The catastrophic drought in East Africa results from cooler waters on the other side of the globe. La Niña conditions in the tropical Pacific dry up the rain in East Africa, said an international team of researchers in a recent issue of Science.
The study found East African rains have been influenced by the warming and cooling of the Pacific for at least the past 20,000 years. Sediments on the bottom of Lake Challa, a crater lake in the foothills of Mount Kilimanjaro, helped the researchers map out East Africa's weather history.
"During La Niña, rainfall is sparse and the winds over Lake Challa are strong. The winds enhance upwelling of nutrients, intensifying the seasonal blooms of algae. After dying and sinking, they form thick layers of light-colored sediments. During El Niño events, on the other hand, rainfall is frequent and the winds are weak, resulting in thinner white layers in the sediment," said author Christian Wolff of the University of Potsdam in Germany in a press release.
The researchers cross-checked the sediment layers with the past 150 years of weather records for the Pacific. The banding pattern matched up with recorded La Niña/El Niño conditions. They then used the bands to work out a record of rainfall going back to the last ice age.
"Surface ocean temperatures play an important role in driving hydroclimate change in this vulnerable region," said co-author Gerald Haug, professor at the ETH (Swiss Federal Institute of Technology) in Zurich, Switzerland.
La Niña is part of a larger climate phenomenon known as the El Niño-Southern Oscillation. El Niño conditions mean a warmer Pacific. But from June 2010 to May 2011, the Pacific has been chilling out with the cooler waters of La Niña.
The drying influence of the latest La Niña over East Africa prolonged what was already a severe drought. Now two years of crop failures have left the region in a state of famine, as the United Nations recently declared.
Climate change is adding further complexity to the situation.
Rising warmer global temperature averages have corresponded with wetter rainy seasons in East Africa in October and November. But having more moisture in the warmer atmosphere doesn't help plant growth if strong La Niña winds keep periodically blowing the moisture elsewhere.
A cool period from 18,000 to 21,000 years ago correlated with drier, but more stable conditions in East Africa. So although the rainy seasons may have been drier, they were dependable.
"Even though rainfall did not vary much during that period, the sediment layers still reflect the beat of El Niño and La Niña cycles," said co-author Axel Timmermann, of the University of Hawaii at Manoa. "Compared with this coldest time, the last 3,000 years have been wetter but more variable, with severe century-long droughts sprinkled throughout."
The increased variability has made it hard for farmers, herders and wildlife to prepare for the next year.
"Will these projected changes affect East Africa's unique biodiversity in its national parks, such as the Serengeti?" asks Timmermann. "We do not yet know, but there are fascinating links to explore further."
IMAGE 1: Oxfam distributing water in the Horn of Africa, which in the period 2010-2011 experienced a severe drought. (Wikimedia Commons)
IMAGE 2: A diagram of the La Niña condition in the Pacific. Warm water is further west than usual. (Wikimedia Commons)