Prepared by Michael C. MacCracken, Executive Director, National Assessment Coordination Office

In 1997, the U. S. Global Change Research Program initiated the "National Assessment of the Potential Consequences of Climate Variability and Change for the United States." This national level assessment included analyses of the importance of climate variability and change in twenty regions around the US, in five cross-cutting sectors focused around natural resources and public health, and for the US as a whole.

Climatically, the hundred-year projections that were used indicated that the climatic conditions of the northern US would become like the central US, the conditions of the central US like those across the southern US, and conditions across the southern US like tropical and subtropical regions. The results of the analyses indicated that there will be significant regional variability in both climate change and resultant consequences across the US. Although overall forest productivity was projected to increase for the next several decades as a result of the carbon dioxide fertilization effect, specific natural ecosystems were projected to be at significant risk because of the very limited potential for their movement and reestablishment.

Although the US has abundant water resources, widespread concerns about water resources arose in every region. As a whole, US agricultural production was projected to increase, although those farming marginal lands are likely to be impacted by further reductions in their economic competitiveness. The extensive coastal regions in the US were projected to be exposed to rising sea levels, increasing the risk to extensive coastal infrastructure, barrier islands, and coastal wetlands. Rising temperatures and the accompanying rise in the absolute humidity, were projected to lead to much more uncomfortable summer weather conditions across the southern and eastern US; analyses indicated in general that health consequences could be minimized by taking a wide array of public health measures.

As the issue of climate change gained attention in the 1980s as a result of scientific advances and early international conferences, the US Congress passed the Global Change Research Act of 1990. This public law recognized the early scientific findings that human activities were starting to change the global climate, asserting that:

• Industrial, agricultural, and other human activities, coupled with an expanding world population, are contributing to processes of global change that may significantly alter the Earth habitat within a few generations
• Such human-induced changes, in conjunction with natural fluctuations, may lead to significant global warming and thus alter world climate patterns and increase global sea levels. Over the next century, these consequences could adversely affect world agricultural and marine production, coastal habitability, biological diversity, human health, and global economic and social well-being.

To address these issues, the US Congress established the U.S. Global Change Research Program (USGCRP) and instructed US federal research agencies to cooperate in developing and coordinating a "comprehensive and integrated United States research program which will assist the Nation and the world to understand, assess, predict, and respond to human-induced and natural process of global change." The USGCRP has led to coordination of the global-change related research efforts of ten federal agencies, including the departments of Agriculture, Commerce (specifically the National Oceanic and Atmospheric Administration-NOAA), Defense, Energy, Health and Human Services, and the Interior along with the agency-based research programs in the Environmental Protection Agency (EPA), National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), and the Smithsonian Institution. While interagency coordination has led to a number of organizational challenges, each agency contributes particular strengths needed to address the very broad issues raised by concerns about global change.

One section of the Congressionally approved Act indicated that the USGCRP: shall prepare and submit to the President and the Congress an assessment which

• Integrates, evaluates, and interprets the findings of the Program and discusses the scientific uncertainties associated with such findings
• Analyzes the effects of global change on the natural environment, agriculture, energy production and use, land and water resources, transportation, human health and welfare, human social systems, and biological diversity
• Analyzes current trends in global change, both human-induced and natural, and projects major trends for the subsequent 25 to 100 years

Fulfilling this mandate for the issue of potential climate change has taken several forms. At the international level, the USGCRP coordinates US participation in the assessments of the Intergovernmental Panel on Climate Change (IPCC). The IPCC assessments in 1990, 1995, and 2000 have, at the global and continental scales, carefully reviewed the scientific literature concerning climate change, its potential consequences, and options for mitigation. With the IPCC finding in 1995 that human activities were having a "discernible influence" on the global climate, the USGCRP undertook a national-level assessment intended to provide detail about potential consequences within the US.

Given this charge, the overall goal of the National Assessment was to analyze and evaluate what is known about the potential consequences of climate variability and change for the US in the context of other pressures on the public, the environment, and the nation's resources. At the same time, the assessment was to be designed to initiate an active dialogue with those who would be affected by the potential consequences in order to help prepare them to more effectively deal with climate variability and change and to sharpen the research focus concerning coping options. Thus was born the National Assessment of the Potential Consequences of Climate Variability and Change for the United States, commonly called simply the National Assessment.

