.
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EXECUTIVE SUMMARY PRIORITIES IN CLIMATE FEEDBACK RESEARCH This report reviews the scientific understanding of key feedback processes in the climate system and suggests research activities that will improve our understanding of these processes and our ability to model them effectively in global climate models. In selecting the priority feedbacks, the following criteria were applied: . the expectation that the feedback process will have a significant effect on the magnitude, timing, or spatial structure of the climate response to human-induced climate forcing during the next century; the likely magnitude of the uncertainty of the effect of the feedback process; and the probability that a well-focused research effort could over the next several years significantly enhance our understanding of and ability to characterize and perhaps quantify the uncertainties associated with the feedback process. In addition to these criteria, discussion is limited to feedback processes that are likely to have large-scale effects that would appear in global averages or averages over large areas of at least continental scale. Better knowledge on these large scales should translate into better understanding on smaller scales, but additional uncertainties in local climate arise from local winds, ocean currents, and geography that are not addressed here. In studying this problem and preparing this report the Panel found that the scientific understanding, observations, and models necessary to understand feedback processes and climate sensitivity overlap significantly with understanding, observing, and modeling climate forcing. Because both factors are changing over time, the transient response of the climate system to gradually increasing forcing must also be considered. Partly for these reasons this report takes a broad view of climate feedback processes and climate sensitivity. It groups feedback processes into three categories: (1) those that primarily affect the magnitude of climate change, (2) those that primarily affect the rate or timing of climate change; and (3) those that primarily affect the spatial patterns of climate change. These categories are also helpful for promoting public understanding of the importance of these processes because they translate into questions like: "How big or important will climate change be?" "How rapidly will climate change?" and "How will climate change in my area?" The Panel has identified the following key climatic processes or closely related phenomena that it judges to be high-priority research areas, based on
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8 UNDERSTANDING CLIMATECHANG~FEEDsACKS the potential contribution to understanding climate evolution over the next 100 years and the potential for rapid scientific progress. The priorities are organized into three categories based on whether their most important effects are on the magnitude, timing, or spatial structure of climate change. More detail supporting these priorities can be found in the body of the report. Feedbacks that primarily affect the magnitude of climate change Cloud, water vapor, and lapse rate feedbacks Ice albedo feedback Biogeochemical feedbacks and the carbon cycle Atmospheric chemical feedbacks Feedbacks that primarily affect the transient response of climate · Ocean heat uptake and circulation feedbacks 3. Feedbacks that primarily influence the pattern of climate change Land hydrology and vegetation feedbacks · Natural modes of climate system variability Over the long term all these areas stand to make valuable contributions to understanding climate change. For the near term the two most important areas are (1) cloud, water vapor, and lapse rate feedback and (2) ice-albedo feedbacks, both of which primarily affect the magnitude of climate change. Feedbacks That Primarily Affect the Magnitude of Climate Change Feedbacks that primarily affect global climate sensitivity Cloud, water vapor, and lapse rate feedbacks as a group and ice-albedo feedback are the feedback processes that seem most important in determining the global mean climate sensitivity. Cloud, Water Vapor, and Lapse Rate Feedbacks Cloud feedback is one of the key uncertainties in projections of future climates, and is responsible for a large fraction of the model-to-model variation in climate sensitivity. Significant uncertainties remain in water vapor and lapse rate feedback, but these are closely coupled to cloud
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EXECUTIVE SUMMARY 9 processes, so we have grouped them together. It is not known whether cloud feedback will increase or decrease global warming, let alone its magnitude. An accelerated and focused effort to test the simulation of cloud, water vapor, and lapse rate feedbacks in climate models, and their role in climate sensitivity is needed. Such an effort is particularly appropriate now because new climate models that predict cloud properties show a large range of cloud feedback strength, new satellite and surface-based measurements exist to test cloud simulations, and cloud-resolving models have emerged as a tool for understanding the interaction of clouds, water vapor, and lapse rate. Effective synergism among efforts to diagnose observations, to model cloud systems, and to model the global climate is essential. A set of observable metrics should be defined and used to test our understanding of cloud, water vapor, and lapse rate feedbacks. Because of its large contribution to current uncertainty estimates and the potential to make significant progress in the near term, the Panel feels that cloud, water vapor, and lapse rate feedback is the highest priority at this time. Ice Albedo Feedback Ice and snow in high latitudes, and in particular sea ice, are important contributors to climate sensitivity through ice albedo feedback, but the magnitude of this feedback remains uncertain. Ice albedo feedback in polar regions is