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Friday, February 2, 2007
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International Scientific Panel on Climate Change Is 90% Sure That Human Actions Have Warmed the Planet
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International scientific panel on climate change is 90% sure that human actions have warmed the planet
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International scientific panel on climate change is 90% sure that human actions have warmed the planet
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In the most exhaustive study of the earth's climate to date, an international panel representing more than 1,000 scientists this morning issued its strongest indictment yet of humanity's role in global warming, calling the evidence "unequivocal."
There is greater than a 90-percent chance that greenhouse-gas pollution has caused much of the warming recorded over the past 50 years, and the pace at which temperatures rise is "very likely" to accelerate in coming decades, warned a report issued by the Intergovernmental Panel on Climate Change, which is better known as the IPCC.
"Warming of the climate system is now unequivocal," said Susan Solomon, a co-chair of the panel's climate-science group, at a news conference in Paris. "That's evident in observations of air and ocean temperatures, melting of snow and ice, and rising global mean sea level."
The report, which took three years to produce, was written by 600 climate scientists from more than 40 countries and was reviewed by some 620 other scientists. This week, the scientific leaders of the panel and representatives of 113 governments met in Paris to agree on the final wording of the report's summary, which was released today.
Achim Steiner, executive director of the United Nations Environment Program, which established the IPCC, said the report's release "will perhaps one day be remembered as the day when the question mark was removed behind the debate about whether climate change had anything to do with human activity on this planet."
"This report represents the most rigorous and comprehensive scientific assessment of global as well as regional aspects of climate change," said Michel Jarraud, secretary general of the World Meteorological Organization, another United Nations agency that established the IPCC. "It has considerably narrowed the uncertainties of the previous reports."
More Extreme Weather on the Horizon
When the panel issued its last major assessment, in 2001, it concluded that there was a 66-percent chance that greenhouse-gas pollution had caused much of the climate warming in recent decades. The panel increased its confidence in making the association because global temperatures have continued to rise since the last report and scientists have had access to a much larger set of computerized climate simulations using more-sophisticated models designed to mimic the ocean, atmosphere, and ice sheets.
The projections by those models suggest broadly that regions closer to the North Pole will warm faster than other parts of the globe, that precipitation will increase in temperate regions and decrease in the tropics, and that weather extremes will strike more often. Some models project that the Arctic Ocean will lose its summertime cover of ice by the latter half of this century, according to the panel.
At the news conference, Gerald Meehl, one of the report's authors, said that by the end of the 21st century, "you'll see a lot more extremes: more intense, longer-lasting heat waves, a lot heavier precipitation, drought increases in a lot of regions."
"Tropical cyclones are projected to get more intense in a lot of areas," he said, referring to hurricanes and similar storms. "We'll see what we've already seen at the end of the 21st century, but everything more severe."
The panel said that, even in the unlikely event that nations immediately cut their emissions of greenhouse gases back to the levels released in 2000, global warming would continue for several decades. In more realistic scenarios of increasing pollution, temperatures are expected to rise by almost four-tenths of a degree Fahrenheit per decade over the next few decades. By the end of the century, the report projects, average global temperatures will climb from 2 to 11.5 degrees Fahrenheit.
The panel's full report, which will amount to more than 1,000 pages when it is released later this year, "must be the biggest, most peer-reviewed document in the history of the science," said Gavin A. Schmidt, a climate modeler at NASA's Goddard Institute for Space Studies in New York City and a researcher at Columbia University.
Are Projections Too Conservative?
But even participants in the process charged that the consensus report is inherently conservative and its conclusions are out of date by the time they appear. The estimates of future sea-level rise, in particular, drew criticism from some researchers who reviewed early drafts of the report.
"The bottom line is that a lot of people who have looked at the most recent data feel like the IPCC projections are too conservative," said R. Steven Nerem, a professor of aerospace-engineering sciences at the University of Colorado at Boulder, who studies changes in sea level.
