Study may unlocks new climate clues
By Summit Voice
FRISCO — A new study may help forecast drought conditions in the oft famine-stricken and geopolitically crucial Horn of Africa. More than 40 million people in the region often live in exceptional drought conditions, most recently in 2010-2011, when the worst drought in decades triggered a humanitarian crisis.
It’s long been clear that El Niño can affect precipitation in the region, very little is known about the drivers of long-term shifts in rainfall. But the study suggests that temperatures in the Indian Ocean may be the key to understanding precipitation patterns in East Africa.
“The problem is, instrumental records of temperature and rainfall, especially in East Africa, don’t go far enough in time to study climate variability over decades or more, since they are generally limited to the 20th century,” said Woods Hole Oceanographic Institution geologist Jessica Tierney, lead author of the paper published in the journal Nature.
Along with colleagues at Columbia University’s Lamont-Doherty Earth Observatory, Tierney used paleoclimate records to study East African climate change over a span of 700 years.
Since there aren’t a lot of tree rings to study, the scientists had to track indicators of moisture balance, including pollen, water isotopes, charcoal, and evidence for run-off events – measured in lake sediment cores. After crunching the data, the researchers discovered a clear pattern showing the easternmost sector of East Africa was relatively dry in medieval times (from 1300 to 1400 a.d.), wet during the “Little Ice Age” from approximately 1600 to 1800 a.d., and then drier again toward the present time.
Matching those records against modeled climate simulations traced the link between sea surface temperatures and atmospheric convection in the Indian Ocean changes rainfall in East Africa. Specifically, wet conditions in coastal East Africa are associated with cool sea surface temperatures in the eastern Indian Ocean and warm sea surface temperatures in the western Indian Ocean, which cause ascending atmospheric circulation over East Africa and enhanced rainfall.
The opposite situation—cold sea surface temperatures in the western Indian Ocean and warmer in the East—causes drought. Such variations in sea-surface temperatures likely caused the historical fluctuations in rainfall seen in the paleorecord.
The central role of the Indian Ocean in long-term climate change in the region was a surprise.
“While the Indian Ocean has long been thought of as a ‘little brother’ to the Pacific, it is clear that it is in charge when it comes to these decades-long changes in precipitation in East Africa,” says Tierney.
Many questions remain, though.
“We still don’t understand exactly what causes the changes in sea surface temperatures in the Indian Ocean and the relationship between those changes and global changes in climate, like the cooling that occurred during the Little Ice Age or the global warming that is occurring now,” says Tierney. “We’ll need to do some more experiments with climate models to understand that better.”
In the past decade, the easternmost region of Africa has seen a drying trend, yet general circulation climate models predict that the region will become wetter in response to global warming.
“Given the geopolitical significance of the region, it is very important to understand whether drying trends will continue, in which case the models will need to be revised, or if the models will eventually prove correct in their projections of increased precipitation in East Africa,” says co-author Jason Smerdon, of the Lamont-Doherty Earth Observatory.
While it’s currently unclear which theory is correct, the discovery of the importance of the Indian Ocean may help solve the mystery.
“In terms of forecasting long-term patterns in drought and food security, we would recommend that researchers make use of patterns of sea surface temperature changes in the Indian Ocean rather than just looking at the shorter term El Niño events or the Pacific Ocean,” says Tierney.
In addition, Tierney and her colleagues lack paleoclimate data from the region that is most directly affected by the Indian Ocean—the Horn of Africa. The paleoclimate data featured in this study are limited to more equatorial and interior regions of East Africa. With support from National Science Foundation, Tierney and her colleagues are now developing a new record of both aridity and sea surface temperatures from the Gulf of Aden, at a site close to the Horn.
“This will give us the best picture of what’s happened to climate in the Horn, and in fact, it will be the first record of paleoclimate in the Horn that covers the last few millennia in detail. We’re working on those analyses now and should have results in the next year or so,” says Tierney.
This research was based on work supported by the National Science Foundation and the National Oceanic and Atmospheric Administration (NOAA).