East Africa suffers regularly at the hands of faraway climatic events such as the warm El Niño or the cool La Niña phases of the El Niño-Southern Oscillation (ENSO). Now, scientists know that the waxing and waning of floods and droughts in the region at the hands of ENSO has been a regular feature dating back 20,000 years.
The catastrophic drought currently plaguing huge regions of Kenya, Ethiopia, Djibouti and Somalia is a result of the La Niña conditions that prevailed from June 2010 to May 2011 in the Pacific Ocean.
The study, published in the August 5 issue of the journal Science, is authored by a group of scientists from Germany, Switzerland, the U.S., the Netherlands, and Belgium. Together, they took samples from Lake Chala, a crater lake in the foothills of Mount Kilimanjaro. What they found was proof that ENSO has been affecting the region’s weather for thousands of years.
Pairs of light and dark coloured striations in the mud at the bottom of the lake ranging between 0.08 and 7 mm in thickness are the result of sedimentary upwelling in the lake. Comparing the thickness of these striations to the tropical Pacific temperatures over the last 150 years match them up to ENSO behaviour.
“During La Niña, rainfall is sparse and the winds over Lake Chala are strong,” explains author Christian Wolff from the University of Potsdam in Germany. “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.”
The thickness and colour of the layers allowed the team to reconstruct East African rainfall back to the Last Ice Age, and show just how dramatically the region’s climate has changed over the 21,000 years. During the coldest period of time, some 18,000 to 21,000 years ago, the climate was dry ad relatively stable.
“Even though rainfall did not vary much during that period, the sediment layers still reflect the beat of El Niño and La Niña cycles,” says co-author Axel Timmermann, Professor at the University of Hawaiʻi at Mānoa’s International Pacific Research Center and School of Ocean and Earth Science and Technology. “Compared with this coldest time, the last 3,000 years have been wetter, but more variable with severe century-long droughts sprinkled throughout.”
These results back up the growing consensus of data that suggests the current global warming trend will affect large-scale rainfall patterns, insomuch as warming will affect the ENSO cycle which in turn will affect region’s like Eastern Africa.
Additionally, the results are consistent with computer models simulating the response to increasing greenhouse gas concentrations.
The climate is likely to warm, which will allow the atmosphere to hold more moisture, and cause the rainy season to become wetter. However it will also increase the variability of the region.
“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.”