coupled strongly to polar cloud processes and ocean heat transport. Improvements are needed in the parameterization of sea-ice growth, associated heat and freshwater fluxes, surface albedo variations, and polar clouds. Better observations of polar ice distributions and associated atmospheric and oceanic properties is needed. Systematic global observations of sea-ice thickness, polar clouds, and the surface albedo in ice- covered areas are especially important, but a system to make ice thickness measurements is as yet unavailable. Further development and distribution of satellite and in situ datasets describing variations of polar ice and polar clouds should be a priority. Processes That Feed Back on Climate Forcings As the climate changes, temperature, precipitation, and circulation changes are likely to change how the climate system deals with the greenhouse gases, aerosols, and surface modifications produced by humans, and this will affect the climate forcing. It is likely that climate change will
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10 UNDERSTANDING CLIMATE CHANGE FEEDBACKS evoke natural responses in the climate system that will magnify or mute human-produced climate forcing through alterations in greenhouse gases and aerosols. Biogeochemical Feedbacks and the Carbon Cycle The global carbon and sulfur cycles contain potentially important feedback processes. There are, however, major gaps in understanding. No definitive explanation has been given for the apparent vast uptake of CO2 by the terrestrial biosphere, and no confident prediction can be given of future biological uptake or release of CO2, particularly over the long term. Few observations are available to guide the necessary scaling of vegetation- climate feedbacks from the scale of an individual leaf to a landscape mosaic of vegetation and soils. In the marine realm the strengths of a wide variety of potential feedback mechanisms related to CO2 uptake and release of dimethylsulfide are yet to be determined. Research into carbon uptake by the land and ocean as outlined in the U.S. Carbon Cycle Plan (Sarmiento and Wofsy, 1999) and North American Carbon Program (Wofsy and Harriss, 2002) should be undertaken to characterize and reduce the uncertainty associated with carbon uptake feedbacks. The goal is to characterize key atmospheric, biospheric, and oceanic processes that influence the abundance of CO2, with special attention given to observations that define large-scale, decadal, and longer- term sources and sinks, and to define the influences on these processes of climate, land use, and socioeconomic policies. A high priority is to understand the nature of the Northern Hemisphere carbon sink, so that the evolution of this sink and its relationship to the evolving climate can be better understood. Research outlined in the Surface Ocean Lower Atmosphere Study Science Plan (Liss et al., 2002) will improve understanding of climate-dimethylsulfide feedbacks. Atmospheric Chemical Feedbacks Improved understanding of atmospheric chemistry feedbacks is important for producing fixture climate projections, for understanding the relationship between measured concentrations of greenhouse gases and their emissions, and for formulating control strategies. Both tropospheric and stratospheric chemical processes interact with temperature, humidity, circulation, and air composition changes and may in turn affect Earth's
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EXECUTIVE SUMAL4RY 11 radiative balance. More research on atmospheric chemical feedback processes is required, with the goal of representing these processes more comprehensively in projections of future climate. The physical and chemical processing of aerosols and trace gases in the atmosphere, the dependence of these processes on climate, and the influence of climate-chemical interactions on the optical properties of aerosols must be elucidated. A more complete understanding of the emissions, atmospheric burden, final sinks, and interactions of carbonaceous and other aerosols with clouds and the hydrologic cycle needs to be developed. Intensive regional measurement campaigns (ground-based, airborne, satellite) should be conducted that are designed from the start with guidance from global aerosols models so that the improved knowledge of the processes can be directly applied in the predictive models that are used to assess future climate change scenarios. The key processes that control the abundance of tropospheric ozone and its interactions with climate change also need to be better understood, including but not limited to stratospheric influx; natural and anthropogenic emissions of precursor species such as NOx, CO, and volatile organic carbon; the net export of ozone produced in biomass burning and urban plumes; the loss of ozone at the surface, and the dependence of all these processes on climate change. The chemical feedbacks that can lead to changes in the atmospheric lifetime of CH4 also need to be identified and quantified. Feedbacks That Primarily Affect the Transient Response of Climate Ocean Heat Uptake arid Circulation Feedbacks Many climate models predict that the rate of warming over the next 30 years will be much larger than the rate of warming observed over the past century. The rate of warming is important for its effect on human affairs and natural ecology, but it is also very important in continuing efforts to understand the relative roles of feedbacks, forcings, and heat storage in setting the observed warming rate. These efforts are important both for detection and attribution of climate change and for improving projections of future climate. The transient response to changed climate forcing involves important feedback processes, because the evolving climate may alter the rate of heat uptake by the ocean through increased thermal stratification of the ocean, or through the effect of changes in surface precipitation and evaporation on ocean salinity and density.
