DRAFT

Climate Change Science: Research Imperatives for the Atmospheric and Related Sciences
(Revised 10/2002)

Executive Summary

There is now clear evidence that the average temperature at the Earth's surface, averaged over the entire globe, is increasing. There is also evidence that this warming is due in part to human activities, such as the burning of fossil fuels and tropical deforestation. The degree to which human activities are responsible remains uncertain, but the following points are not in serious dispute among climate scientists:

• The theory of how greenhouse gases directly influence the earth's energy balance 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. Atmospheric concentration of carbon dioxide has increased by more than 30%, methane by more than 150%, and nitrous oxide by 17% since 1750.
  
  
• A doubling of carbon dioxide relative to pre-industrial levels is likely to occur by the middle or late 21st 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 pre-industrial levels, unless some hitherto unknown carbon sink emerges.
  
  
• Because of the long lifetime of carbon dioxide in the atmosphere, centuries will be required after the peak concentration occurs for concentrations to be reduced to their current levels.

While significant progress has been made toward a better understanding of the climate system and toward improved projections of long-term climate change, uncertainty remains in several key aspects, including the magnitude and timing of anticipated changes. These uncertainties can and should be reduced by advances in the following areas:

• Understanding of climate system 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.

The following text elaborates on these areas.

1. Introduction

Human activities have become a major source of environmental change. Of great current interest are the climate consequences of the increasing concentration of greenhouse gases in the atmosphere primarily resulting from the use of fossil fuels and tropical deforestation. These radiatively active gases interact strongly with the earth's thermal radiation, 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 increases in greenhouse gases. For many regions of the world, 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 pose unprecedented challenges to our wisdom and foresight. It is a long-term problem that requires a long-term perspective. Important decisions are required by current and future national and world leaders.

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 be done to address the issues. This difficulty arises because of the long time scale over which the buildup of greenhouse gases has been occurring and the widely differing societal perceptions concerning the potential seriousness of greenhouse warming and related environmental changes. The differing views result, at least in part, from the wide range of uncertainty in the projections of climate change and its impact upon society and the environment. An improved scientific understanding of the effect of increasing concentrations of greenhouse gases in the atmosphere is therefore essential for shaping informed public opinion and developing scientifically informed national and international policy responses to the prospect 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

The scientific community is expected to provide unbiased assessments of scientific information that are essential for scientifically informed policy decisions of government and industry. 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 Intergovernmental Panel on Climate Change (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 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 (1, 2). 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 century, over a variety of greenhouse gas emission scenarios and model runs that were considered, ranged between 1.4°C 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."

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(3). 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, suggest 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.^*

^*It is noted that the Administration is developing a Climate Change Research Initiative, which together with a Climate Change Technology Initiative, is proposed to take the next steps in response to advances in understanding the earth's climate system.

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 and timing 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 (4, 5, 6), the U.S. National Assessment, as well as the IPCC reports. Among other things, the NRC reports have identified three keys to progress: data, computer resources, and organization.

a. Infrastructure Priorities

Observations are the building blocks of climate science and applications. Reliable, long-term 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. The Third Conference of the Parties of the United Nations Framework Convention on Climate Change (1997) concluded that the global capacity to observe the earth's climate system was inadequate and deteriorating worldwide. It is imperative to establish as soon as possible an integrated global climate observing system (GCOS), 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 World Meteorological Organization (WMO) and many individual nations have been under way for several years, but progress has been slow.

Computers are the equivalent of a scientific 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 U.S. continuously upgrade computing capability for climate research and projections.

A well-coordinated, multi-agency research effort is also vital for progress. Climate change research in the U.S. is rich and diverse, and crosscuts the historic missions of various government agencies. Consequently, research initiatives often can be fragmented and poorly coordinated. The need for improved coordination of high priority climate research imperatives within the U.S., 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 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 highest current priority. These can be grouped into several broad and interrelated categories.

i) Develop an improved understanding of climate system interactions and feedbacks

Progress toward developing an improved understanding of the climate system interactions and feedbacks discussed below hinges on a better understanding of the many basic 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 factors that determine the atmospheric concentrations of greenhouse gases and aerosols. 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 aerosols to the atmosphere, most notably sulfate particles and black carbon (soot). Aerosols have short lifetimes and are unevenly distributed. They impact radiation budgets, both directly and indirectly, but their relative importance remains highly uncertain. The greatest uncertainty is their indirect effect on clouds and cloud processes. It is believed that aerosols tend to cause global cooling which may have offset some of the warming due to greenhouse gases.

Climate system feedbacks are fundamental in determining the sensitivity of climate to increasing greenhouse gas concentrations. The term "feedback" refers to natural processes internal to the climate system that can amplify (positive feedback) or damp (negative feedback) the direct response to greenhouse gasses. Estimates of the indirect contribution of climate system feedbacks to global warming indicate they may equal or exceed the direct effect of increasing greenhouse gas concentrations. Water vapor feedback is the most important positive feedback. Water vapor is a natural greenhouse gas which increases with increasing global temperatures. Ice albedo feedback is also an important positive feedback, which results 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.

The full suite of potentially important feedback processes is yet to be adequately understood and quantified. 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 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 time scales are needed to discriminate between the sources of natural climate variability and identify an 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 changes. 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 then depend on the collection and analysis of relevant proxy information, for example, 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 by solar radiation (external) and purely internal modes that are expressions of internal feedback mechanisms. The best known of these is the El Niņo-Southern Oscillation phenomenon, which is a major contributor to interannual variability. A better understanding of the interactions and feedbacks associated with 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 global warming will occur, the crucial details of magnitude, timing, and specific regional responses are still very much in doubt. The uncertainties arise due to model approximations of natural processes and incomplete knowledge and modeling of all factors that can significantly influence future climate. Rapid application of new research findings to model improvement is important to progress. 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. A helpful and non-controversial 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 events, 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 mitigation and adaptive strategies based on the improved predictions.

The assessment of regional environmental and social impacts is a multidisciplinary task that involves both natural and social scientists. 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 US National Assessment report 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 more clearly understood and 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 can rarely be assessed with absolute certainty. The recent NAS report, Climate Change Science: an Analysis of Some Key Questions (5) 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 a 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 affecting climate change to some degree, we have a collective responsibility to develop and undertake carefully considered corrective 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 physical, chemical, biological, and social sciences, in tandem with technological and engineering, proceed side by side in a broad multidisciplinary effort. Broadly acceptable response strategies must rest on a rational scientific and technology foundation. Caution is required in adopting response strategies to assure that its total effect will not create an unacceptable environment and/or societal legacy. Recognizing the increasing ability of engineering to provide powerful solutions, but at the same time recognizing the risk of unintended environmental or social and/or environmental consequences, the National Academy of Engineering is leading an "Earth System Engineering" approach to the problem which brings to bear 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 American Meteorological Society'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.