Ask farmers about rain, and they will tell you that the timing matters more than how much falls in a given year. In Eastern Africa, the months of the “long rains” or “belgs” are between March and June.
A new study in the journal Climate Dynamics suggests that these months will be much drier in the future in Kenya, Ethiopia and other East African nations because of climate change. Some 17.5 million people in the Horn of Africa already face food insecurity in the region, with stagnating agricultural production, population growth and recent drought.
The study contradicts predictions by the Intergovernmental Panel on Climate Change (IPCC) that climate change would lead to more precipitation in the region.
Scientists from the U.S. Geological Survey and the University of California, Santa Barbara, studied the flow of air currents over the Indian Ocean.
Over the past 60 years the Indian Ocean has warmed rapidly due to greenhouse gas emissions from human activity. The warmer Ocean heats up the air above, causing it to rise until it hits a cooler patch in the atmosphere. At that point, the hot air condenses and falls as rain (the process of convection). Unfortunately, the rain falls in middle of the tropical Indian Ocean and not over land. This region is part of a global atmospheric current called the Walker circulation.
The air, which has now lost its moisture, flows westward and descends over eastern Africa. The winds bear little rain.
Scientists say that between 1980 and 2009, oceanic heating has reduced precipitation over eastern Africa during the vital “long-rains” season.
“While there appear to be many factors that govern interannual variability in east African long-rains precipitation, convective activity during [the March to June season] has steadily declined in eastern Africa for the past 30 years as the convective branch of the Walker circulation has become more active over the Indian Ocean,” the paper states.
The authors say the IPCC report got it wrong because of the many unknowns that govern rainfall. Long rains have many inputs that are not well understood.
Conventional modeling suggests that the tropical Walker circulation will become weaker due to climate change, resulting in more rainfall in eastern Africa. In the IPCC report, 18 out of 21 models predicted greater precipitation in the region. But recent studies, including this one, argue for a strengthening of the Walker circulation. This study uses observational data to show that the Walker circulation has extended westward, which makes precipitation more likely over the Indian Ocean and droughts the norm in eastern Africa.
“Nonetheless, the projections of increased rainfall in eastern tropical Africa are taken seriously by food aid agencies, and hundreds of millions of dollars in development aid for agriculture, environmental conservation, and water resource planning are predicated on such uncertain IPCC projections,” states the paper. “If the climate continues to tilt toward an intensified Walker circulation, or a westward extension of its western branch, rainfall should continue to decrease in the most food insecure region of the world.”