Despite any reservations they might have about the new paper, virtually all climate scientists agree with Dr. Hansen’s group that society is not moving fast enough to reduce emissions of greenhouse gases, posing grave risks. 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 publishing climate scientist and, in recent years, as a political activist. He has been arrested at rallies, and he has joined with a group of young people who sued the federal 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.

That stance has made him a hero to college students fighting climate change, but some fellow scientists fear he has opened himself to the charge that he is skewing his scientific research for political purposes.

In 2009, nations agreed to try to limit the planetary warming to 3.6 degrees Fahrenheit, or 2 degrees Celsius, above the preindustrial level. The Earth has already warmed by about half that amount. The climate appears to be destabilizing, 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 Earth warmed naturally, about 120,000 years ago, when the temperature reached a level estimated to have been only slightly higher than today. Large chunks of the polar ice disintegrated then, and scientists have established that the sea level rose 20 to 30 feet.