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 warned on Tuesday that permitting a warming of that magnitude would actually be quite 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.