In the latest report from the Intergovernmental Panel on Climate Change, released Aug. 9, scientists are unequivocal that climate change is bad, that many impacts are already being felt, and that human greenhouse gas emissions are to blame. As more data accumulates from ancient and contemporary sources, and as computer climate models become more refined, scientists’ certainty is rapidly increasing about how different levels of greenhouse gas emissions will shape the future.
For Africa, scientists have high confidence that the continent is already warming faster than the global average, and will see an increase in extreme heatwaves and coastal sea level rise. But one key weather pattern with major economic implications—monsoon rains in the Sahel, the semi-arid band that stretches across the continent and divides the tropics from the Sahara desert—remain an area where the future remains relatively uncertain.
“Monsoons are an area of significant disagreement among climate models,” says Richard Seager, a climate scientist specializing in drought at Columbia University’s Lamont-Doherty Earth Observatory, who was not an IPCC author. “And even if they agree, they may not be right. So that’s still a big concern.”
Why monsoon rain patterns are hard to predict
Monsoons are a seasonal rain pattern produced when shifting wind patterns cause a collision between moist air blowing in from an ocean and air coming over land. In the Sahel, there are two monsoons, one in the east that produces rain in the spring (March to May) and autumn (October to December), and another in the west that produces rain in the summer (June to September). Because they are the chief source of rain in the Sahel, they are vital to the economy: Agriculture accounts for more than one-third of west Africa’s GDP, and across the continent as a whole 95% of farmland is rainfed.
The IPCC reports that in general, in a warming world there should be an overall increase in rainfall across the Sahel. A key driver of that trend is the difference between ocean and land temperatures. The ocean warms more slowly than the land, and the greater that gap, the stronger the monsoons are likely to be, said Francois Engelbrecht, a climate scientist at the University of the Witwatersrand in South Africa and IPCC author.
But the total volume of annual rainfall matters less to farmers and livestock herders than the seasonal distribution: A few catastrophic downpours are far less useful than predictable, periodic rain. Climate models suggest that the timing of monsoon rains will likely shift, Engelbrecht said, but why, and by how much, remains uncertain. One problem is that local rain patterns, compared to other forms of weather, can be highly random, and are subject to a complex tangle of climate influences. Another is that they tend to occur on scales smaller than the resolution of most climate models (typically around 10 square kilometers), which makes them hard to reproduce.
“No one understands exactly how the monsoon functions,” Engelbrecht said. “It’s natural to expect that the west and east African monsoons will become generally stronger, but there’s less information about seasonality than there is about annual rainfall totals.”
And no matter how it’s distributed, Engelbrecht said, more rain may not do much good if temperatures are also much higher.
“I would caution against seeing a rainfall increase as a benefit to agricultural economies in the Sahel,” he said. “Even when it rains more, if it becomes warmer the soil becomes drier. The potential benefits may to a very large extent be offset.”
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