Presenting a new NOAA report on 2011 extreme weather, Dr. Karl said that extremes of precipitation have increased as the planet warms and more water evaporates from the oceans. He also said models suggest that as carbon dioxide builds up in the atmosphere and heats the planet, droughts will increase in frequency and intensity.

“But it is difficult and unlikely to discern a human fingerprint, if there is one, on the drought record of the United States,” he said.

Some other climate scientists were more categorical about the human contribution to extreme climate events.

Kevin Trenberth, distinguished senior scientist at the National Center for Atmospheric Research, a university-sponsored institute in Boulder, Colo., said that when the greenhouse effect caused by burning fossil fuels is added to the natural variability of climate, weather disasters can be expected to occur more frequently.