The sun provides the energy that determines the climate and weather. Solar radiation passes through space and is largely absorbed by components of the global climate system (the atmosphere, oceans, and land, as well as the biosphere, which includes all living things); the remaining radiation is reflected. The solar radiation absorbed by the earth’s surface is released as infrared radiation. Some of this radiation passes back through the atmosphere, and some is absorbed in the atmosphere by the molecules of gas—principally water vapor, carbon dioxide, methane, and chlorofluorocarbons—known collectively as greenhouse gases. These gas molecules act as a partial thermal blanket, trapping much of the heat energy and redirecting it to the earth’s surface and lower atmosphere. This naturally occurring process, called the greenhouse effect (see fig. 1), helps to maintain the earth’s temperature at an average of approximately 60 degrees Fahrenheit.is reflected by the Earth and the atmosphere. by greenhouse gas molecules. The effect of this is to warm the Earth’s surface and the lower atmosphere. atmosphere. by the Earth’s surface and warms it. Earth’s surface. Additional atmospheric warming—called the enhanced greenhouse effect or global warming—appears to be associated with human activities. During the past century, as industry, agriculture, and transportation have grown, so, too, have atmospheric concentrations of heat-trapping greenhouse gases (see app. I). At the same time, the earth has gotten warmer, according to historical data. Recorded temperatures for the period from 1860 through 1993 show a warming trend that generally coincides with the increased use of fossil fuels during the Industrial Revolution—and, hence, with the increased emission of greenhouse gases. During the past 50 to 100 years, volcanic eruptions have combined with the increased combustion of fossil fuels and emission of greenhouse gases to increase the concentration of aerosols in the lower atmosphere. Scientists believe that because these aerosols deflect sunlight, they have partially offset the effects of global warming. As a result, scientists surmise, temperatures have not reached the levels projected by GCMs, which do not include the aerosols’ effects. To help understand the global climate system’s response to emissions of greenhouse gases, scientists use three types of GCMs: atmospheric, oceanic, and coupled. In general, atmospheric GCMs predict the physical behavior of the atmosphere. Oceanic GCMs represent the physics of the ocean. Coupled GCMs, which scientists regard as the most advanced of the models, physically join atmospheric and oceanic GCMs and treat the evolution of the climate in both domains. To improve predictions of the future climate, modelers are also striving to couple, and to some degree have coupled, (1) the land surface to the atmosphere and (2) the Antarctic sea ice to both the ocean and the atmosphere. All types of GCMs process vast quantities of data on variables affecting climate. Using complex mathematical equations to represent the actions and interactions of these variables, the GCMs process the data to project patterns of climatic conditions. (App. II shows how a coupled GCM works.) To test the accuracy of a model’s projections, modelers run the model with their best estimates of historical climatic data and compare the resulting projections with records of actual climatic conditions for the period being modeled. Modelers assume that if the model can accurately simulate actual climatic conditions for prior periods, then it can be used to accurately project future climatic conditions. Although the earth’s gradual warming since the mid-19th century is generally consistent with GCMs’ estimates of the effects of greenhouse gases (when adjusted for the effects of aerosols), scientists have not been able to attribute the warming conclusively to the enhanced greenhouse effect or to quantify its effects. Specifically, they have not been able to uniquely and quantitatively distinguish the effects of higher concentrations of greenhouse gases from the effects of other factors that can change the climate. Such factors include natural fluctuations in the global climate system, increases in atmospheric ozone, air pollution, and aerosols emitted into the atmosphere from volcanic eruptions. Until more is known about the relative influence of these various factors on the earth’s climate, GCMs’ estimates of global warming will remain uncertain. Over the last decade, GCMs have accurately simulated many elements of the observed climate, providing useful indications of some future climatic conditions. For example, atmospheric models have demonstrated some skill in portraying aspects of atmospheric variability, such as the surface temperature of the sea. Oceanic models have also simulated the general circulation of the ocean, including the patterns of the principal currents. Coupled models, though still prone to small-scale errors, have simulated the current climate on a large scale as well as portrayed large-scale atmospheric and oceanic structures. This progress notwithstanding, the models remain limited in their ability to estimate, with desired accuracy, the magnitude, timing, and regional distribution of future climatic changes. These limitations stem from scientists’ imperfect understanding of the global climate system and computers’ insufficient capacity to perform more detailed simulations. More specifically, the accuracy of the models’ predictions is limited by (1) incomplete or inadequate representations of the processes affecting climate and (2) insufficient computer power. Research is being conducted to overcome both the scientific and the technical limitations affecting the accuracy of GCMs’ estimates. According to the U.S. Global Change Research Program, most GCMs include the most important processes that affect climate, such as radiation, convection, and land surface exchanges. However, some models do not include or fully incorporate some processes, and even the most advanced models do not adequately represent the interactions of some processes. None of the models fully incorporates certain components of the global climate system, called feedbacks or feedback mechanisms, and none adequately represents the interactions of these mechanisms with greenhouse gases, called feedback processes. Atmospheric and oceanic GCMs include fewer processes than coupled GCMs, and their simulations are, therefore, more limited and, in some cases, less accurate. Atmospheric models do not fully portray the influence of oceanic pressures (currents) and fluctuations in climate, while oceanic models do not fully account for the effects of atmospheric surface winds. The omission or incomplete incorporation of some processes may introduce errors into these models’ projections. For example, atmospheric models tested in 1991 produced systematic errors in their projections of sea level pressure, temperature, zonal wind, and precipitation. Compared with atmospheric and oceanic GCMs, coupled GCMs include more processes and interactions at the ocean-atmosphere interface, but even they do not include critical biospheric and chemical interactions with the atmosphere. The U.S. Global Change Research Program is supporting efforts by modeling groups to include more complete sets of processes in their models and to identify systematic errors in the models. Although coupled GCMs produce more comprehensive simulations of current climatic conditions than either atmospheric or oceanic GCMs, their simulations still differ from actual conditions. Modelers believe that these models are impaired by a condition known as climatic drift, which results from imbalances in the models’ analyses of heat and moisture variables. These imbalances cause the models’ estimates of temperature and precipitation to deviate from actual conditions. For example, in an experiment conducted by the National Center for Atmospheric Research in 1988, the models estimated wintertime ocean temperatures that were 7 degrees warmer than observed temperatures for the icebound region of Antarctica and 9 degrees colder than observed temperatures for the tropics. Modelers either accept climatic drift or try to correct its effects by inserting adjustments, called flux adjustments. Because flux adjustments artificially improve the models’ performance, their use is controversial. Scientists believe that an increased understanding of the interactions between atmospheric and oceanic variables—and, hence, a more accurate mathematical representation of these interactions—may eventually remove the need for flux adjustments. Reducing the need for flux adjustments is an objective of coupled model research. GCMs include many of the most important feedback mechanisms, such as vegetation, water vapor, ice cover, clouds, and the ocean. However, the models do not yet adequately represent the interactions of these mechanisms with greenhouse gases. Such interactions can amplify, dampen, or stabilize the warming produced by increased concentrations of greenhouse gases. The influence of feedback mechanisms on climate is likely to increase as concentrations of greenhouse gases increase; however, modelers do not fully understand the effects of these mechanisms and have not learned how to represent them with sufficient accuracy in models. Although they have clarified the role of water vapor and improved their ability to model its effects, they are still seeking to understand and accurately model the effects of clouds, which have the greatest potential of all the feedback mechanisms to amplify or moderate global warming. Recent studies have shown that different schemes for modeling cloud formation processes can lead to substantially different projections of the earth’s temperature. In 1989, for instance, two simulations, which varied only in their treatment of the cloud feedback process, produced estimates of the increase in the earth’s annual average surface temperature of 4.9 and 9.4 degrees, respectively. Insufficient computer power affects the accuracy of GCMs’ estimates because even the most powerful computers are limited in their ability to store and analyze the vast quantity of data required to accurately simulate changes in the global climate. Modelers have tried to overcome these limitations by introducing assumptions into their models that deliberately oversimplify some operations in order to free the GCMs’ capacity and time for other, more critical operations. For example, modelers have assumed that the ocean was not warmed by emissions of greenhouse gases before 1985. Although this assumption gains capacity for the GCMs, it introduces an error, called the cold start error, that increases the uncertainty of the GCMs’ predictions. Another oversimplification, the division of the earth into relatively large grids for analytical purposes, prevents the GCMs from accurately predicting regional changes in climate. Simulations by coupled GCMs that are calculated on the assumption that the ocean was not warmed by increased emissions of greenhouse gases before 1985 do not adequately account for the ocean’s reduced capacity to absorb these emissions. In fact, the ocean will reach its capacity for absorbing these emissions sooner—possibly decades sooner—than the coupled GCMs calculate. It will then deflect more of the heat-trapping emissions to the atmosphere, thereby enhancing global warming more rapidly than the models predict. While recognizing that the cold start error artificially delays the onset of global warming in GCMs’ predictions, scientists do not know by how much or by how long it distorts the predictions. Overall, they believe that it causes the models to underestimate the change in temperature that will result from the emissions. Modelers have shown that the cold start error can cause projections of the earth’s average annual temperature to differ by as much as 0.7 degrees after 50 years. According to scientists, an extraordinary commitment of computer time would be required to project the timing of future temperature changes more accurately. Completing the number of computer runs needed to arrive at more precise timing projections could take many months even on a state-of-the-art supercomputer. Still another limitation affecting the accuracy of GCMs’ estimates is the relatively large size of the grids into which the models divide the earth. These grids typically cover an area about the size of South Carolina. Although their use enables GCMs to depict larger-scale regional effects in relatively large, homogeneous regions, it does not allow modelers to incorporate detailed regional features. Consequently, the use of large grids prevents the models from accurately forecasting climatic changes for smaller, less homogeneous regions. The use of smaller grids would permit the incorporation of more detailed features that could be used to project regional changes more precisely. However, models using smaller grids would take longer to run. Each grid contains a single value for each variable for the entire area represented. Today’s grids are smaller than those we described in our 1990 report on global warming, but they are not yet small enough to produce the information policymakers and planners need to develop strategies for adapting to regional changes. Researchers believe that the combination of greater computer power, which would permit the use of smaller grids, and greater understanding of cloud formation processes, which would permit the incorporation of this important but often excluded feedback mechanism, would produce more accurate projections of regional climatic changes. To improve the accuracy of GCMs’ estimates, scientists are developing models that incorporate more of the processes affecting the climate system (particularly cloud formation processes) and better reflect interactions among various components of the climate system, including interactions between or among the ocean and the atmosphere; the land surface, the biosphere, and the atmosphere; and the cryosphere (frozen regions), the ocean, and the atmosphere. They are also developing larger and faster computers that can manipulate data for longer periods of time and smaller grids. In addition, they are collecting more data and conducting more research on the processes affecting climate and improving the international exchange of such data. Various international programs, such as the World Climate Research Programme and the Global Climate Observing System, currently have efforts under way to address these actions. In commenting on a draft of this report, the Director of the Office of the U.S. Global Change Research Program and agency officials stated that the program has several ongoing efforts to address the limitations of GCMs discussed in this report. For example to address the models’ inadequate representation of processes affecting the climate, the program is devoting approximately 30 percent of its $1.8 billion budget for fiscal year 1995 to conduct research aimed at improving scientific understanding of these processes. In addition, to address the need for increased computer power, the program has, through NSF, established a dedicated computing facility for modeling the climate system, known as the Climate Simulation Laboratory, in cooperation with the National Center for Atmospheric Research. This facility will provide state-of-the-art computer resources and data storage systems for use in major modeling research simulations. The goals and funding for the U.S. Global Change Research Program’s fiscal year 1995 research programs are summarized in appendix III. Further information on the program’s efforts to reduce the uncertainties of GCMs’ projections appears in a letter from the Subcommittee on Global Change Research, which is reproduced in appendix VI. Five federal agencies reported spending an estimated $122.6 million during fiscal years 1992 through 1994 to fund modeling activities to improve predictions of the future climate. As shown in table 2, the agencies reported spending approximately $36.9 million, $40.5 million, and $45.3 million for these projects in fiscal years 1992, 1993, and 1994, respectively. Of the five agencies, DOE had the largest climate change modeling program, representing about 36 percent of the total cost for all 3 years. Appendix IV presents background and cost information on each agency’s climate modeling program. Most of the agencies’ climate modeling research was contracted out to universities and research laboratories throughout the United States. These modeling activities were conducted at five major modeling centers in the United States: 1) the National Center for Atmospheric Research in Boulder, Colorado; 2) NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey; 3) NASA’s Goddard Institute for Space Studies in New York, New York; 4) NASA’s Goddard Space Flight Center in Greenbelt, Maryland; and 5) DOE’s Lawrence Livermore National Laboratory in Livermore, California. Although the accuracy of general circulation models’ estimates of climatic change has improved over the past decade, these estimates are still limited by incomplete and inaccurate representations of the processes affecting climate and by insufficient computer power. These limitations prevent scientists from carrying out analyses that would yield more precise information about the magnitude, timing, and regional effects of predicted increases in warming. Ongoing efforts to collect and analyze data, improve representations of climatic processes, and, develop and apply more powerful computers should improve the accuracy of the models’ estimates. Whether these estimates will provide policymakers with the information they need to respond to possible future climatic changes will depend on the degree of certainty expected from the models, the resources provided to improve the models, and advances in scientists’ fundamental understanding of the climate system. We obtained comments from representatives of DOE, NASA, NSF, NOAA, and EPA. According to these comments, which were coordinated by the Director of the Office of the U.S. Global Change Research Program, the agencies found, overall, that this report provided an interesting and useful perspective on the most important factors that limit the credibility of general circulation models’ projections of future climatic conditions. However, the agencies believed that the report would be more useful if it provided some perspective on what the modeling community has learned about the models’ limitations and what efforts are under way to address them. The agencies also believed that the report focused too heavily on the limitations of the models while remaining largely silent on their accomplishments. We have responded to the agencies’ comments by adding information about ongoing research to overcome the models’ scientific and technical limitations and about recent positive results achieved with the models. Additionally, the agencies believed that the report should include, in full, the report of the Forum on Global Climate Change Modeling (Forum), which was developed to inform policymakers about the issues associated with using general circulation models. While we believe that the Forum’s document is useful, we did not include it in this report because its major points are summarized in the agencies’ detailed comments and are included in the body of this report, insofar as they pertain to the objectives of this assignment. Furthermore, since the Forum’s report is available to the public from the Office of the U.S. Global Change Research Program (USGCRP Report 95-01, May 1995), we believe that persons desiring the additional detail may request the document. The agencies’ comments and our response appear in appendix VI. We conducted our work between September 1994 and June 1995 in accordance with generally accepted government auditing standards. We reviewed various scientific documents that discussed the models’ limitations and the implications of these limitations. Through the Director of the Office of the U.S. Global Change Research Program, we collected data on costs from the five agencies that fund U.S. global climate change modeling. We did not independently verify the validity of the cost data. Appendix V more fully discusses our scope and methodology. As arranged with your office, unless you publicly announce its contents earlier, we plan no further distribution of this report until 30 days after the date of this letter. At that time, we will send copies to the Director of the U.S. Global Change Research Program and other interested parties. We will make copies available to others upon request. Please call me at (202) 512-6111 if you or your staff have any questions. Major contributors to this report are listed in appendix VII. Over the past century, human activities have significantly increased atmospheric concentrations of carbon dioxide, methane, and nitrous oxide—known, together with water vapor, as greenhouse gases. Emissions of carbon dioxide, the most abundant greenhouse gas after water vapor, increased by about 25 percent from preindustrial times until 1993. Currently, the growth in emissions is primarily attributable to the increased use of fossil fuels, whereas, in the 19th and early 20th century, it was due to deforestation and the expansion of agriculture. Methane emissions increased by about 9 percent between 1978 and 1987 and have more than doubled since preindustrial times. Nitrous oxide emissions increased by about 9 percent from preindustrial times until 1993. Table I.1. details the increases in greenhouse gas emissions, the periods when the increases occurred, and the sources of the emissions. The relative contribution of each gas to the enhanced greenhouse effect is determined by the ability of the gas to absorb infrared radiation and by its atmospheric abundance. Atmospheric abundance is determined by the quantity of gas emitted and by its atmospheric life span. For example, although a methane molecule is a more effective absorber of infrared radiation than a carbon dioxide molecule, it contributes only about a third as much to the enhanced greenhouse effect because it is less abundant. Carbon dioxide is believed to have contributed 70 percent of the enhanced greenhouse effect from the beginning of the Industrial Revolution up to 1990. Figure I.1 depicts the relative contributions to the enhanced greenhouse effect from the cumulative contributions of carbon dioxide, methane, and nitrous oxides. Chlorofluorocarbons are not included in the figure because, unlike carbon dioxide, methane, and nitrous oxide, their atmospheric concentrations vary considerably across the globe and are difficult to quantity. General circulation models (GCM) are the most advanced tool that scientists have to model climate and predict climatic change. These models comprise complex mathematical equations that describe various physical processes and interrelationships, including seasonal changes in sunlight, global air currents, and other factors that affect the climate. Because the equations are so complex, modelers cannot solve them exactly and consequently must segment the earth into a discrete number of grids to approximate the solutions. The coupled model depicted in figure II.1 calculates solutions for 18 layers above each grid (extending from the ocean’s surface to the top of the atmosphere) and 20 layers below each grid (extending from the surface to the floor of the ocean). Figure II.1: How One General Circulation Model Works Earth is divided into a gridwork of 50, 520 “boxes”. The atmosphere above each box is divided into 18 layers. The ocean under each box is divided into 20 layers. Each layer’s program represents a set of variables (such as winds and temperature) and formulas for basic physical laws (such as the conservation of energy). The computer calculates how processes in each layer affect conditions in each neighboring layer and feeds that data into adjoining layers. The computer repeatedly recalculates as modeled days pass into months. As seasons change, it varies the amount of sunlight. The U.S. Global Change Research Program (USGCRP) provides insight into the causes and effects of changes in the earth’s climate system, especially those related to human activities, and is developing tools to assess options for responding to global change. As the depth of understanding grows, the research results are intended to provide increasingly valuable support for formulating national and international policy, as well as for evaluating the impact and effectiveness of the actions taken. The research activities are grouped by major focus in six broad categories: observing the earth’s climate system through land, ocean, and satellite managing data and information to ensure that they are preserved and available for national and international researchers to use; understanding global change processes, ranging from cloud formation and hydrologic processes to the accumulation of atmospheric ozone; predicting the magnitude, timing, and extent of global change; evaluating the consequences of global change by analyzing the impact of global change on the environment and on society; and assessing policies and options for responding to global change. The President’s fiscal year 1995 budget for the research activities is summarized in table III.1. Observing the earth’s climate system Predicting change (modeling) The earth’s environmental system encompasses the atmosphere; the oceans and marine life; the land surface and biosphere (plant and animal life); and the cryosphere (snow, glaciers, sea ice, and icecaps). Because this complex, interconnected system cannot be reconstructed and experimented with in the traditional laboratory sense, numerical models are used to simulate the behavior of the earth’s system and its fluctuations, variations, and responses to disturbances, including the effects of human activities. Coupled atmospheric/oceanic GCMs are, within the limits of available resources and ingenuity, designed to include as much of the important and relevant physics, chemistry, and biology as is understood and as is needed to address particular questions posed to the models about future climatic change. A wide array of modeling activities supports the need to provide society with the best possible predictions of weather; anomalous seasonal events, such as floods and droughts; fluctuations in the frequency of climatic extremes; and long-term changes in climate. These activities, which are conducted at research centers, universities, and government laboratories, are supported by government agencies that have responsibility for scientific research, including (1) the Department of Energy (DOE), (2) the National Aeronautics and Space Administration (NASA), (3) the National Science Foundation (NSF), (4) the National Oceanic and Atmospheric Administration (NOAA), and (5) the Environmental Protection Agency (EPA). These agencies support research using full models of the global climate to improve, test, and, in some cases, project the future climate and its potential changes. The agencies’ roles in modeling climatic change are discussed below. DOE’s modeling program focuses on changes and variations in the earth’s climate—especially those caused by human activities—that may occur over periods ranging from decades to centuries. DOE’s program (1) tests the performance of models from around the world by comparing their ability to represent the recent climate, (2) simulates the effects of carbon dioxide emissions on the climate, and (3) develops global models by taking advantage of the new generations of highly parallel computers. These activities are intended to develop the coupled models of the earth’s oceans, atmosphere, and land surface that are needed to project the climate more accurately from tens to hundreds of years. During fiscal year 1994, DOE funded modeling research at 24 universities and research centers. NASA’s modeling program focuses on developing and applying a four-dimensional model that places special emphasis on the role of data from satellites in providing research-quality information on the climate system. NASA’s program supports efforts to (1) better understand the relative roles of the various factors that have changed or are changing the earth’s climate; (2) analyze the global effects of feedback mechanisms, such as clouds, that can amplify or moderate climatic change; and (3) develop tools for integrating data from satellites and other sensors into a coherent record of atmospheric behavior. During fiscal year 1994, NASA supported modeling research on climatic change at its Goddard Institute for Space Studies and Goddard Space Flight Center and at two universities. NSF’s modeling program focuses on climatic change that occurs over seasons to centuries and provides computer resources to the research community. Specifically, NSF’s programs emphasize research on coupling models of the atmosphere, oceans, land surface, and cryosphere into a single integrated model that can simulate the global climate system over the long term. NSF also supports wide-ranging research activities, including simulations of climates of the distant past, of natural variations in the present climate, and of the interactions of the various processes and influences. During fiscal year 1994, NSF funded major modeling research projects at 10 universities and research centers. NOAA’s modeling program focuses primarily on seasonal to interannual (year to year) predictions and on better understanding long-term climatic variation and change. NOAA’s activities include (1) developing and improving models of the atmospheric-oceanic system, (2) comparing models’ simulations to observations and analyses of the processes that most influence climate, (3) simulating the potential climatic effects of increased concentrations of greenhouse gases, and (4) separating the effects of natural climatic variations from the effects of human activities on climate. In addition, NOAA has tried to develop models capable of predicting the seasonal to interannual fluctuations that cause extreme rainfall and other similar disruptions to regional climates. During fiscal year 1994, NOAA supported global general circulation modeling research on 10-year and longer time scales at its Geophysical Fluid Dynamics Laboratory and four universities and research centers. EPA focuses its modeling research on chemical and environmental interactions within the biosphere. It supports research to improve GCMs’ representations of ecosystems and of the relationship between chemicals and plant and animal life in an area so that the effects of climatic change on the biosphere and of biospheric change on climate can be projected. During fiscal year 1994, EPA funded modeling research at three universities and research centers. As previously stated, five agencies support research on modeling global climate change. This research—through which models of the global climate are improved, tested, and, in some cases, used to project the future climate and the ways it may change—can be grouped into two broad areas: modeling to predict changes in climate that may occur over decades and modeling to simulate the current climate. The cumulative estimated costs of the five agencies’ modeling activities was approximately $123 million during fiscal years 1992 through 1994, as table IV.1 shows. The Ranking Minority Member of the House Committee on Commerce asked us to review the factors that affect the accuracy of GCMs’ estimates of future climatic changes and determine the costs of federally funded GCMs for fiscal years 1992 through 1994. We conducted our work between September 1994 and April 1995 in accordance with generally accepted government auditing standards. To determine the factors that affect the accuracy of GCMs’ estimates of future climatic changes, we reviewed information that we previously reported in Global Warming: Emission Reductions Possible as Scientific Uncertainties Are Resolved (GAO/RCED-90-58, Sept. 28, 1990). We also met with headquarters officials at DOE, NASA, NSF, NOAA, and EPA and with the Director of the Office of U.S. Global Change Research Program (USGCRP) to discuss these factors. From these meetings, we obtained various scientific assessments of GCMs’ strengths and limitations. In October 1994, the Subcommittee on Global Change Research held the Forum on Global Change Modeling with modelers from throughout the United States. The intent of the forum was to address requests from the White House Office of Science and Technology Policy and the General Accounting Office to produce a consensus document on issues concerning the use of climate models to inform policy on future climatic changes. This forum, whose participants included agency officials, scientists, and academicians involved in studying the global climate, provided information on the strengths and weaknesses of GCMs, as well as other relevant topics. In addition, we searched four scientific data bases to identify additional assessments of the models’ limitations. Throughout our review, we met with the Director of the Office of USGCRP to clarify technical issues associated with the models’ limitations. To identify federal funding for GCMs during fiscal years 1992 through 1994, we obtained cost data by agency from USGCRP. We worked with the Director of the Office of USGCRP to develop an instrument to capture all relevant cost components. We did not independently verify the validity of the cost data. On May 12, 1995, we met with the Director of the Office of USGCRP, the Manager of the Climate Modeling Program at NASA, the Deputy Director of the Office of Global Programs at NOAA, and the Manager of the Global Change Research Program at EPA to obtain their comments on a draft of this report. On May 22, 1995, the Chair of the Subcommittee on Global Change Research provided us with written comments on the draft. These comments integrated the responses of the five agencies included in our review (see app. VI). We have addressed the comments in the text of this report, where appropriate. The following are GAO’s comments on the Subcommittee on Global Change Research’s letter dated May 22, 1995. 1. Under the heading “Factors Limiting the Accuracy of Models’ Estimates,” we discussed some of the successes of GCMs to create a context and provide balance for our discussion of the models’ limitations. Later, under the heading “Improving GCMs’ Estimates,” we discussed the efforts that are currently under way to address these limitations and referred to the agencies’ discussion of such activities in this letter. Under the heading “Agency Comments,” we explained why we did not reproduce the report of the U.S. Global Change Model Forum in this report. 2. We added a footnote on page 3 of the report to better explain USGCRP’s role in coordinating federal research on global climate change. 3. We revised our discussion of the models’ limitations (pp. 8-14 of our draft report) as necessary to address the agencies’ specific comments. We changed the heading “Exclusion of Critical Processes,” cited in the agencies’ comments, to “Some Processes Not Included or Fully Incorporated in Some Models” to better describe the supporting text. Mary A. Crenshaw, Site Senior The first copy of each GAO report and testimony is free. Additional copies are $2 each. Orders should be sent to the following address, accompanied by a check or money order made out to the Superintendent of Documents, when necessary. Orders for 100 or more copies to be mailed to a single address are discounted 25 percent. U.S. General Accounting Office P.O. Box 6015 Gaithersburg, MD 20884-6015 Room 1100 700 4th St. NW (corner of 4th and G Sts. NW) U.S. General Accounting Office Washington, DC Orders may also be placed by calling (202) 512-6000 or by using fax number (301) 258-4066, or TDD (301) 413-0006. Each day, GAO issues a list of newly available reports and testimony. 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Summary: Pursuant to congressional request, GAO reviewed the accuracy of general circulation models (GCM) in forecasting global warming trends, focusing on the: (1) factors limiting the accuracy of GCM estimates of future climatic changes; and (2) federal expenditures for GCM for fiscal years (FY) 1992 through 1994. GAO found that: (1) although GCM have improved their ability to predict future climatic changes over the last decade, their estimates are still limited by their incomplete or inaccurate representations of climate-affecting processes and by insufficient computer power; (2) scientists do not fully understand how the climate system responds to potentially important physical, chemical, and biological processes; (3) the lack of computer power requires scientists to use simplified assumptions and structures that increase the uncertainty of the models' predictions; (4) scientists are conducting research to overcome the limitations of the computer models; and (5) five federal agencies spent about $122.6 million for various global modeling projects, which represented about 3 percent of the global change research program's budget for FY 1992 through 1994.