The United States experienced some of the most extreme weather s in its history this spring, including deadly outbreaks of tornadoes(龙卷风), near- record flooding, drought and wildfires.
Damages from these (62) have already passed $ 32 billion, and the hurricane season, which is just beginning, is (63) to be above average, according to the National Oceanic and Atmospheric Administration.
Government scientists said Wednesday that the () of extreme weather has increased over the past two decades, (65) as a result of global warming caused by the (66) of carbon dioxide in the atmosphere.
But they were careful not to (67) humans for this year’s rash of (68) s, saying that in some ways weather (69) were returning to those seen at the beginning of the last century.
"Looking at long-term patterns since 1980, indeed, (70) climatic and meteorological (气象的) s have increased," said Thomas R. Karl, director of NOAA’s National Climatic Data Center. (71) a new NOAA report on 2011 extreme weather, Dr. Karl said that extremes of precipitation (降雨量)have increased as the planet warms and more water evaporates from the oceans. He also said models suggest that as carbon dioxide (72) in the atmosphere and heats the planet, droughts will increase in frequency and (73)
"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 (74) to extreme climate s.
Kevin Trenberth, a (75) senior scientist, said that when the greenhouse effect caused by burning fossil fuels is added to the natural (76) of climate, weather disasters can be (77) to occur more frequently.
"Global warming is contributing to the (78) incidence of extreme weather because the environment in which all storms (79) has changed from human activities," Dr. Trenberth said in a telephone interview Wednesday. "Records are not just (80) , they are smashed. It is as clear a warning as we are going to get about (81) for the future.\