STOCKHOLM — The world’s top climate scientists on Friday formally embraced an upper limit on greenhouse gases for the first time, establishing a target level at which humanity must stop spewing the gases into the atmosphere or face irreversible and potentially catastrophic climatic changes. They warned that the target was likely to be exceeded in a matter of decades unless steps are taken soon to reduce emissions.
Unveiling the latest U.N. assessment of climate science, the experts cited a litany of changes that are already underway, warned that they are likely to accelerate, and expressed virtual certainty that human activity is the main cause.
“Climate change is the greatest challenge of our time,” said Thomas F. Stocker, co-chairman of the Intergovernmental Panel on Climate Change, the U.N.-sponsored group of scientists that produced the report. “In short, it threatens our planet, our only home.”
The panel, in issuing its most definitive assessment yet of the risks of human-caused warming, hoped to give impetus to international negotiations toward a new climate treaty, which have languished in recent years in a swamp of technical and political disputes. The group made clear that time was not on the planet’s side if emissions continued unchecked.
“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.”
Despite the urgency of the panel’s findings, the course of future international negotiations remains unclear, and heated opposition to established climate science and to strong actions to reduce emissions is certain to continue.
Going well beyond its four previous analyses of the emissions problem, the panel endorsed a “carbon budget” for humanity — a limit on the amount of the primary greenhouse gas, carbon dioxide, that can be produced by industrial activities and the clearing of forests.
To stand the best chance of keeping the planetary warming below an internationally agreed target of 3.6 degrees Fahrenheit (2 degrees Celsius) above the level of preindustrial times, the panel found, no more than 1 trillion metric tons of carbon can be burned and the resulting gas released into the atmosphere.
Just over half that amount has already been emitted since the beginning of the Industrial Revolution. At the rate energy consumption is growing, the trillionth ton will be released somewhere 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.
To keep using fossil fuels beyond the trillionth ton of emissions, companies would have to develop potentially expensive technology to capture and store carbon dioxide from emissions sources like power plants. Such efforts have been lagging; 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 report is a 30-page synopsis of a larger, 900-page report that is to be released next week on the physical science of climate change. That will be followed by additional reports in 2014 on the likely impacts and on possible steps to limit the damage.
The group has now issued five major reports since 1990, each of them finding greater certainty that the world is warming and greater likelihood that human activity is the chief 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 scientists emphasized, however, that those uncertainties cut in both directions and that the only way to limit the risk is to limit emissions.
The group won the Nobel Peace Prize in 2007, along with Al Gore, the former vice president, for highlighting the climate problem. But it came under attack in recent years by climate-skeptic organizations, who assailed the new report as alarmist even before it was published.
The Heartland Institute, a Chicago organization that once compared climate scientists to the Unabomber, issued a document last week saying that any additional global warming would likely be limited to a few tenths of a degree and this “would not represent a climate crisis.”
The new report did not take direct aim at such climate doubters, but it did dismiss some of their favorite theories, including the notion that cosmic rays are warming the planet.
On an issue much cited by the climate doubters, a slowdown in global warming that has occurred over the past 15 years, the report acknowledged that it was not fully understood, but said such pauses had occurred in the past and the natural variability of climate was a likely explanation.
“People think that global warming means every year is going to be warmer than the year before,” said Gerald A. Meehl, an American scientist who helped write the report. “It doesn’t work that way. It’s more like a stair-step kind of thing.”
Climate scientists not involved in writing the new report said that the authors had made a series of cautious choices in their assessment of the scientific evidence.
Regarding sea level rise, for instance, they gave the first firm estimates ever contained in an intergovernmental panel report, declaring that if emissions continued at a rapid pace, the rise by the end of the 21st century could be as much as 3 feet. They threw out a string of published papers suggesting a worst-case rise closer to 5 feet.
Similarly, the authors went out of their way to include a recent batch of papers suggesting that 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 lowered the bottom end of the range of potential warming that could be expected to occur over the long term if the carbon dioxide level in the atmosphere were to double, reversing 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.
In Washington, the White House praised the new report. President Barack Obama’s science adviser, John P. Holdren, cited increased scientific confidence “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.”
Ban Ki-moon, the United Nations secretary-general, spoke to delegates at the meeting Friday by video link, declaring his intention to call a meeting of heads of state in 2014, in a risky attempt to push such a treaty forward. The last such high-level meeting, in Copenhagen in 2009, ended in disarray.
“The heat is on,” Ban declared. “Now we must act.”