Droughts like those that parched Afghanistan and parts of the United States in the last several years are likely to recur in the years to come, a recent study published in the journal Science warns.
Central and southwest Asia went through a crippling three-year drought beginning in the late 1990s. At the same time, rainfall was well below normal in broad areas of the United States, southern Europe and North Africa.
That caught the attention of Martin Hoerling, a climate scientist at the U.S. National Oceanic and Atmospheric Administration. "That was a curiosity to us, because it raised the question whether perhaps these regional droughts may have had a common source or a common influence," he says.
Mr. Hoerling and his colleague, Arun Kumar, thought the common source could be found in the Pacific Ocean. They observed that during the drought years of 1998 to 2002, the waters of the eastern tropical Pacific Ocean were cooler than normal. It's a phenomenon known as La Nińa. At the same time, the western Pacific and the Indian Oceans were warmer than normal.
Mr. Hoerling and Arun Kumar plugged the climate data from 1998 through 2002 into computer models and ran more than 50 different simulations. "What we found was that it was virtually unanimous," he says. "The outcome of it being dry was very strongly determined by the conditions of the ocean."
Warm air rising off a warm ocean pushes around high-altitude winds called the jetstream. The jetstream carries weather patterns around the globe. Mr. Hoerling says the warm western Pacific, together with La Nińa's unusually cool waters to the east, moved the jetstream away from its normal route. He says those two factors joined forces to move rainfall away from central and southwest Asia. "Both ocean conditions were combining to have the same impact, they were additive. The sum was greater than looking at each influence separately," he says.
And Mr. Hoerling's research suggests these droughts could be coming more often. La Nińa comes and goes. There were 16 La Nińa events last century. But global warming is driving up temperatures in the Indian Ocean and western Pacific, which Mr. Hoerling says could mean more droughts in central and southwest Asia.
Climate scientist Matt Barlow, with the research firm Atmospheric and Environmental Research Incorporated, was not involved with Mr. Hoerling's research, but he agrees with its conclusions. "The trend is toward warmer sea surface temperatures in the west Pacific. So we are certainly concerned that if that trend continues, which it may well, that that would lead to a drier climate, basically. So such episodes of drought may be more frequent in the future," he says.
But both Mr. Barlow and Mr. Hoerling agree the exact same drought won't happen every time La Nińa comes around. They say the Earth's atmosphere is much more complicated than that. Mr. Hoerling says that's why they ran their models more than 50 times. "It's like rolling the dice 50 times," he says. "What we found out was that the dice were loaded. However, the same number didn't come out of each roll. The die were loaded to be dry, to be sure. But there could have been a range."
That means the next La Nińa cycle could be less dry, or it could be drier. Or it could affect somewhat different regions. But Mr. Hoerling says the odds are central and southwest Asia, as well as portions of the United States, will face droughts in the years ahead.