STOCKHOLM, Sweden — 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 climatic changes.
They warned that the target is likely to be exceeded in a matter of decades unless steps are taken soon to reduce emissions.
Unveiling the latest United Nations assessment of climate science, the experts cited a litany of changes already under way, 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 Stocker, co-chairman of the Intergovernmental Panel on Climate Change, the U.N.-sponsored group of scientists that produced the report.
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 a swamp of technical and political disputes.
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.
No more than 1 trillion metric tons of carbon could be burned and the resulting gases released into the atmosphere, the panel found, if planetary warming is to be kept below 3.6 degrees Fahrenheit above the level of preindustrial times. That temperature is an internationally agreed upon target, above which scientists believe the most dangerous effects of climate change would begin to occur.
Just over a half-trillion tons have been burned since the beginning of the industrial revolution, and at the rate energy consumption is growing, the trillionth ton will be burned sometime around 2040, according to calculations by Myles Allen, a scientist at the University of Oxford and one of the authors of the new report.