The panel said there was no solid scientific understanding of how rapidly the vast stores of ice in Greenland and other polar regions will begin to erode.
PARIS, Feb. 2 — In a bleak and powerful assessment of the future of the planet, the leading international network of climate change scientists has concluded for the first time that global warming is "unequivocal" and that human activity is the main driver, "very likely" causing most of the rise in temperatures since 1950.
They said the world is already committed to centuries of warming, shifting weather patterns and rising seas, resulting from the buildup of gases in the atmosphere that trap heat. But the warming can be substantially blunted by prompt action, the panel of scientists said in a report released here today.
The report summarized the fourth assessment since 1990 by the group, the Intergovernmental Panel on Climate Change of the United Nations, sizing up the causes and consequences of climate change. But it is the first in which the group asserts with near certainty — more than 90 percent confidence — that carbon dioxide and other heat-trapping greenhouse gases from human activities have been the main causes of warming since 1950.
In its last report, in 2001, the panel, consisting of hundreds of scientists and reviewers, put the confidence level at between 66 and 90 percent. Both reports are online at http://www.ipcc.ch.
If carbon dioxide concentrations in the atmosphere reach twice their pre-industrial levels, the report said, the global climate will probably warm by 3.5 to 8 degrees. But there would be more than a 1-in-10 chance of much greater warming, a situation many earth scientists say poses an unacceptable risk.
Many energy and environment experts see such a doubling as a foregone conclusion sometime after midcentury unless there is a prompt and sustained shift away from the 20th-century pattern of unfettered burning of coal and oil, the main sources of carbon dioxide, and an aggressive quest for expanded and improved nonpolluting energy options.
Even an increased level of warming that falls in the middle of the group’s range of projections would likely cause significant stress to ecosystems and alter longstanding climate patterns that shape water supplies and agricultural production, according to many climate experts and biologists.
While the new report projected a modest rise in seas by 2100 — between 7 and 23 inches — it also concluded that seas would continue to rise, and crowded coasts retreat, for at least 1,000 years to come. By comparison, seas rose about 6 to 9 inches in the 20th century.
John P. Holdren, an energy and climate expert at Harvard University, said that the “report powerfully underscores the need for a massive effort to slow the pace of global climatic disruption before intolerable consequences become inevitable.” [Read a report by Mr. Holdren. (PDF format)]
“Since 2001 there has been a torrent of new scientific evidence on the magnitude, human origins and growing impacts of the climatic changes that are underway,” said Mr. Holdren, who is the president of the American Association for the Advancement of Science. “In overwhelming proportions, this evidence has been in the direction of showing faster change, more danger and greater confidence about the dominant role of fossil fuel burning and tropical deforestation in causing the changes that are being observed.”
The conclusions came after a three-year review of hundreds of studies of clues illuminating past climate shifts, observations of retreating ice, warming and rising seas, and other shifts around the planet, and a greatly expanded suite of supercomputer simulations used to test how earth will respond to a building blanket of gases that hold heat in the atmosphere.
The section released today was a 20-page summary for policymakers, which was approved early this morning by teams of officials from more than 100 countries after three days and nights of wrangling over wording with the lead authors, all of whom are scientists.
It described far-flung ramifications for both humans and nature.
“It is very likely that hot extremes, heat waves and heavy precipitation events will continue to become more frequent,” said the summary.
Generally, the scientists said, more precipitation will fall at higher latitudes, which are likely also to see lengthened growing seasons, while semi-arid, subtropical regions already chronically beset by drought could see a further 20-percent drop in rainfall under the midrange scenario for increases in the greenhouse gases.
The summary added a new chemical consequence of the buildup of carbon dioxide to the list of mainly climatic and biological impacts foreseen in its previous reports: a drop in the pH of seawater as oceans absorb billions of tons of carbon dioxide, which forms carbonic acid when partly dissolved. Marine biologists have said that could imperil some kinds of corals and plankton.
The report essentially caps a half-century-long effort to discern whether humans, through the buildup of carbon dioxide and other gases released mainly by burning fuels and forests, could influence the earth’s climate system in potentially momentous ways.