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12 UNDERSTANDING CLITIC TE CHANGE FEEDBACKS To better represent the exchange of heat and carbon dioxide at the air- sea interface, physical representations of upper ocean processes need to be improved in climate models based on experimental studies of the vertical structure of temperature, absorption of solar radiation, and salinity representative of different ocean environments, including high northern and southern latitudes. Improved definition of the time-dependent temperature and salinity distribution in the global ocean is essential, including the air-sea fluxes of heat and freshwater. This will require full implementation of a system with the capabilities of the current and planned ocean-observing satellites, the Argo global array of profiling floats, the in situ tropical ocean observation networks, and a strategy for monitoring key regions of the ocean where deep-water formation occurs, such as the Labrador, Greenland- Iceland-Norwegian, Weddell, and Ross Seas. Feedbacks That Primarily Influence the Pattern of Climate Change Although the change in global mean climate is important and in some ways easier to project, regional changes are of great practical significance and may provide important clues to understanding the climate system. Land Hydrology and Vegetation Feedbacks Feedback processes over land are critically important to understanding the climate response over land and its effect on humans. Global climate change may initiate local changes in hydrology and surface albedo that feed back to produce larger or smaller local changes in temperature, precipitation, evaporation, soil moisture, and vegetation. The responses of the hydrologic and energy cycles over land play a critical role in determining the impacts of climate change on water resources, carbon stocks, and agriculture, yet these responses vary widely among different climate models. Basic processes such as the response of the land-atmosphere system to diurnal variations of insolation are poorly simulated in current climate models. The melting of snow and ice and associated hydrologic and radiative consequences also tend to be poorly simulated. Dynamic vegetation modeling is also in its very early stages. An integrated analysis is required of the diurnal and annual cycles of the energy, water, and carbon budgets at the land-surface and through the atmospheric boundary layer for different ecosystems and climatic regimes, including managed ecosystems like irrigated cropland. This analysis aimed
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EXECUTIVE SUMA~1RY 13 at improving theoretical understanding and model parameterizations needs to fully integrate land and atmosphere processes and use carefully designed observational metrics to test modeled processes. These models must account for time-varying land surface properties. Sustained multiyear observations of terrestrial ecosystems, their functioning, and their role in the climate system that will contribute to the development and improvement of process-oriented vegetation models for use with climate models should be encouraged. Natural Modes of Climate System Variability Radiatively induced greenhouse warming is not the only effect of greenhouse gas buildup. There is a growing body of evidence that suggests that human activities may also be capable of changing the time-averaged states of the natural modes of variability of the climate system, most notably, the El Nino-Southern Oscillation (ENSO) and the high-latitude northern and southern hemisphere annular modes. An understanding of these modes and how they react to anthropogenic forcing is essential for detection and attribution of global climate change and for interpreting the role of feedbacks. In addition, the natural variability of these modes on a year-to- year time scale provides a testbed for model parameterizations of feedbacks. A tightly integrated effort is needed to close the major gaps in the understanding and modeling of the relationships between natural modes of climate variability and climate change. This effort should integrate data acquisition, analysis, and modeling and should include interactive interfaces among national and international programs that are pursuing seasonal forecasting, climate change feedbacks research, climate change simulation, and climate change detection and attribution (NRC, 2001d). Chapter 1 provides introductory materials as context for understanding the need for a national research strategy in climate feedbacks research. Chapters 2 through 8 provide expanded discussions of the key climate feedback processes that the Panel believes are most in need of study. Chapter 9 summarizes the main recommendations from the chapters. Each chapter is structured somewhat differently, in part because the research needs are different in each area. However, each discussion is intended to leave the reader with a sense of the key processes that are important role in determining the climatic response to a greenhouse gas forcing. Each discussion outlines some of the most important first steps that should be taken to better characterize and hopefully reduce the uncertainty associated with the various feedbacks. These steps include, in general terms,
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14 UNDERSTANDING CLITIC TE CHANGE FEEDBA CKS the types of observations and metrics that can be used to improve both understanding and model representations, as well as to test simulations.
Representative terms from entire chapter: