There is now clear evidence that the mean annual temperature at the Earth's surface, averaged over the entire globe, has been increasing in the past 200 years. There is also clear evidence that the abundance of greenhouse gases in the atmosphere has increased over the same period. In the past decade, significant progress has been made toward a better understanding of the climate system and toward improved projections of long-term climate change. Several national and international studies published in 2001 have provided reviews and assessments of the science of climate change. A National Research Council report concluded that "[g]reenhouse gases are accumulating in the Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. . . . The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability" (National Research Council 2001a). The Intergovernmental Panel on Climate Change (IPCC) reported that recent regional climate changes, particularly temperature increases, have already affected many physical and biological systems (McCarthy et al. 2001), and a national assessment on climate change impacts on the United States concluded that "natural ecosystems, which are our life support system in many ways, appear to be the most vulnerable to the harmful effects of climate change," but, "highly managed ecosystems appear more robust" (National Assessment Synthesis Team 2001).
The report by the IPCC stated that the global mean temperature is projected to increase by 1.4°C-5.8°C in the next 100 years (Houghton et al. 2001). Uncertainties remain in several key aspects, including the magnitude, timing, and regional distribution of anticipated climate change. These uncertainties can and should be better understood and quantified, and reduced by advances in the following research areas:
- Understanding of climate system forcing, interactions and feedbacks;
- Projections of global and regional climate change and their environmental and social impacts;
- Methods for quantifying the uncertainty of climate change projections and articulating our understanding of the uncertainties to policy makers.
The infrastructure required to meet these research priorities include
- reliable, long-term climate information from paleorecords as well as from existing and future observational systems, designed or adapted for the study of climate variability and change;
- adequate computational resources for climate research and projections and for ensembles of calculations to quantify uncertainties in the projections;
- a focused and well-coordinated multiagency and international research and applications structure.
Because human activities are contributing to climate change, we have a collective responsibility to develop and undertake carefully considered response actions. The fundamental challenge is to understand and respond to the risk represented by climate change in the larger context of overall societal issues and environmental stresses. Development of effective response strategies requires a holistic approach, in which the natural and social sciences, in tandem with technology and engineering, proceed side by side in a broad multidisciplinary effort. The federal agencies involved are now organized under a presidential initiative that mandates cooperative activities in climate change research and climate change technology (U.S. Climate Change Science Program 2002).
The atmospheric and related sciences have a central role to play in providing scientific knowledge needed to inform and evaluate many aspects of proposed response strategies.
1. Introduction. Human activities have become a major source of environmental change. Of great urgency are the climate consequences of the increasing atmospheric abundance of greenhouse gases and other trace constituents resulting primarily from energy use, agriculture, and land clearing. These radiatively active gases and trace constituents interact strongly with the Earth's energy balance, resulting in the prospect of significant global warming. When used in this context, the term "global warming" includes all climate and environment effects arising from natural climate variability as well as from anthropogenic changes in atmospheric composition and land use. For many nations, the possibility of substantial climate change is viewed as likely to have a serious impact on the global environment and on human welfare over the course of the next few decades to centuries. Because greenhouse gases continue to increase, we are, in effect, conducting a global climate experiment, neither planned nor controlled, the results of which may present unprecedented challenges to our wisdom and foresight as well as have significant impacts on our natural and societal systems. It is a long-term problem that requires a long-term perspective. Important decisions confront current and future national and world leaders.
The atmosphere synthesizes, transforms, and communicates exchanges at its boundaries. Hence, the issues of climate change are global in nature and must be addressed in a global context. However, it is proving difficult to achieve national and international consensus as to what should and could be done to address the issues. This difficulty arises not just because of the long timescale over which the buildup of greenhouse gases has been occurring, but also because of the wide range of climate change projections and the timing and severity of their impact upon human welfare. An improved scientific understanding of the changing atmospheric composition and its climatic significance is therefore essential for informing the public and developing scientifically informed national and international policy responses to the prospects of climate change.
The purpose of this statement is to provide a broad overview of the climate change research imperatives and challenges that are most relevant to the atmospheric and related oceanographic and hydrologic sciences.
2. Background. Prior to the industrial age, natural processes such as solar variability and volcanic outgassing were the dominant forcing factors producing long-term climate changes over periods of decades, centuries, and millennia. However, there is convincing evidence that since the industrial revolution, human activities, resulting in increasing concentrations of greenhouse gases and other trace constituents in the atmosphere, have become a major agent of climate change.
During the industrial period, the atmospheric abundance of carbon dioxide, methane, nitrous oxide, halocarbons (e.g., chlorofluorocarbons), and tropospheric ozone have increased as a direct result of human activity. The increase of carbon dioxide in the atmosphere has come largely from fossil fuel combustion and land clearing. Methane is produced in anoxic environments in wetlands, agriculture, and landfills, and is also inadvertently released to the atmosphere during energy exploration and delivery. Nitrous oxide is formed by microbial reactions in soils and waters, including those acting on the increasing amounts of nitrogen contained in fertilizers. The major sources of chlorofluorocarbons are refrigerants, industrial uses, and aerosol propellants. Tropospheric ozone is formed by photochemical reactions involving emissions of methane and pollutants and is the third most important greenhouse gas after carbon dioxide and methane. However, the relative effect of carbon dioxide dominates all other anthropogenic increases of greenhouse gases since the industrial revolution.
An overwhelming majority of scientists agree on the following facts relating to the global warming issue.
- The theory of how greenhouse gases directly interact with atmospheric radiation is not controversial. If no other factors counter their influence, increases in their concentration will lead to global warming.
- A steady rise in the concentration of greenhouse gases began over 200 years ago and is continuing. Atmospheric concentration of carbon dioxide, the principal greenhouse gas, has increased from pre-industrial concentrations of 280 ppmv (parts per million by volume) to over 367 ppmv in 2000, an increase of more than 30%; methane has increased from 0.7 to about 1.8 ppmv, an increase of more than 150%; nitrous oxide has increased from 0.27 to over 0.31 ppmv, an increase of 16%. Tropospheric ozone is estimated to have increased by 35% since the industrial revolution.
- At current rates and modes of energy use, doubling of carbon dioxide relative to preindustrial levels is likely to occur by the mid- or late twenty-first century.
- If we were to proceed on our present course until the current global inventory of known oil and coal deposits are exhausted, carbon dioxide concentrations could reach 4-6 times those of preindustrial levels.
- Because carbon dioxide is not destroyed chemically in the atmosphere, centuries and longer will be required before the added carbon dioxide is removed from the atmosphere to the deep ocean and geologic reservoirs.
Informed policy decisions of government and industry demand unbiased assessments of scientific results by the scientific community. The nature of science is such that there is rarely total agreement among scientists. Individual scientific statements and papers-the validity of some of which has yet to be assessed adequately-can be exploited in the policy debate and can leave the impression that the scientific community is sharply divided on issues where there is, in reality, a strong scientific consensus. The IPCC was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environmental Program (UNEP) to fulfill the critical role of providing objective scientific, technical, and economic assessments of the current state of knowledge about various aspects of climate change. IPCC assessment reports are prepared at approximately five-year intervals by a large international group of experts who represent the broad range of expertise and perspectives relevant to the issues. The reports strive to reflect a consensus evaluation of the results of the full body of peer-reviewed research. A large number of U.S. scientists are on the international Working Groups of the IPCC that prepare and review these reports. They provide an analysis of what is known and not known, the degree of consensus, and some indication of the degree of confidence that can be placed on the various statements and conclusions. These reports have become the prime scientific basis for international political decisions about climate change.
The Third Assessment Report (TAR) of the IPCC was published in 2001 (Houghton et al. 2001; McCarthy et al. 2001). The conclusions of the report have been widely publicized and extensively discussed. The IPCC Working Group I was charged, among other things, with assessing the observed climate changes over the past century. Working Group I concluded that "an increasing body of observations gives a collective picture of a warming world and other changes in the climate system." In addressing the question of attribution, they further concluded that "there is new and stronger evidence that most of the warming observed over the past 50 years is attributable to human activities." The projections of global mean temperature change during the next hundred years, over a variety of greenhouse gas emission scenarios and model runs that were considered, ranged between 1.4° and 5.8°C
Working Group II, which assessed the sensitivity, adaptive capacity, and vulnerability of natural and human systems to climate change, concluded that "recent regional climate changes, particularly temperature increases, have already affected many physical and biological systems." They further concluded that "the potential for large-scale and possibly irreversible impacts poses risks that have yet to be reliably quantified."
The White House requested the U.S. National Academy of Sciences (NAS) to "identify the areas in the science of climate change where there are the greatest certainties and uncertainties." The NAS Committee on the Science of Climate Change concluded, among other things, that "[g]reenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we can not rule out that some significant part of these changes is also a reflection of natural variability" (National Research Council 2001a).
At the request of Congress, the United States Global Change Research Program (USGCRP) delivered the first national assessment on "Climate Change Impacts on the United States: The Potential Consequences of Climate Variability and Change" to the nation in a report released in 2001 (National Assessment Synthesis Team 2001). This assessment was the first attempt by the scientific community, in partnership with stakeholders from a wide spectrum of geographic regions and economic sectors across the nation, to assess impacts, vulnerabilities, and adaptation strategies related to globally induced, regional climate change. Key findings from the report suggested that "natural ecosystems, which are our life support system in many ways, appear to be the most vulnerable to the harmful effects of climate change," but "highly managed ecosystems appear more robust." Since regional climate changes are likely to interact with other environmental stresses and socioeconomic factors, an integrated research approach was recommended to narrow the outstanding uncertainties related to the impact of regional climate change.
3. Research imperatives. While significant progress has been made toward a better understanding and improved projection of climate change and its impacts, uncertainty remains regarding the magnitude, timing, and regional distribution of anticipated changes. The research imperatives needed to narrow and better quantify these uncertainties have been addressed in a number of National Research Council (NRC) Reports (National Research Council 1999, 2001a, b), the U.S. National Assessment (National Assessment Synthesis Team 2001), as well as the IPCC reports (Houghton et al. 2001; McCarthy et al. 2001). Among other things, the NRC reports have identified three keys to progress: data, computer resources, and organization.
A. INFRASTRUCTURE PRIORITIES. Environmental observations and information are the building blocks of climate science and applications. Reliable, long-term paleorecords, historical, and in situ and remotely sensed instrumental data are needed to better characterize the nature of natural variability, identify the sources of past climate variability and change, monitor and diagnose current climate conditions, evaluate climate models, and provide initial conditions for climate model projections. Thus, data requirements crosscut almost every aspect of climate change science.
Limited time-space observational programs have provided and continue to provide vital information on climate processes. However, most of the data that have been used for climate purposes were and still are obtained from observation systems designed for other purposes (e.g., weather prediction). These data are often inadequate for the analysis and description of climate change because of the higher accuracy requirements for observations used to detect very low amplitude climate changes over long time periods. The Third Conference of the Parties of the United Nations Framework Convention on Climate Change (United Nations 1997) in 1997 concluded that the global capacity to observe the Earth's climate system is inadequate and deteriorating worldwide. It is imperative to establish an integrated global climate observing system (GCOS) as soon as possible, designed to comprehensively observe, with the required accuracy and long-term stability, the key variables of the climate system, including its forcing factors. Efforts led by the WMO and many individual nations have been under way for several years, but progress has been slow.
Computers are the equivalent of an experimental laboratory for weather and climate scientists. A high-performance, state-of-the-art computing capability is essential for the development of improved climate system models and for their full exploitation for climate projection. Effective use could be made of computers a thousand times more powerful than those now available for climate and weather research and forecasting. As computing power continues to increase, it is imperative that the United States continuously upgrades computing capability for climate research and projections.
A well-coordinated, multiagency research effort is also vital for progress. Climate change research in the United States is rich and diverse, and crosscuts the historic missions of various government agencies. Consequently, research initiatives can often be fragmented and poorly coordinated. The need for improved coordination of high-priority climate research imperatives within the United States, as well as major U.S. participation in and leadership of international climate research activities such as the World Climate Research Programme, has been pointed out in several NRC reports.
B. RESEARCH PRIORITIES. The Earth's climate system is tightly coupled and awe inspiring in its complexity. Understanding and modeling the myriad physical, chemical, and biological forcing, interactions, and feedbacks of the system is a daunting task that will continue to occupy researchers for the foreseeable future. It is important to focus on questions of the highest current priority. These can be grouped into several broad and interrelated categories.
i) Develop an improved understanding of climate system forcing, interactions, and feedbacks.
Progress toward developing an improved understanding of the climate system forcing, interactions, and feedbacks discussed below hinges on a better understanding of external forcing factors and the many basic interactive physical, chemical, and biological processes of ocean, land, cryosphere, and atmosphere that are involved. These include radiative transfer, cloud physics, ocean heat storage and transport, and the nature of the physical, chemical, and biological processes at the interface between atmosphere and the underlying ocean, land, and ice.
More certain projections of global climate change will require major advances in understanding and modeling the forcing factors of climate change (i.e., factors that determine the atmospheric concentrations of greenhouse gases and aerosols and their radiative properties.) Of the greenhouse gases that are directly influenced by human activity, the most important are carbon dioxide, methane, ozone, nitrous oxide, and chlorofluorocarbons (CFCs). Human activity also contributes to the aerosols that are in atmosphere, most notably sulfate particles and black carbon (soot). Aerosols have short lifetimes and are unevenly distributed. They impact the radiation budget; however, unlike the greenhouse gases, their radiative properties and spatial distributions are not as well known. A greater uncertainty about aerosols is their influence on clouds and cloud processes and, hence, their indirect effect on the Earth's energy balance. It is believed that the net effect of aerosols tends to cause global cooling, which may have offset some of the warming due to greenhouse gases. Land-use change is another important human-induced regional forcing factor, but accurate histories of land-use change are spotty and estimates of future land-use changes are very difficult. Solar variability is a natural forcing that is strongly correlated with climate variations in the past; however, its relative importance in recent climate change, especially when compared with human-induced greenhouse forcing, appears to be low. More research into feedback mechanisms that might amplify or attenuate the climate's response to individual forcings is needed.
Climate system feedbacks are fundamental in determining the climate response to forcing by changing the abundance of greenhouse gases and aerosols. The term "feedback" refers to processes internal to the climate system that can amplify (positive feedback) or dampen (negative feedback) the climate forcing. Research in the past several decades suggests that climate system feedbacks are at least comparable and likely to be up to two to three times the direct effects of climate forcing. Water vapor feedback is the most important positive feedback: warm air can hold more water vapor, which is itself a natural greenhouse gas. Albedo feedback in regions of surface ice cover is also an important positive feedback resulting from increasing surface absorption of solar radiation as ice cover decreases. Cloud feedback may also be very important, but its magnitude and even its sign remain uncertain. Climate-induced changes of the land surface may in turn feed back on the climate itself. Many processes are involved, including changes in soil characteristics (e.g., soil moisture, holding capacity), vegetation, radiative characteristics (e.g., albedo), and surface-atmosphere exchanges of water vapor, other gases (e.g., CO2), particulates (e.g., dust) and momentum. Some of these processes remain poorly understood and difficult to measure and model, and thus require continued research.
The full suite of potentially important feedback processes is yet to be adequately understood and quantified. For example, although observations of global mean temperatures indicate that the surface has warmed by about 0.6°C over the past 100 years, the mean tropospheric temperature trends, for which we have observations only over the past 40 years or so, are more complex and not well understood. Although there remains significant observational uncertainties, global mean tropospheric temperatures warmed faster in the 1960s and 1970s, and slower in the 1980s and 1990s, than the global mean surface temperature. These differences indicate the need for research into feedback processes that might explain this reversal in tropospheric temperature trends. Additional poorly understood feedbacks of potential importance are carbon cycle feedback, due to warming-induced changes in the land and ocean carbon reservoirs; atmospheric chemistry feedback, due to chemical interactions affecting ozone concentrations, aerosol formation, and atmospheric heating profiles; and ocean circulation feedback, arising from ocean circulation changes that affect ocean-atmosphere heat and freshwater exchange.
A better documentation and understanding of the global hydrologic cycle are also fundamental for a better understanding and projection of climate change and its impacts. The global hydrologic cycle is intertwined in a fundamental way with the energy processes of the climate system and the sensitivity of climate to increasing greenhouse gases. Furthermore, changes in regional hydrology (i.e., precipitation, evaporation, soil moisture, vegetation, and runoff) are among the most significant elements of climate change and its impacts.
A better description and understanding of past climate variations on decadal and longer timescales are needed to discriminate among the causes of natural climate variability and to fingerprint the anthropogenic component of change. This includes secular changes in the nature of shorter-term variability (e.g., El Niņo episodes, weather extremes) and abrupt climate change. The relatively short instrumental time series, generally less than a century in length, is often too short to adequately characterize longer-term variability. Progress will thus depend on the collection and analysis of relevant proxy information (e.g., coral data to characterize secular variations in El Niņo events.)
The climate system exhibits natural modes of spatial and temporal variability. These include the diurnal and annual cycles forced (externally) by solar radiation and modes of variation that are expressions of internal nonlinear dynamics of the Earth system. The best known of these internal modes is the El Niņo-Southern Oscillation phenomenon, which arises from a fluid dynamical instability of the coupled atmosphere-ocean system. A better understanding of mechanisms of these modes will enhance efforts to assess, understand, and model the climate system, and may be important in determining the regional responses to anthropogenic climate change.
While there is general agreement among the various climate system models that significant global warming will occur in the next 50 years, many crucial details of magnitude, timing, and specific regional responses-especially for hydrological variables, such as precipitation-are still very much in doubt. The uncertainties arise due to incomplete identification and understanding of processes significant in future climate, and their necessary approximations in models. Rapid application of new research findings to model improvement is important to reducing uncertainties in climate projections. This requires constructive interfaces between the modeling community and scientists in the broader physical, chemical, and biological communities.
ii) Improve projections of global and regional climate change and their environmental and social impacts.
Changes in the global mean annual temperature conceal complex, large amplitude patterns of regional climate change. In addition, vulnerability to, and potential impacts of, climate change vary regionally and locally. Climate changes in some regions may be benign or beneficial, while in others they can have disruptive social and economic effects. Among the potentially disruptive effects of climate change are changes in the nature of short-term climate variability, most notably the nature, frequency, and intensity of extreme weather and short-term climate events. The relationship between climate change and many crucial aspects of these events is poorly understood and research in this area is exacerbated by the dearth of reliable climatologies of these episodic phenomena. An aid in coping with such variability is the expected improvements in weather and short-term climate prediction, most notably of the prediction of probabilities of extreme weather, such as floods, hurricanes, and droughts. The environmental and social vulnerability of a region to the potential impact of global warming can be decreased through timely implementation of mitigation and adaptive strategies based on the improved predictions.
The assessment of regional environmental and social impacts is a multidisciplinary task that involves natural and social scientists working in tandem with policy makers. It can begin with an assessment of the impacts of various climate change scenarios, but an assessment of anticipated impacts is far more difficult. Current assessment tools, including climate system models with regional resolution, do not as yet provide reliable results.
The first U.S. National Assessment report (National Assessment Synthesis Team 2001) concluded with a series of recommended "research pathways," which will aid in answering questions such as, "how is our environment being altered by climate change and how much confidence can we place in future projections, given our ability to understand past changes and variations?" and, "what are the likely costs of adapting to increases in average temperature and heat index and to what degree can cities take climate change into account in planning for new infrastructure, such as water distribution and routing, bridges, and peak power demands?" The report also recommended a sustained commitment to high-quality observations required for detecting changes in important aspects of our environment.
iii) Develop improved methods for quantifying the uncertainty of climate change projections.
As the processes that shape climate become identified and more clearly understood, and as climate models improve, the uncertainties in model projections can be expected to narrow. However, climate models will never be perfect. Only a portion of natural climate variability is predictable, and the inherent degree of predictability is not yet well understood. Emission scenarios involve future human decisions, which are difficult to anticipate, and may be interactive with the resulting projections of climate change for which they serve as input. Consequently, projections of climate change will always need to be couched in probabilistic terms.
For issues such as global warming, the interface between science and policy formulation is typically framed in terms of risk management. Whether action is deemed to be warranted depends upon how the risks and benefits are framed (consideration of "decisions of least regret"), how far in the future they lie, and whether response options may be foreclosed if decisions are delayed. Natural scientists are called upon to estimate the risks of harmful consequences and the benefits that might be realized under various policy scenarios. Economists, in turn, are called upon to estimate societal costs and other consequences inherent in those risks, and alternately the costs that would be incurred in taking preemptive actions designed to mitigate the risks.
Risk cannot be assessed with absolute certainty. The recent NAS report, "Climate Change Science: An Analysis of Some Key Questions," emphasized the importance for policy makers of providing measures of the uncertainty of climate change projections ("confidence limits and probabilistic information, with their basis, should always be considered as an integral part of the information that climate scientists provide to policy and decision makers"). It is important that scientists develop objective measures of uncertainty that will assist in the transformation of model results to probabilistic information that is more directly useful for decision making by the public, the business community, and local, state, and federal governments. The application of ensemble modeling techniques offers one means to put the estimation of uncertainty on a more quantitative basis. As one measure of uncertainty, it makes use of the spread among a number of forecasts obtained by slightly modifying the initial conditions. Unfortunately, ensemble prediction requires significantly greater investment in computer resources than is the case at present.
4. EVALUATION OF RESPONSE STRATEGIES. Because human activities are contributing to and accelerating climate change, we have a collective responsibility to develop and undertake carefully considered response actions. The fundamental challenge is to manage the risk represented by climate change in the larger context of overall societal issues and environmental stresses. Development of effective strategies requires a holistic approach, in which the natural and social sciences, in tandem with technology and engineering, proceed side by side in a broad multidisciplinary effort. Broadly acceptable response strategies must rest on a rational scientific and technological foundation. Caution is required in adopting response strategies to assure that the total effect will not create an unacceptable environment and/or societal legacy. Given the complexity and uncertainties surrounding the scientific and societal issues involved in climate change, we should consider a research agenda applied in a parallel approach to 1) increase scientific understanding (leading to better characterization and reduction of scientific uncertainty) and inform mitigation policy deliberations, and 2) provide answers to "if . . . then" questions involving potential technological measures and/or adaptive strategies applied to reduce environmental and societal vulnerability.
The federal agencies involved in the broad areas of environmental science, technology, and policy are now organized under a presidential initiative that mandates cooperative activities in climate change research and climate change technology (U.S. Climate Change Science Program 2002).
Recognizing the increasing ability of engineering to provide powerful solutions, but at the same time recognizing the risk of unintended social and/or environmental consequences, the National Academy of Engineering is leading an "Earth system engineering" approach to the problem, which brings tools from engineering and the physical and social sciences. The atmospheric and related sciences have a central role to play in providing scientific information needed to inform and evaluate many aspects of proposed response strategies.
Extensive multidisciplinary research is needed to narrow the many formidable knowledge gaps. The AMS's annual Global Change and Climate Variations Symposium provides a mechanism for the communication of research results, needs, and future directions to the physical, chemical, biological, and social scientists and their students involved in global change research. The AMS Policy Program serves to inform and interact with the public and local, state, and federal decision makers on issues related to the science of climate change and its social and economic impacts. The atmospheric and related oceanographic and hydrologic sciences represented in the AMS are central to this multidisciplinary effort. Consequently, climate change research can be expected to occupy a prominent place in these sciences for the foreseeable future.
Houghton, J. T., Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell, and C. A. Johnson, Eds., 2001: Climate Change 2001: The Scientific Basis. Cambridge University Press, 892 pp.
McCarthy, J. J., O. F. Canziani, N. A. Leary, D. J. Dokken, K. S. White, Eds., 2001: Climate Change 2001: Impacts, Adaptation, and Vulnerability. Cambridge University Press, 1032 pp.
National Assessment Synthesis Team, 2001: Climate Change Impacts on the United States: The Potential Impacts of Climate Variability and Change. Cambridge University Press, 618 pp.
National Research Council, 1999: Global Environmental Change: Research Pathways for the Next Decade. National Academy Press, 595 pp.
--, 2001a: Climate Change Science: An Analysis of Some Key Questions. National Academy Press, 29 pp.
--, 2001b: Improving the Effectiveness of U.S. Climate Modeling. National Academy Press, 125 pp.
United Nations, 1997: Kyoto Protocol to the United Nations Framework Convention on Climate Change. Article 3, Annex B. [Available online at www.unfccc.de.]
U.S. Climate Change Science Program, cited 2002: Climate Change Science Program (CCSP) Research Plan. [Available online at www.climatescience.gov/.]