The devastating string of tornadoes, droughts, wildfires and floods that hit the United States this spring marks 2011 as one of the most extreme years on record, according to a new federal analysis.

Just shy of the halfway mark, 2011 has seen eight $1-billion-plus disasters, with total damages from wild weather at more than $32 billion, according to the National Oceanic and Atmospheric Administration. Agency officials said that total could grow significantly, since they expect this year's North Atlantic hurricane season, which began June 1, will be an active one.

Overall, NOAA experts said extreme weather events have grown more frequent in the United States since 1980. Part of that shift is due to climate change, said Tom Karl, director of the agency's National Climatic Data Center.

"Extremes of precipitation are generally increasing because the planet is actually warming and more water is evaporating from the oceans," he said. "This extra water vapor in the atmosphere then enables rain and snow events to become more extensive and intense than they might otherwise be."

But for some kinds of extreme weather, teasing out a contribution from climate change is more difficult.

The second half of April brought a swarm of tornadoes that leveled parts of the Midwest, including the twister that killed 151 people in Joplin, Mo. So far, 2011 has seen the sixth-highest number of tornado deaths on record, prompting many people to wonder whether climate change has played a role. So far, scientists say there's no good evidence for or against a climate change influence on tornado behavior.

Meanwhile, computer models predict that droughts -- like those that have scorched large swaths of Texas, New Mexico and Arizona this year -- will become stronger and more frequent as climate change continues. But because patterns of drought vary widely from decade to decade, that makes it "very difficult and unlikely that we're going to be able to discern a human fingerprint, if there is one, on the drought record in the foreseeable future," Karl said.

'Sometimes Mother Nature just blasts us'

Katherine Hayhoe, a climate scientist at Texas Tech University, used a medical analogy to explain how climate change could affect extreme weather.

"If you get a really bad bug, it could be that you were really run down," she said. "The bug is what is making you sick, but the background conditions -- being run down -- make you more susceptible to that virus. We have to acknowledge that we are changing the background conditions."

Warmer temperatures provide more energy and water in the atmosphere to feed storms, she said, noting evidence that heavy precipitation events are becoming more frequent in some parts of the globe.

But not everyone is convinced. Bill Patzert, a climate scientist at NASA's Jet Propulsion Laboratory, says he believes that climate change is real, but "it's too simple an answer to say there is more moisture in the atmosphere, so storms are more violent."

"Sometimes we have a quiet year, and sometimes Mother Nature just blasts us," he said.

More obvious influences on this year's wild weather, experts said, were La Niña and an unusual blast of cold Arctic air that reached as far south as the central United States last winter.

The droughts and associated wildfires seen across the southern United States this year, as well as increased rain and snow in the northern half of the country, are characteristic side effects of La Niña, a global weather pattern scientists recognize by the telltale pattern of cooling the equatorial Pacific Ocean. The most recent La Niña, which ended late last month, was a strong one, said Michelle L'Heureux, a climate scientist who leads the El Niño-La Niña forecasting team at NOAA's Climate Prediction Center.

"The dry seemed to be particularly dry, and the wet seemed to be particularly wet," she said.

La Niña and other events make climate influence unclear

But the unusually wet December in southern California and heavy snowpack in the northern United States are also signs of Arctic air that dipped south as another weather pattern, the North Atlantic Oscillation, hit a negative phase, experts said. That sent cold air and winter storms farther south than normal until the weather pattern started fading away in mid-January.

"The effects of La Niña were sort of muddled together with the effects of the North Atlantic Oscillation," said Ed O'Lenic, chief of the operations branch at NOAA's Climate Prediction Center. "It's very difficult to separate the two."

Further muddling the picture, some research suggests that the unusual behavior of the North Atlantic Oscillation this year and during the winter of 2009-2010 may be a consequence of declining sea ice in the Arctic.

But it's not clear whether climate change has affected or will affect the behavior of La Niña and her counterpart, El Niño. "It's possible that [the El Niño cycle] impacts could be aided and abetted by climate," L'Heureux said, who said the ambiguity can be just as frustrating for climate scientists as it is for the public trying to make sense of unusual weather.

"At this point, it's too difficult to make that real-time attribution," she said. "That's really what our field struggles at -- people want this information on demand. They want to know now what's causing this extreme rainfall event."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500