Study Links Droughts Across the Globe

Global Warming, La Nina Set the Stage for Dry Conditions

Afghan farmer plows barren land

A farmer plows his barren land in the outskirts of Kabul. Scientists think unprecedented warming in the western Pacific Ocean may be to blame for drought conditions in Afghanistan. Reuters Limited hide caption

itoggle caption Reuters Limited
computer model of average global temperatures from 1998-2002

Computer models suggest that abnormally warm temperatures (in red) in the western Pacific and Indian Oceans, combined with cold La Nina waters (in blue) in the eastern Pacific, may have been the perfect recipe for drought. Courtesy Martin Hoerling/ Science hide caption

itoggle caption Courtesy Martin Hoerling/ Science
Drought dries up parts of the Rio Grande

In Bernardo, N.M., portions of the Rio Grande have nearly dried up, the result of a four-year-long drought in the American Southwest. USGS hide caption

itoggle caption USGS

El Nino and Its Sister, La Nina

El Nino is a disruption of the ocean-atmosphere system in the Pacific Ocean. It occurs when westward-blowing trade winds weaken, diverting the warm waters that usually flow into the western Pacific eastward toward South America.

• The warmer-than-normal waters that result in the tropical Pacific Ocean can have a dramatic impact on global climate.

• Rain follows the warm water eastward, causing drought in Indonesia and Australia and increased rainfall across the southern tier of the United States.

La Nina is the flip side of El Nino. It occurs when cooler-than-normal temperatures invade the tropical Pacific. La Nina's climate impacts tend to be the opposite of El Nino's, causing increased rainfall for some and drought for others.

The drought that gripped much of the United States over the past four years was not an isolated event. Afghanistan was also devastated, receiving just two years’ worth of rainfall over the past four years. The United Nations Environment Program reported this week that the drought has set back attempts to rebuild that nation after years of war and political turmoil. And as NPR's Richard Harris reports, new evidence now suggests global warming is a factor that links the two droughts.

Across the American Southwest and Southeast, farmers and gardeners have been struggling to keep their plants alive over the past four years. The same has been true for people in central Asia and southern Europe, where rainfall has been far below normal. In Afghanistan, sustenance crops have wilted and died, and even drinking water has dried up.

Suspicious that these droughts may be related, Martin Hoerling, a researcher with the National Oceanic and Atmospheric Administration’s Climate Diagnostics Center in Colorado, decided to investigate a possible link.

Because the Pacific and Indian Oceans affect global weather patterns, Hoerling and a colleague examined water-temperature records for these oceans from 1998 to 2002. During that period, they found that cooler waters — a result of the weather phenomenon known as La Nina — prevailed off the American coast.

La Nina is known for its ability to create drought in the United States. But Hoerling found another powerful trend — the waters in the Indian Ocean and the western Pacific were unusually hot, far warmer than the range of normal variation.

Previous studies have linked higher ocean temperatures to global warming. But Hoerling’s research, which appears in Thursday’s issue of the journal Science, goes one step further. He found that the warm oceans, coupled with the effects of La Nina, worked together to trigger the droughts in Afghanistan and Southern Europe, as well as the United States.

Kevin Trenberth of the National Center for Atmospheric Research agrees, adding that climate change is doing more than simply reducing rainfall. Because it dries out the soil, climate change can also make droughts worse and last longer.

For the moment, there's relief in sight for the eastern United States, where soaking rains and snowfall have eased arid conditions. But when considered against the long-term effects of warmed ocean waters, Hoerling says, the precipitation offers only a temporary respite.

And when La Nina returns, as it inevitably will, the scene will once again be set for what Hoerling calls "the perfect ocean for drought."

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