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Dallas, Texas – The U.S. National Assessment of climate change won’t provide policymakers or the public with useful information because it relies on computer climate models that are incapable of making accurate regional predictions of global warming, according to a study released today by the George C. Marshall Institute.

The study’s author, Dr. David Legates, who is Associate Professor of Climatology in the Center for Climatic Research at the University of Delaware, noted that “these models, which are intended to describe climate only on a very large scale, are currently used by the National Assessment to describe possible scenarios of regional climate change in the U.S.”

“Because current models cannot accurately represent the existing climate without manipulation,” Legates noted, “they are unlikely to render reliable global climate scenarios or provide useful forecasts of future climate changes in regions of the United States as small as the Midwest, West or South.”

The study explains how General Circulation Models (GCMs) work and why the attempt to forecast complex climate factors such as atmospheric changes, the interaction of land, sea, and air, and the role of clouds in climate is so difficult. The strengths and weaknesses of climate models are discussed and the report shows how GCMs are used to answer important questions about global warming.

The two climate models used in the U.S. National Assessment are then described with reference to their similarities and differences. The limitations of these models — the Canadian Global Coupled Model and the Hadley Climate Model from Great Britain — are outlined with emphasis on their inability to provide useful regional scenarios of climate change.

The Marshall report concludes with an analysis of how well these two models reproduce the present-day climate as a benchmark for their ability to predict future climate.

Key findings in the report include:

— Our incomplete understanding of the climate system and our inability to represent this imperfect understanding mathematically limit the usefulness of the current GCMs.

— The two models have significant problems in representing the land surface: for example, the western United States is represented simply as one large slope beginning at the Pacific Ocean and descending into the Great Plains, so that the role of the coastal plain, the most densely populated area of the western states, is ignored.

— It is common practice to “tune” GCMs to make them represent current conditions more accurately, but the very fact that the GCMs need this manipulation casts serious doubt on their ability to predict future conditions. Because all factors are interconnected in climate modeling, an error in one field will adversely affect the simulation of every other variable.

— To reduce complexity and computational time, GCMs treat surfaces as uniform and average the flows of moisture and energy between the land surface and the atmosphere over large areas. But the extensive variability of the land surface and the effects that even small-scale changes can have make modeling land-surface interactions quite difficult.

The National Assessment itself recognized that both models that it selected provide a more extreme climate change scenario than other models that were available and that had been developed in the U.S. Both models offer incomplete modeling of the effects of individual greenhouse gasses, including water vapor and atmospheric sulfates. The CGCM1 in particular fails to model sea ice dynamics and offers a simplistic treatment of land-surface hydrology.

Predicted temperature increases over various regions of the United States differ considerably between the two models; these predictions fail to correspond with observed precipitation variability and contradict each other. In general, the Hadley model simulation is closer to the observed climate in the United States than the Canadian simulation, although both models produced considerable differences from observations. This, again, casts serious doubt on the models’ ability to simulate future climate change. Given these uncertainties, the report concludes, using the available GCMs to assess the potential for climate change in specific regions is not likely to yield valid and consistent results.

GCMs can provide possible scenarios for climate change, but at the present level of sophistication, they are not reliable enough to be used as the basis for public policy. Using GCMs to make predictions about local climate change in the United States is deeply flawed, and can only lead to deeply flawed policies.

The U.S. National Assessment of the Potential Consequences of Climate Variability and Change for the Nation intends to “provide a detailed understanding of the consequences of climate change for the nation.” The Assessment’s findings, called the National Assessment Synthesis Report, will be published in the near future.

The George C. Marshall Institute is a not-for-profit science and public policy research group located in Washington, D.C. Detailed information on the Institute can be obtained on its web site ( http://marshall.org ) or by calling 202-296-9655.

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