WASHINGTON - Some of the nation's leading climate scientists have told the White House that greenhouse gas emissions will continue to contribute to global warming through the end of this century, but that much uncertainty remains in their field.
The report from a committee at the National Research Council comes as the Bush administration prepares to walk away from a 1997 treaty mandating cuts in emissions of carbon dioxide and other pollutants from power plants, vehicles and other sources in favor of a plan built around voluntary reductions.
The White House had asked for the review of science's current understanding of climate change from the council, part of the National Academy of Sciences, as part of a review of global warming policy.
"We know that greenhouse gases are accumulating in Earth's atmosphere, causing surface temperatures to rise," said Ralph Cicerone, a professor of earth system science and chancellor of the University of California-Irvine and chairman of the committee, after briefing administration officials Wednesday.
"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," Cicerone said.
Scientists generally agree that temperatures at Earth's surface rose by about 1 degree Fahrenheit during the 20th century. Most readings show 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 both negative and positive effects for many plant and animal species.
"The changes observed over the past several decades are likely mostly due to human activities," the committee reported, "but we cannot rule out that some significant part of these changes are also a reflection of natural variability."
The panel said that based on assumptions that greenhouse-gas emissions will accelerate and conservative assumptions about how the climate will react, computer models suggest that average global surface temperatures will rise between 2.5 and 10.4 degrees Fahrenheit by the end of this century.
But the committee also noted that while this range of predictions offered by the Intergovernmental Panel on Climate Change reflects current thinking of the scientific community, the summaries the international body has been giving to policymakers have placed less emphasis on scientific uncertainties and caveats.
Scientific temperature records have only been kept around most of the world for the last 150 years or so, leaving researchers to rely on information obtained from natural sources such as ice cores drilled from Antarctica and Greenland, tree rings and other long-enduring markers.
While those records show there have been big swings in the planet's temperatures over the past 400,000 years (up to 3.6 degrees per 1,000 years when the last ice age was ending), they also show that both carbon dioxide and methane are more abundant in the atmosphere today than at any other time covered by the records.
Uncertainties in climate change predictions can only be reduced by devoting more resources to basic research, as well as supporting construction of better climate models driven by supercomputers and ground and satellite systems for observing global climate, the scientists said.
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