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But they were careful not to blame humans for this year’s rash of deadly events, saying that in some ways weather patterns were returning to those seen at the beginning of the last century.

“Looking at long-term patterns since 1980, indeed, extreme climatological and meteorological events have increased,” said Thomas R. Karl, director of NOAA’s National Climatic Data Center. “But in the early part of the 20th century, there was also a tendency for more extreme events followed by a quiet couple of decades.”

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.

So far this year, there have been nearly 1,400 preliminary tornado reports nationwide; those reports will most likely be whittled down to about 900 confirmed tornadoes, the second-highest annual total recorded in modern times. The record is 1,011 confirmed tornadoes in 2008.

The year also is on track to be one of the deadliest, with 536 fatalities so far from tornadoes, placing 2011 in sixth place in United States history and the deadliest since 1936, NOAA reported.