Michael MacCracken, chief scientist for climate change at the Climate Institute in Washington, D.C., sent a letter to leaders of the IPCC last week warning that the draft he had reviewed did not sufficiently explain the likely magnitude of how much oceans would swell in the future. As the former head of the federal government's office in charge of climate research, Mr. MacCracken oversaw a review of the IPCC's 2001 assessment.
The current assessment projects that warming temperatures will cause ocean water to expand and glacial ice to melt, raising sea levels from 7 to 23 inches by the year 2100. The IPCC report six years ago gave a much broader range of 3 to 35 inches.
But the estimates do not include the potential acceleration in melting from Greenland and Antarctica because the processes controlling their ice sheets are too poorly known. Nonetheless, recent changes have caused many glaciologists and sea-level specialists to predict a faster rate of sea-level rise. Even as the IPCC report was going through its long review process, scientists have reported in the past year that melting along the flanks of Greenland has increased markedly.
"A lot of people in my field say that a meter of sea-level rise by 2100 is probably closer to being on target," said Mr. Nerem.
Assessing Accuracy of Past Projections
Coincidentally, several leading climate researchers published a paper in today's issue of Science comparing the previous IPCC projections with what actually has happened. Stefan Rahmstorf, of Germany's Potsdam Institute for Climate Impact Research, and his colleagues reported that the IPCC had underestimated the changes over the past 16 years. The IPCC uses different scenarios to make its projections, depending on various rates of economic growth, gauges of pollution emissions, and estimates of how sensitive the climate is to greenhouse gases. When Mr. Rahmstorf's team analyzed the rates at which the real globe's temperature and sea levels had risen, it found that the trends matched the upper boundary of the IPCC estimates.
Andrew J. Weaver, a professor of climate dynamics at the University of Victoria, in British Columbia, called the Science paper "a clever piece." But he said it was unclear how to extrapolate those findings into the future. "It underscores the conservative nature of the IPCC process," he said. Still, Mr. Weaver said he agreed generally with the panel's sea-level projections, and estimated that the actual sea-level rise over the next century would be close to the IPCC's upper limit.
The panel was able to make more-definitive statements this year than it did six years ago because the changes observed around the globe are all consistent and much more apparent. "It's so striking," Mr. Weaver said. "This has got to be the most studied phenomenon in the history of the planet."
Established in 1988 under the United Nations, the IPCC has issued three previous major assessments of the earth's climate. Each new edition has grown more definitive, as the evidence of climate change has strengthened and scientific understanding of key processes has improved. The first assessment, released in 1990, helped spur nations two years later to adopt the world's first climate treaty, the Framework Convention on Climate Change, which bound countries in a general way to stabilize "greenhouse-gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system."
The second assessment by the IPCC, issued in 1995, provided the scientific basis for countries in 1997 to adopt the Kyoto Protocol, which committed developed nations to legally binding emissions targets. The United States signed the treaty but never ratified it. In its third assessment, in 2001, the IPCC proclaimed that "there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."
Previous IPCC reports have drawn criticism from scientists and politicians who question the evidence that human beings are causing significant changes on the planet. Last year, the Republican-controlled Congress investigated the IPCC process and denounced some of its findings. The National Academy of Sciences and other scientific leaders in the United States, however, have come out in support of the IPCC's past conclusions.
Background articles from The Chronicle:
- Climate Science on Trial (9/8/2006)
- House Panel Heatedly Debates Research Behind Global-Warming Theory (8/4/2006)
- A Climate for Not Much Change (3/31/2006)
- Warming Climate Has Greenland's Glaciers on the Run (2/24/2006)
- Hurricanes Have Grown More Intense Since 1970, Researchers Find (9/23/2005)
- Hot Air and Cold Fear (5/23/2003)
- The Storm at the Center of Climate Science (11/10/2000)
- The Great Melt: Is It Normal, or the Result of Global Warming? (7/14/2000)