美国大旱与气候变化相关

As one of the worst droughts ever continues to grip major portions of the United States, a new study links this summer’s record-setting dry spell, and other extreme weather events, to the world’s warming climate.

Parched earth

In Texas, the earth is parched. Rivers have dried up, and pasture land has turned brown from the heat. It’s been this way since January 2011. The southwestern state is the fourth largest producer of rice in the United States but the drought could cut production by half.

"Our total agri-income, farm gate value of our commodities here, were usually right around $290 million, and a large percentage of that comes from rice production," Texas agricultural extension agent Peter McGuill says. "You’re talking about a big chunk of money that’s not going to be circulating within the economy.”

James Bradbury, a climate scientist with the World Resources Institute, a global environmental think tank, explains that La Nina, a natural weather pattern that periodically cools the Pacific Ocean, helped trigger the drought by bringing warmer, dryer weather to the American South, which has been hardest hit by the drought.

“Time will tell the extent to which rising temperatures and global climate change contributed to this specific event and the severity of it," Bradbury says. "I think there is a good likelihood that the temperatures that we’re seeing and the heat wave that we’re seeing is all consistent with a warmer world, that that's exacerbating these drought conditions.

Human influence

Peter Stott, who leads the climate monitoring team for the Met Office Hadley Centre, a climate research institution in southwest Britain, says La Nina is only part of the story. He co-authored the American Meteorological Society study which links climate change with the Texas drought and other extreme weather events.

“We did find clear evidence for human influence on the Texas heat wave and also in the very unusual temperatures we had in the United Kingdom in 2011,” he says.

The study finds the 2011 Texas drought was 20 times more likely to occur than in the 1960s as a result of human-induced climate changing emissions in the atmosphere. The heat wave last November in England was 62 times more likely to have occurred than 50 years ago, according to the report.

While not all extreme weather events can be linked to climate change, Stott and his colleagues found evidence that they are more probable in a warmer world.

“What we must remember is that it is the combination of natural variations of climate that is important here," Stott says. "We saw that in La Nina in Texas, but, over and above that, there is this additional climate effect that can and has indeed in the last year led to a greater vulnerability to extreme weather.”

Worsening conditions

Drought continues to parch other parts of the U.S., sparking wildfires and damaging crops in one third of the nation’s counties. U.S. scientists predict that these conditions could even get worse in the coming months, which doesn’t bode well for Iowa farmer Tom Zaputil’s corn crop, which hasn’t had a significant rain since June.

"This here is strictly dryness here," Zaputil says, referring to his crop. "These stalks will cannibalize themselves to pull moisture out of it in order to feed that ear, and these will get brittle and very susceptible to high wind damage later on in the season.”

Stott says the new findings are a wake-up call that the adverse impact of a warming climate can be reduced by acting now to cut carbon-dioxide emissions from cars, factories and buildings.

“So hopefully people can understand the implication of future climate change and relate that to what’s happening at the moment.”

Stott says the study is the first of what he hopes will be annual reports examining the connection between global warming and specific extreme weather events.