Reduced US air pollution to boost rainfall in Africa’s Sahel

Reduced sulphur dioxide emissions in the US are expected to substantially increase rainfall in Africa’s semi-arid Sahel, according to new research that highlights the wider benefits of clean air policies.

Scientists at Columbia University’s Lamont-Doherty Earth Observatory have made a surprising discovery: if US sulphur dioxide emissions are cut to zero by 2100, rainfall over the Sahel could increase up to 10 per cent from 2000 levels.

Emissions of sulphur dioxide – a toxic gas that contributes to acid rain and premature deaths from respiratory and cardiovascular diseases – have dropped dramatically in the US since pollution filters were placed on coal-fired power plants starting in the 1970s.

“Reducing emissions in one region can influence rainfall far away because our global atmosphere is interconnected,” the study’s lead author, Dan Westervelt, said in a statement. “We show that the health and environmental benefits of US clean air policies extend to global climate as well.”

Sulphur dioxide simultaneously cools and dries earth’s climate by reflecting sunlight back to space and suppressing heat-driven evaporation near the ground. But they also have a clear effect on the tropical rain belt.

According to the study, the rain belt ordinarily shifts north when the northern hemisphere heats up during summer. When sulphur emissions are high, however, cooler temperatures in the north stop the rain belt from migrating as far.

Cutting US emissions to zero was enough to move the rain belt roughly 35 kilometres north, placing more of the Sahel in its path, the researchers found.

“We did not expect to see such a clear, significant influence on the Sahel,” said Westervelt. “This northern shift of the tropical rain belt could mean that cropland at the Sahel’s northern edge could become more productive in the future.”


Image credit: Pablo Tosca/Oxfam

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