The group operates under the aegis of the United Nations and was chartered in 1988 — a year of record heat, burning forests, and the first big headlines about global warming — to provide regular reviews of climate science to governments to inform policy choices.
Government officials are involved in shaping the summary of each report, but the scientist-authors, who are unpaid, have the final say over the thousands of pages in four underlying technical reports that will be completed and published later this year.
Big questions remain about the speed and extent of some impending changes, both because of uncertainty about future population and pollution trends and the complex interrelationships of the greenhouse emissions, clouds, dusty kinds of pollution, the oceans and earth’s veneer of life, which both emits and soaks up carbon dioxide and other such gases.
But a broad array of scientists, including authors of the report and independent experts, said the latest analysis was the most sobering view yet of a century of transition — after thousands of years of relatively stable climate conditions — to a new norm of continual change.
Should greenhouse gases continue to accumulate in the atmosphere at even a moderate pace, average temperatures by the end of the century could match those last seen 125,000 years ago, in the previous warm spell between ice ages, the report said.
At that time, the panel said, sea levels were 12 to 20 feet higher than they are now. Muych of that extra water is now trapped in the ice sheets of Greenland and Antarctica, which are eroding in some places.
The panel said there was no solid scientific understanding of how rapidly the vast stores of ice in polar regions will melt, so their estimates on new sea levels were based mainly on how much the warmed oceans will expand, and not on contributions from the melting of ice now on land.
Other scientists have recently reported evidence that the glaciers and ice sheets in the Arctic and Antarctic could flow seaward far more quickly than estimated in the past, and they have proposed that the risks to coastal areas could be much more imminent. But the I.P.C.C. is proscribed by its charter from entering into speculation, and so could not include such possible instabilities in its assessment.
Michel Jarraud, the secretary general of the United Nations World Meteorological Organization, said the lack of clarity should offer no one comfort. “The speed with which melting ice sheets are raising sea levels is uncertain, but the report makes clear that sea levels will rise inexorably over the coming centuries,” he said. “It is a question of when and how much, and not if,” he said, adding: “While the conclusions are disturbing, decision makers are now armed with the latest facts and will be better able to respond to these realities.”
Achim Steiner, the executive director of the United Nations Environment Program, which oversees the I.P.C.C. along with the meteorological group, said society now had plenty of information on which to act.
“The implications of global warming over the coming decades for our industrial economy, water supplies, agriculture, biological diversity and even geopolitics are massive,” he said. “This new report should spur policymakers to get off the fence and put strong and effective policies in place to tackle greenhouse gas emissions.”
The warming and other climate shifts will be highly variable around the world, with the Arctic particularly seeing much higher temperatures, said Susan Solomon, the co-leader of the team writing the summary and the section of the I.P.C.C. report on basic science. She is an atmospheric scientist for the National Oceanic and Atmospheric Administration.
The kinds of vulnerabilities are very much dependent on where you are, Dr. Solomon said in a telephone interview. “If you’re living in parts of tropics and they’re getting drier and you’re a farmer there are some very acute issues associated with even small changes in rainfall — changes we’re already seeing are significant,” she said. “If you are an Inuit and you’re seeing your sea ice retreating already that’s affecting your lifestyle and culture.”
The 20-page summary is a sketch of the findings that are most germane to the public and world leaders.
The full I.P.C.C. report, thousands of pages of technical background, will be released in four sections through the year — the first on basic science, then sections on impacts and options for limiting emissions and limiting inevitable harms, and finally a synthesis of all of the findings near year’s end.
In a news conference in Paris, Dr. Solomon declined to provide her own views on how society should respond to the momentous changes projected in the study.
“I honestly believe that it would be a much better service for me to keep my personal opinions separate than what I can actually offer the world as a scientist,” she said. “My stepson, who is 29, has an utterly different view of risks than I do. People are going to have to make their own judgments.”
Some authors of the report said that no one could honestly point to any remaining uncertainties as justification for further delay.
“Policy makers paid us to do good science, and now we have high very scientific confidence in this work — this is real, this is real, this is real,” said Richard B. Alley, one of the lead authors and a professor at Penn State University. “So now act, the ball’s back in your court.”
Elisabeth Rosenthal reported from Paris, and Andrew C. Revkin from New York. Read full post and comments:
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