Climate Scientists Advise White Rouse on Global Warming.

In a report requested by the Bush administration, a committee of the National Research Council (NRC) sums up the current understanding science has of global climate change. "Climate Change Science: An Analysis of Some Key Questions" characterizes the global warming trend over the last 100 years and examines what may be in store for the 21st century, as well as the extent to which warming may be attributable to human activity. The committee--made up of 11 of the nation's top climate scientists, including seven members of the National Academy of Sciences, one of whom is a Nobel-Prize winner--also emphasized that much more systematic research is needed to reduce current uncertainties in climate-change science.

"We know that greenhouse gases are accumulating in Earth's atmosphere, causing surface temperatures to rise," said committee chair Ralph Cicerone, chancellor of the University of California at Irvine. "We don't know precisely how much of this rise to date is from human activities, but based on physical principles and highly sophisticated computer models, we expect the warming to continue because of greenhouse gas emissions."

Based on assumptions that emissions of greenhouse gases will accelerate and conservative assumptions about how the climate will react to that, computer models suggest that average global surface temperatures will rise between 2.5 and 10.4[degrees]F (1.4 and 5.8[degrees]C) by the end of this century.

With respect to the basic question of whether climate change is occurring, the report notes that temperatures at the Earth's surface rose by about 1[degrees]F (about 0.6[degrees]C) during the 20th century. This warming process has intensified in the past 20 years, accompanied by retreating glaciers, thinning arctic ice, rising sea levels, lengthening of the growing season in many areas, and earlier arrival of migratory birds.

The committee said the conclusion of the Intergovernmental Panel on Climate Change (IPCC) that the global warming that has occurred in the last 50 years is likely the result of increases in greenhouse gases accurately reflects the current thinking of the scientific community. It cautioned, however, that uncertainties about this conclusion remain; the sources of uncertainty are the natural variability inherent in the climate on time scales that range from decades to centuries, questions about the ability of models to simulate natural variability on such long time scales, and questions about the degree of confidence that can be placed in estimates of temperatures going back thousands of years and based on evidence from tree rings or ice cores.

The greenhouse gas of most concern is carbon dioxide, since the naturally occurring chemical also is generated by the continued burning of fossil fuels, can last in the atmosphere for centuries, and "forces" more climate change than any other greenhouse gas. Other significant greenhouse gases include methane, nitrous oxide, water vapor, tropospheric ozone, and chlorofluorocarbons (CFCs), which together have a "forcing" effect on climate change approximately equal to that of carbon dioxide. Man-made sources of methane, nitrous oxide, and ozone have resulted in substantially increased concentrations in the atmosphere in the 20th century, although each of these gases also has natural sources. CFCs are entirely synthetic compounds.

The best information about past climate variability comes from ice cores drilled miles deep in Antarctica and Greenland, which reveal that temperatures have changed substantially over the past 400,000 years. Although most of these changes occurred over thousands of years, some rapid warmings took place over periods of decades.

The ice cores also trapped carbon dioxide and methane, showing that those gases were present in the atmosphere at their lowest levels during cold eras and at higher levels during warm eras. Carbon dioxide did not rise much above 280 parts per million by volume (ppmv) until the industrial revolution. By the end of the 20th century, it had reached 370 ppmv, with an average increase in the last two decades of 1.5 ppmv per year. Both carbon dioxide and methane are more abundant in the atmosphere now than at any time during the 400,000-year ice core record.

The committee noted that IPCC has examined a range of scenarios concerning future greenhouse gas emissions. Such scenarios are valuable, according to the committee, because they provide a warning of the magnitude of climate change that may occur if emission rates continue to climb at a rate similar to that of the last century. Alternative scenarios are needed, though, to illustrate the sensitivity to underlying assumptions, particularly with regard to future technological development and energy policy.

The committee also was asked by the White House to examine whether there were any substantive differences between the IPCC reports and their abridged technical and policy-maker summaries. The IPCC was established by the United Nations and the World Meteorological Organization in 1988, and its reports and summaries have been influential in international negotiations related to the Kyoto protocol.

The report finds that the full IPCC Working Group 1 report does an admirable job of reflecting research activities in climate science and is adequately summarized in the technical summary. The corresponding summary for policy makers, it adds, places less emphasis on the scientific uncertainties and caveats. Looking to the future, the committee suggested that improvements to the IPCC process may need to be made to ensure the best scientific representation possible and to keep the process from being seen as too heavily influenced by governments that "have specific postures with regard to treaties, emissions controls, and other policy instruments."

To reduce some of the uncertainties inherent in current climate change predictions, a strong commitment must be made to basic research as well as to improving climate models and building a global climate-observing system, the committee said. More comprehensive measurements of greenhouse gases and increased computational power also will be needed.

Although potential impacts from global warming were looked at in the report, it was not part of the committee's charge to make policy recommendations for dealing with them. The White House requested this fast-track review of the state of climate science in preparation for international discussions on global warming. "In view of the critical nature of this issue, we agreed to undertake this study and to use our own funds to support it," said Bruce Alberts, president of the National Academy of Sciences and NRC chair. The study was completed in a month.

Copies of "Climate Change Science: An Analysis of Some Key Questions" are available from the National Academy Press by phone at [phone] or [phone], or from the Internet at [less than]http://www.nap.edu[greater than].