NEW YORK • The nations of the world agreed years ago to try to limit global warming to a level they hoped would prove somewhat tolerable. But leading climate scientists have warned that permitting a warming of that magnitude would actually be highly dangerous.
The likely consequences would include killer storms stronger than any in modern times, the disintegration of large parts of the polar ice sheets, and a rise of the sea sufficient to begin drowning the world's coastal cities before the end of this century, the scientists declared.
"We're in danger of handing young people a situation that's out of their control," said Dr James Hansen, the retired Nasa climate scientist who led the new research. The findings were released on Tuesday by European science journal Atmospheric Chemistry and Physics.
The basic claim of the paper is that by burning fossil fuels at a prodigious pace and pouring heat-trapping gases into the atmosphere, humanity is about to provoke an abrupt climate shift.
The authors believe that water pouring into the oceans from melting land ice will set off a feedback loop that will cause parts of the great ice sheets in Greenland and Antarctica to disintegrate rapidly.
But some scientists are unconvinced by the specific assertions the authors are making.
"Some of the claims in this paper are indeed extraordinary," said Dr Michael Mann, a climate scientist at Pennsylvania State University. "They conflict with the mainstream understanding of climate change to the point where the standard of proof is quite high."
However, virtually all climate scientists agree that society is not moving fast enough to reduce emissions of greenhouse gases. An agreement reached late last year in Paris seeks to cut emissions, but it is not remotely ambitious enough to limit global warming to the degree Dr Hansen regards as necessary.
Among Dr Hansen's colleagues, some of the discomfiture about the new paper stems from his dual roles as a climate scientist and a political activist. He has been arrested at rallies and has joined a group of young people who sued the US government over what they said was its failure to limit global warming.
Dr Hansen argues that society is in such grave peril that he feels morally compelled to go beyond the normal role played by a scientist and to sound a clear warning.
In 2009, nations agreed to try to limit planetary warming to 2 deg C, above the pre-industrial level. The earth has already warmed by about half that amount. The climate appears to be destabilising, virtually all land ice on the planet has started to melt, and the oceans are rising at an accelerating pace.
The paper, written by Dr Hansen and 18 other authors, dwells on the last time the earth warmed naturally, about 120,000 years ago, when the temperature reached a level only slightly higher than today. Large chunks of the polar ice disintegrated then, and scientists have established that the sea level rose 20 feet to 30 feet (6m to 9m).
Climate scientists agree that humanity is about to cause an equal or greater rise in sea level, but they have tended to assume that such a large increase would take centuries, at least. The new paper argues that it could happen far more rapidly, with the worst case being several feet of sea-level rise over the next 50 years, followed by increases so precipitous that they would force humanity to beat a hasty retreat from the coasts.
"That would mean loss of all coastal cities, most of the world's large cities and all their history," Dr Hansen said in a video statement that accompanied the paper.
NEW YORK TIMES