A Wetter Sahel, But will it Last?

The Sahel is a semiarid region south of the Sahara Desert that stretches from the Atlantic Ocean to the Red Sea. In the 1970s and 1980s it was hit by a series of persistent droughts and recurring famines, epitomized by the 1984 famine in Ethiopia. The Sahel remains one of the poorest and least developed regions in the world. It's also one of the most vulnerable to climate change and variability. One bright spot for the region is that since the mid-1980s, average rainfall has increased steadily [see this animation]. People engaged in sustainable land management techniques such as agroforestry have been able to take advantage of it, rebuilding their livelihoods.

Scientists investigated many theories about the cause of the droughts in the years and decades since their inception. Initially they blamed it on land degradation from decades of poor land management made worse by rapid population growth. But ten years ago, Alessandra Giannini, a scientist at the International Research Institute for Climate and Society conclusively showed that changes in global tropical sea-surface temperature were are all needed to explain the shifting rainfall patterns in the Sahel.

Since then, Giannini and others have been trying to understand what specifically about the oceans and their behavior could lead to droughts in one decade, and rainfall in the next.

Giannini's latest study, published in Environmental Research Letters, makes a strong case that the answer lies in the North Atlantic--more specifically, how it behaves compared to tropical oceans elsewhere. She and her colleagues analyzed climate model simulations of the 20th century as well as projections for the 21st century. They discovered that by simply looking at what North Atlantic sea-surface temperatures were doing relative to those in the rest of the world's tropical oceans, they could explain not only the Sahel's climatic ups and downs of the 20th century but also tie in to the observed trend of increasing rainfall in the region, and to projections of wetter conditions.

"For the first time, we have a unified, sensible explanation of the past, present and future climate of the Sahel," Giannini says. She explains her results in this video abstract.

Bucking the Trend

In the 20th century, while the global tropical oceans grew consistently warmer because of greenhouse gas emissions, the North Atlantic changed little and in some areas even became cooler.

"Why? Because sulfate aerosols--essentially pollution from fossil fuel burning in North America and Europe--stopped the sun's radiation from reaching the surface there," Giannini says. "Normally, the North Atlantic will feed moist air in monsoon flow into the African continent, which is ultimately carried across the Sahel and gives it its rainfall."

From the 1960s through the 1980s, Giannini and colleagues hypothesize that the aerosols over the North Atlantic ocean significantly reduced evaporation, cutting off the supply of moisture to the Sahel and throwing the region into a state of persistent drought.

Since the passing of the U.S. Clean Air Act and other environmental legislation in the U.S. and Western Europe, aerosol concentrations have steadily decreased. As a result, the North Atlantic has been warming up, actually outpacing the warming occurring in the global tropical oceans. With its moisture supply back online, the Sahel's average annual rainfall has crept back up.

"The North Atlantic makes or breaks the deep convection that brings rain over the Sahel," Giannini says.

"The mechanism proposed by Dr. Giannini should be further evaluated in both models and observations, but already the connection of rainfall to a warmer Atlantic should allow us to better gauge the reliability of future projections and to understand the sources of current uncertainty in our models," says Michela Biasutti, a climate scientist at the Lamont-Doherty Earth Observatory who studies the Sahel and was not involved in the study.

'Seeing' Climate Change

Climate change scenarios for the region indicate that average annual rainfall will increase throughout this century if the North Atlantic continues to warm.

Giannini's coauthors, Seyni Salack and Tiganadaba Lodoun, Ph.D. students at universities in Dakar and Ouagadougou, respectively, found that these projections were remarkably consistent with current rainfall trends. They looked at actual rainfall measurements across West Africa over the last 60 years and found that the increased average rainfall is caused by fewer, more intense bursts rather than more rainy days.

'Not only has rainfall been increasing, but interestingly, the increase is better explained by increased intensity of rain events, rather than by more rainy days,' says Giannini.

This is markedly different than the wetter Sahel of the 1940s and 1950s, before the droughts. Back then, the higher rainfall came from more frequent rain events. But it is consistent with the general expectation from climate change that a warmer, moister atmosphere may lead to more intense downpours.

The increase in rainfall doesn't necessarily bring only good news for the people of the region, however. The more intense downpours have led to recurrent flooding in recent years, causing loss of life, crops and infrastructure.

This is why it's critical we understand the nature of climate variability and change in the Sahel, and help develop sound adaptation policies, so that people aren't pinned between drought-induced famine one year and losing their homes to floods the next.

In addition to this scientific research, in part funded by the National Science Foundation, IRI has a number of ongoing training and data-sharing activities in the Sahel that are helping the region build its capacity to better predict changes in rainfall and temperatures over the short, medium and long term. This work is being funded through a number of partners, including the U.S. Agency for International Development, the CGIAR Research Program on Climate Change, Agriculture and Food Security, and the National Oceanic and Atmospheric Administration.