‘The rate of the recent drying in the Horn of Africa is unprecedented in the last 2,000 years …’
The Horn of Africa, encompassing Somalia, Djibouti and Ethiopia, has been drying in sync with increases in global greenhouse gas emissions and could experience for frequent and intense droughts as global warming continues, scientists said after studying layers of sediments from the Gulf of Aden.
The detailed core samples enabled the scientists to create a highly accurate temperature record going back 2,000 years. The Horn of Africa has suffered deadly droughts every few years in recent decades and the study findings suggest the region will get progressively less rain during the crucial “long rains” season of March, April and May.
“What we see in the paleoclimate record from the last 2,000 years is evidence that the Horn of Africa is drier when there are warm conditions on Earth, and wetter when it is colder,” said lead author Jessica Tierney, a University of Arizona associate professor of geosciences. “The rate of the recent drying in the Horn of Africa is unprecedented in the last 2,000 years,” Tierney said.
The team found their reconstructions of local temperature and aridity were in step with an independent reconstruction of Northern Hemisphere temperatures stretching back to the year A.D. 1. All the data point to the Horn of Africa becoming warmer and drier in the last 100 years.
The sediment layers within the core are so well-preserved that researchers can peer back in time decade by decade. Previous research also suggests that the Sahara, which once bloomed with regular rainfall, suddenly dried out about 5,000 years ago in just a century or two, and not more gradually, as many researchers had assumed. The finding provides evidence that climate shifts can happen suddenly.
The team’s report, “Past and future rainfall in the Horn of Africa,” was scheduled for online publication October 9 in the journal Science Advances. Co-author Caroline Ummenhofer is at Woods Hole Oceanographic Institution in Massachusetts. The National Science Foundation funded the research.
To figure out the Horn of Africa’s 2,000-year history of temperature and aridity, the researchers analyzed the chemical contents of the core layer by layer. The chemical compounds that indicate past temperature and aridity came from particular organisms that change their chemistry depending on their environment.
Past temperatures were inferred by analyzing chemicals left in the sediment by single-celled marine organisms known as archaea. The archaea alter the chemical composition of their cell membranes depending on the water temperature.
To track past levels of aridity, Tierney and her colleagues analyzed fatty acids from the leaf wax of terrestrial plants. Because leaf litter and soil are blown into or wash into the nearby Gulf of Aden, the wax ends up in the sediment.
When the climate is drier, the fatty acids in the leaf wax have a higher proportion of a heavy form of hydrogen known as deuterium. Although the Horn of Africa had experienced a wet period during the Little Ice Age (A.D. 1450-1850), the researchers found an increasing shift toward heavy hydrogen in the last century, indicating the climate was drying.
Global-scale models used to predict future changes under global warming suggest the region will become wetter, primarily during the “short rains” season from September to November. But the new study by Tierney and her colleagues suggests those gains may be offset by declining rainfall during the March-to-May “long rains” season, which is particularly important for the region’s agriculture.
“It is getting drier right now in the 20th century and we expect it to continue to get dry,” Tierney said. “If we can simulate rainfall in these arid tropical and subtropical regions better, we can understand the future impact of climate change.”