Humans Faulted for Global Warming

By Juliet Eilperin
Washington Post Staff Writer
Saturday, February 3, 2007

An international panel of climate scientists said yesterday that there is an overwhelming probability that human activities are warming the planet at a dangerous rate, with consequences that could soon take decades or centuries to reverse.

The Intergovernmental Panel on Climate Change, made up of hundreds of scientists from 113 countries, said that based on new research over the last six years, it is 90 percent certain that human-generated greenhouse gases account for most of the global rise in temperatures over the past half-century.

Declaring that "warming of the climate system is unequivocal," the authors said in their "Summary for Policymakers" that even in the best-case scenario, temperatures are on track to cross a threshold to an unsustainable level. A rise of more than 3.6 degrees Fahrenheit above pre-industrial levels would cause global effects -- such as massive species extinctions and melting of ice sheets -- that could be irreversible within a human lifetime. Under the most conservative IPCC scenario, the increase will be 4.5 degrees by 2100.

Richard Somerville, a distinguished professor at the Scripps Institution of Oceanography and one of the lead authors, said the world would have to undertake "a really massive reduction in emissions," on the scale of 70 to 80 percent, to avert severe global warming.

The scientists wrote that it is "very likely" that hot days, heat waves and heavy precipitation will become more frequent in the years to come, and "likely" that future tropical hurricanes and typhoons will become more intense. Arctic sea ice will disappear "almost entirely" by the end of the century, they said, and snow cover will contract worldwide.

While the summary did not produce any groundbreaking observations -- it reflects a massive distillation of the peer-reviewed literature through the middle of 2006 -- it represents the definitive international scientific and political consensus on climate science. It provides much more definitive conclusions than the panel's previous report in 2001, which said only that it was "likely" -- meaning between 66 and 90 percent probability on a scale the panel adopted -- that human activity accounted for the warming recorded over the past 50 years.

Some of the report's most compelling sections focused on future climate changes, because the buildup of carbon dioxide in the atmosphere would exert an effect even if industrialized countries stopped emitting greenhouse gases tomorrow. Gerald Meehl, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo., who helped oversee the chapter on climate projections, said that in the next two decades alone, global temperatures will rise by 0.7 degrees Fahrenheit.

"We're committed to a certain amount of warming," said Meehl, who worked with 16 computer-modeling teams from 11 countries. "A lot of these changes continue through the 21st century and become more severe as time goes on."

Meehl added, however, that a sharp cut in greenhouse gas emissions could still keep catastrophic consequences from occurring: "The message is, it does make a difference what we do."

For the first time, IPCC scientists also looked at regional climate shifts in detail, concluding that precipitation in the American Southwest will decline as summer temperatures rise, just as precipitation in the Northeast will increase. Linda Mearns -- another NCAR senior scientist who was also one of the lead authors -- said these changes could cause water shortages and affect recreational activities in the Southwest. Developing countries in Africa and elsewhere could also experience severe droughts.

Governments and scientific organizations across the globe nominate scientists to produce and review the IPCC assessment without pay under the auspices of the United Nations. A group of key authors and government officials met in Paris this week to finalize the document, which reflects three years of work.

"Every government in the world signed off on this document, including the U.S.," said World Bank chief scientist Robert T. Watson, who chaired the last round of deliberations. Watson added that compared with the 2001 report, "the difference is now they have more confidence in what they're doing."

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