STOCKHOLM — For the first time, the world's top climate scientists on Friday formally embraced an upper limit on greenhouse gases while warning that it is likely to be exceeded within decades if emissions continue at a brisk pace, underscoring the profound challenge humanity faces in bringing global warming under control.
A panel of experts appointed by the United Nations, unveiling its latest assessment of climate research, reinforced its earlier conclusions that global warming is real, that it is caused primarily if not exclusively by human emissions, and that it is likely to get substantially worse unless efforts to limit those emissions are rapidly accelerated.
"Human influence has been detected in warming of the atmosphere and the ocean, in changes in the global water cycle, in reductions in snow and ice, in global mean sea level rise, and in changes in some climate extremes," the report said. "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."
Going well beyond its four previous analyses of the emissions problem, the Intergovernmental Panel on Climate Change endorsed a "carbon budget" for humanity — an upper limit on the amount of the primary greenhouse gas, carbon dioxide, that can be emitted from industrial activities and forest destruction.
To stand the best chance of keeping the planetary warming below an internationally agreed target of 3.6 degrees Fahrenheit above preindustrial levels and thus avoiding the most dangerous effects of climate change, the panel found, only about 1 trillion tons of carbon can be burned and the resulting gas spewed into the atmosphere.
Just over half that amount has already been emitted since the beginning of the Industrial Revolution, and at current rates of energy consumption, the trillionth ton will be released around 2040, according to calculations by Myles R. Allen, a scientist at the University of Oxford and one of the authors of the new report. More than 3 trillion tons of carbon are still left in the ground as fossil fuels.
Limiting the warming to the agreed-upon target "is technically doable, but at the moment we're not going in the right direction," Dr. Allen said in an interview. "I don't think we'll do it unless we bite the bullet and start talking about what we're going to do with that extra carbon that we can't afford to dump into the atmosphere."
To keep using fossil fuels beyond the trillionth ton of emissions, companies would have to develop potentially expensive technology to capture carbon dioxide from emissions sources like power plants and store it underground. Such efforts have been lagging badly; only last week, Norway scaled back one of the most ambitious such projects because of soaring costs.
But a considerable body of research suggests that in principle it could be done, and in the United States, the Obama administration is moving toward rules that would essentially require utilities to develop the technology if they want to keep burning coal to produce electricity. In response, the president's Republican opponents have accused him of waging a "war on coal."
The new report from the intergovernmental panel was released on Friday, after an all-night editing session that followed a week of discussion behind closed doors in Stockholm. The substantive points did not change greatly from a draft that was leaked in August, though the new version was extensively revised for clarity.
Since 1990, the intergovernmental panel has been the primary scientific body advising the world's governments about the risks of global warming. Every five or six years, hundreds of scientists on the committee assess thousands of published papers about climate change, giving their view of which are most likely to be accurate.
The group has now issued five major reports, each of them finding greater certainty that the world is warming and greater likelihood that human activity is the principal cause. The new report finds a 95 to 100 percent chance that most of the warming of recent decades is human-caused, up from the 90 to 100 percent chance cited in the last report, in 2007.
But the new document also acknowledges that climate science still contains huge uncertainties, including the likely magnitude of the warming for a given level of emissions, the rate at which the ocean will rise, and the likelihood that plants and animals will be driven to extinction.
The group won the Nobel Peace Prize in 2007, along with Al Gore, the former U.S. vice president, for highlighting the climate problem. But it came under attack in recent years by climate-skeptic organizations, who began to assail to the new report as alarmist even before it was published. The Heartland Institute, a Chicago organization that once compared climate scientists to the Unabomber, claimed in a report it released last week that any additional global warming would likely be limited to a few tenths of a degree and this "would not represent a climate crisis."
Climate scientists not involved in writing the new report said that, in reality, the authors had made a series of conservative choices in their assessment of the scientific evidence. Regarding sea level, for instance, they gave the first firm estimates ever contained in an intergovernmental panel report, declaring that if emissions continue at a runaway pace, the rise by the end of the 21st century could be as much as three feet. They threw out a string of published papers suggesting a worst-case rise closer to five feet.
Similarly, the authors went out of their way to include a recent batch of papers suggesting the earth might be somewhat less sensitive to carbon dioxide emissions than previously thought, even though serious questions have been raised about the validity of those estimates.
The new report lowers the bottom end of the range of potential warming that could be expected to occur if the carbon dioxide level in the atmosphere were to double, overturning a decision the panel made in the last report and restoring a scientific consensus that had prevailed from 1979 to 2007. Six years ago, that range was reported as 3.6 to 8.1 degrees Fahrenheit; the new range is 2.7 to 8.1 degrees.
"The I.P.C.C. is far from alarmist — on the contrary, it is a highly conservative organization," said Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany, whose papers on sea level were among those that got discarded. "That is not a problem as long as the users of the I.P.C.C. reports are well aware of this. The conservatism is built into its consensus structure, which tends to produce a lowest common denominator on which a large number of scientists can agree."
In Washington, the White House praised the new report. President Obama's science adviser, John P. Holdren, said in a statement that it "conveys scientists' strengthened confidence in projections that the kinds of harm already being experienced from climate change will continue to worsen unless and until comprehensive and vigorous action to reduce emissions is undertaken worldwide."
The United States was for many decades the world's largest emitter of greenhouse gases, though it was surpassed a few years ago by China. It refused to agree to a treaty called the Kyoto Protocol that sought to limit global emissions, greatly weakening that effort, and efforts to pass a comprehensive climate policy for the United States failed soon after Mr. Obama took office.
But he has recently embraced the issue anew, declaring his intention to use executive authority under the Clean Air Act to limit emissions. Those steps in the United States, as well as rising ambition in many other countries, have led to renewed hopes for an ambitious global climate treaty in 2015.
Addressing the delegates in Stockholm by video link, the United Nations secretary general, Ban Ki-moon, said he would call heads of state to a worldwide meeting next September to try to create momentum for a global agreement.
"The heat is on," Mr. Ban declared. "Now we must act."
"Continuing rapid emissions now is kicking the climate can down the road, leaving climate change for our children and grandchildren," said Christopher B. Field, an American scientist heading a subgroup of the intergovernmental panel that will issue a report next year on climate impacts. "But it is kicking a can that gets to be bigger, heavier and harder to move with each kick."
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