Climate Change is Greening the Sahel? Not so Fast…
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 that killed more than 100,000 people. The region remains one of the poorest and least developed in the world. It’s also one of the most vulnerable to climate change and variability.
Since the mid-1980s, average rainfall in the Sahel has partially recovered, possibly contributing in places to a ‘re-greening’ of vegetation so widespread that it can be observed from space.
Recently, a paper published in Nature Climate Change argued that climate change from rising greenhouse-gas emissions can explain most of the increase. The paper made global headlines like this one.
IRI climate scientist Alessandra Giannini has serious reservations about the study. “The direct and indirect effects of greenhouse gases cannot be easily separated,” she writes in a response piece in the August print issue of Nature Climate Change.
Giannini argues that the study and its results were based on climate model simulations from one model–a model that is not capable of reproducing the well established historical connection between Sahel rainfall and sea-surface temperatures. This is a significant flaw, says Giannini, who conclusively demonstrated this connection in 2003, and has been trying to unravel the mysteries of a Sahelian climate since.
In 2013, she published a paper which showed 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, the climatic ups and downs of the Sahel during the 20th century could not only be explained, but also tied to both the observed trend of increasing rainfall in the region and to projections of wetter conditions. “That work marked the first unified, sensible explanation of the past, present and future climate of the Sahel,” Giannini says.
Other model-based studies—some more than a decade old—had already shown how the Sahel could become wetter based on the increase in the surface temperature of Africa north of the equator in response to the increase in atmospheric carbon-dioxide concentrations alone. “But we know that the direct influence of CO2 is not the only influence on Sahel rainfall in the real world,” Giannini says. “The influence of CO2 mediated by warming of the global oceans, and the regional response of sea surface temperatures to aerosols also play a role.”
Even the region’s current wet phase is not without its concerns. “Rainfall in the Sahel is anything but consistent nowadays,” Giannini says. “During the wet 1950s and 1960s, the higher rainfall came from more frequent rain events. Today, there’s greater variability from one year to the next and from one season to the next, and the increase is better explained by increased intensity of rain events, rather than by more rainy days,” she says.
The more intense downpours have led to recurrent flooding in recent years, causing loss of life, crops and infrastructure, she says.