The US National Assessment had three major components:

There were a number of unique organizational approaches used in the conduct of the Assessment. First, responsibility for the National Assessment process was widely distributed and broadly inclusive, with each of the regional, sectoral, and synthesis activities being comprised of experts from universities, government agencies, public and private sector organizations, and from other stakeholder communities. Second, the assessment process was financially supported in a shared manner by the set of USGCRP agencies, with each agency assuming responsibilities for which they had particular interests. Third, the assessment is viewed by the agencies as the start of a long-term engagement bringing together the scientific community and those affected by climate change. Together, these steps have been designed to promote broad understanding of the issue and its importance and to encourage early recognition of the need to start adapting to climate change.

Conduct of the Assessment focused on addressing issues of importance to people in particular places (the regions) and with particular interests (the sectors). Starting with the broad array of public concerns about the environment, the Assessment explored the degree to which existing and future variations and changes in climate might affect issues that people really care about. A short list of questions guided the process:

• What are the current environmental stresses and issues that form the backdrop for potential additional impacts of climate change?
• How might climate variability and change exacerbate or ameliorate existing problems? What new problems and issues might arise?
• What are the priority research and information needs that can better prepare the public and policy makers for reaching informed decisions related to climate variability and change?
• What coping options exist that can build resilience to current environmental stresses, and also possibly lessen the impacts of climate change?

In analyzing the potential consequences, the National Assessment teams utilized three approaches to generate plausible future climatic conditions. Historic data sets were assembled and used to evaluate the potential consequences of a reoccurrence of past variations in the climate, recognizing that there would also be an underlying warming trend. Computer-generated scenarios from the Canadian and Hadley (U.K.) climate modeling centers were used to provide a self-consistent set of plausible future conditions. These model scenarios spanned the range from warm/moist to hot/dry conditions that are typical of the wider set of climate models. Should these conditions occur, the US would experience about a 5 to 10F (about 3 to 6C) warming and modest to strong precipitation increases across the US.

Such changes would be equivalent to shifting warm climatic zones northward such that, over the 21st century, the future climate of the central US would become like the current climate of the southern US. The third approach, only used in a few instances in this initial assessment phase, was to consider the amount of change that would significantly increase existing vulnerability by, for example, crossing a threshold or triggering a non-linearity. In addition to the various approaches to treating prospective climate variability and change, ecosystem models were used to generate prospective changes in vegetation cover, and socio-economic scenarios were generated to consider possible changes in population and economic activity.

The various regional and sectoral studies provided a diverse picture of potential consequences, with a mixture of beneficial and detrimental consequences. While potential adaptation measures were identified that could moderate adverse consequences for many societal activities, disruptions were found to be likely to affect the complex distribution of ecosystems across the US, with some types of rare ecosystems (e.g., alpine meadows, coral reefs) being particularly impacted because there was little potential for natural or assisted adaptation. Because of the likelihood of intensified evaporation and changes in the amounts and duration of mountain snowpack, water resources arose as an issue in regions across the US, from the dry regions of the southwest to the more abundant regions of the Pacific Northwest.

Timber and agricultural production were found to be likely to increase due to CO2 fertilization, but, should warming occur near the upper bounds of estimates, increased water stress and potential disruptive threats (e.g., fires for forests, pests for agriculture) could create significant vulnerability. Coastal and permafrost regions were found to be likely to suffer increasing stress, creating problems for buildings and other infrastructure. While the warmer, moister conditions would raise the heat index, reducing the quality of life, the health threat from such conditions is currently being reduced through increased reliance on air-conditioning. More generally, sustaining the health of the US population was found to require intensified attention to public health practices and community structure (e.g., building codes).

Quite clearly, many important questions and issues remain to be addressed. Given the initial nature and limited scope of the assessment, it was also recognized that uncertainties, including possible surprises, are also likely. For this reason, it is expected that assessment activities will become an ongoing part of the USGCRP.