Scientists Increase Estimate of Global Warming’s Severity
Global warming may boost world temperatures by up to 11 degrees Fahrenheit by the end of the 21st century, a figure substantially higher than previous estimates, according to a confidential draft report prepared by an influential group of climate scientists sponsored by the United Nations.
Moreover, “there is now stronger evidence for human influence on global climate,” the scientists concluded in their preliminary report, which was distributed to more than 100 governments this week for review.
Several scientists familiar with the new report, prepared by an international group known as the Intergovernment Panel on Climate Change, said its findings significantly strengthen the case for a human role in climate change. Although there is general agreement that the climate is warming, the question of how much of the change is caused by human action has been a major topic of scientific inquiry.
The issue has also figured in the presidential campaign. Vice President Al Gore has frequently asserted that global warming is a major problem on which the government must begin taking action.
Texas Gov. George W. Bush has been more skeptical. “I don’t think we’ve got all the facts,” he said in the second campaign debate earlier this month. “I think it’s an issue that we need to take very seriously, but I don’t think we know the solution to global warming yet.”
In the new draft report, the scientists conclude that it is “likely” that human actions “have contributed substantially” to the observed warming. The major human contribution is the release of so-called greenhouse gases into the atmosphere through the burning of coal, oil, natural gas and other fossil fuels.
That “is a stronger conclusion” than was offered by earlier assessments, said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research in Boulder, Colo. “This is not the work of one individual scientist. This is a consensus reached across scientists in the international community. It has gone through extensive reviews.”
The report is not likely to quiet all debate on the issue, however. Some longtime critics of projections about global warming said that they remain skeptical.
Paradoxically, the report also suggests that some pollution control efforts may unintentionally be making the planet a hotter place.
Greenhouse gases can contribute to global warming by trapping solar heat and preventing it from being reflected back into space.
Mounting evidence suggests that the Earth’s atmosphere has been steadily warming for nearly 150 years as a result of carbon gas produced by burning oil, gas and coal, with the warmest years on record occurring in the last decade.
Overall, the panel’s draft predicts that temperatures worldwide may increase 2.1 to 11 degrees Fahrenheit (1 to 5.8 degrees Celsius). Earlier assessments projected an increase of 2.1 to 7.2 degrees Fahrenheit (1 to 3.5 degrees Celsius).
An 11-degree shift in average temperatures would be a major change in climate. This week, for example, 11 degrees was the difference in average temperatures between Los Angeles and Seattle.
The greater increase is projected in large measure because of efforts to control pollution from industrial facilities and power plants. Pollution-control measures have greatly reduced the amount of sulfate particles that cause acid rain and a variety of health problems. But those particles also have a cooling effect in the atmosphere because they deflect the sun’s heat. As sulfate levels drop, the temperature will effectively rebound.
“These sulfate particulates have had a masking effect,” said one atmosphere expert who has seen the report but who asked not to be identified. “We are cleaning up this air pollution, and that is making global warming worse.”
The new report, which will not be made final until it has been approved early next year, is the first formal update in five years of an assessment prepared by the climate change panel. The panel is a technical group sponsored by the U.N. and the World Meteorological Organization and comprising hundreds of scientists who assess scientific, social and economic aspects of global climate change. The panel does no original research of its own but attempts to arrive at a measured technical assessment of often-conflicting studies on climate change.
The contents of the report were first disclosed Wednesday by the Associated Press. Several scientists familiar with it agreed to discuss the findings with The Times, on condition they not be identified.
With international negotiations now underway to limit the amount of such greenhouse gases that nations may release, almost any attempt to reach a scientific consensus on climate change is controversial. So far, no major industrial nation has ratified an agreement negotiated in Kyoto, Japan, in 1997 to stave off global warming by reducing greenhouse emissions.
The U.N. group’s first report on climate change, released in 1995, generated considerable criticism over allegations that political bias had colored its assessments.
Scientists who worked on the report were at pains to rebut such charges in advance this time. “This is a cold-eyed, objective rendition of the science,” said one such climate scientist. “We could not do it any better.”
To ensure that the new draft is based fairly on the scientific data, “there have been skeptics involved, as authors and reviewers,” Trenberth said.
The report will be the subject of an international meeting next year in China, where the dozens of participating governments will all have the opportunity to review and perhaps temper its conclusions.
“Until that point, the whole report is not considered final,” Trenberth said. The final report, he said, “involves a negotiation between the scientists who determine what can be said and the governments who determine how it can be said.”
