Global Warming: All That Hot Air Must Be Having an Effect
NDREW C. REVKIN
January 12, 2003
Political antagonists of President Bush attacked what they described as his lack of sufficient action on global warming last week, but his administration held firm, saying cautious measures were best while the science surrounding climate change and the human factors contributing to it remained uncertain.
Mr. Bush has called for nothing beyond voluntary measures for the time being to slow growth in emissions of heat-trapping greenhouse gases. But Senators John McCain, Republican of Arizona, and Joseph I. Lieberman, the Connecticut Democrat, have joined in proposing legislation that would require mild reductions by 2010 and sharper ones by 2016.
At a Commerce Committee hearing organized last week by Mr. McCain, the administration's critics said they had rising concerns about the dangers of global warming and complained that the administration was hiding behind the need for more study.
Some pointed out that climate change is an issue that will always need more study, particularly to pinpoint the regional impact of changing precipitation or drought patterns, melting ice and rising seas.
They also pointed to the steady march toward certainty in the lengthening list of studies on global warming conducted over the last 15 years.
Those who doubt humans are contributing significantly to global warming often complain that such reports give inadequate voice to skeptics, who say that aggressive efforts to cut emissions are scientifically unfounded and too costly.
Still, many experts on the climate system say that the slow, subtle evolution of the language in the most influential reports reflects an incremental, but significant shift toward certainty - at least on the basic question: are people contributing to global warming?
The Intergovernmental Panel on Climate Change has produced three climate reports since it was formed in 1988 under United Nations auspices, with a fourth analysis just getting under way.
The National Academy of Sciences has addressed various aspects of human-caused climate change in reports over the years, including one prepared at the behest of the White House in 2001.
All of the reports are laced with caveats and discuss unresolved questions, but authors and other scientists say that is standard in such research.
What is most important, they say, is the shift in confidence in the conclusions, which can be traced in the following excerpts from five reports issued between 1990 and 2001.
(I.P.C.C. stands for the Intergovernmental Panel on Climate Change. N.A.S. is the National Academy of Sciences.)
1990, First Assessment Report of the I.P.C.C.
"The unequivocal detection of the enhanced greenhouse effect from observations is not likely for a decade or more."
1992, Policy Implications of Greenhouse Warming: Mitigation, Adaptation and the Science Base (N.A.S.)
"Increases in atmospheric greenhouse gas concentrations probably will be followed by increases in average atmospheric temperature."
1995, Second Assessment Report of the I.P.C.C.
"The balance of evidence suggests a discernible human influence on global climate."
2001, Third Assessment Report of the I.P.C.C
"There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."
2001, Climate Change Science: An Analysis of Some Key Questions (N.A.S.)
"Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability. . . .
"Despite the uncertainties, there is general agreement that the observed warming is real and particularly strong within the past 20 years."
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