Africa > Africa’s Sahel Could See Sharp Increase In Rainfall

Africa: Africa’s Sahel Could See Sharp Increase In Rainfall

2017/07/08

Climate change could turn one of Africa’s driest regions into a very wet one by suddenly switching on a Monsoon circulation.

For the initial time, scientists find evidence in computer simulations for a possible abrupt change to heavy seasonal rainfall in the Sahel, a region that so far has been characterized by extreme dryness. They detect a self-amplifying mechanism which may kick-in beyond 1.5-2 degrees Celsius of world warming – which happens to be the limit for world temperature rise set in the Paris Climate Agreement. Although crossing this new tipping point is potentially beneficial, the change could be so large, it would be a major adaptation challenge for an by presently troubled region.

“Additional rain in a dry region can be good news,” said lead-author Jacob Schewe from the Potsdam Institute for Climate Impact Research (PIK). “Climate change due to greenhouse gases from burning fossil fuels really has the power to shake things up. It is driving risks for crop yields in a lot of regions and generally increases dangerous weather extremes around the globe, from presently on in the dry Sahel there seems to be a luck that further warming may indeed enhance water availability for farming and grazing.”

Co-author Anders Levermann from PIK and LDEO of New York’s Columbia University added: “We don’t know what the impacts on the ground will be, this is beyond the scope of our study; but imagine the luck of a greening Sahel. Still, the sheer size of the possible change is mindboggling – this is one of the very few elements in the Earth system that we may witness tipping any minute at this time. Once the temperature approaches the threshold, the rainfall regime could shift within just a few years.”

Regions like the central parts of Mali, Niger, and Chad – which are practically part of the Sahara desert – could receive as much rainfall as is today registered in central Nigeria or northern Cameroon which boast a richly vegetated tropical climate.

A new tipping element in the climate system

Dozens of cutting-edge climate computer simulation systems indicate, on average, a weak wet trend for the Sahel under unabated climate change, so it is well known that there’ll likely be some additional rain in the region in a warming world. The scientists presently took a closer look at those simulations that show the greatest increase, plus 40 to plus 300 % additional rain, while others show only a mild increase or even slight decreases. They find that in these wet simulations, as the surrounding oceans warm, Sahel rainfall increases suddenly and substantially.

During the same time the monsoon winds that blow from the Atlantic ocean to the continental interior get stronger and extend northwards. This is reminiscent of periods in earth’s history during which, according to paleoclimatic findings, African and Asian monsoon systems alternated between wet and dry, sometimes completely abruptly.

The scientists before identified a self-amplifying mechanism behind the sudden rainfall changes. At the same time as the ocean surface temperature increases, additional water is evaporated. The moist air drifts onto land, where the water is released. At the same time as water vapor turns into rain, heat gets released. This increases the temperature difference between the generally cooler ocean and the warmer landmasses, sucking additional moist winds into the continent’s interior. This again will produce additional rain, and so on.

“Temperatures have to rise beyond a certain point to start this process,” explained Schewe. “We find that the threshold for this ‘Sahel monsoon’ is remarkably similar across different models. It seems to be a robust finding.”

Huge adaptation challenge for an by presently troubled region

“The enormous change that we may see would clearly pose a huge adaptation challenge to the Sahel,” said Levermann. “From Mauritania and Mali in the West to Sudan and Eritrea in the East, additional than 100 million people are potentially affected that by presently at this time are confronted with a multifold of instabilities, inclunding war. Particularly in the transition period between the dry climatic conditions of today and the conceivably much wetter conditions at the end of our century, the Sahel may experience years of hard-to-handle variability between drought and flood. Obviously, agriculture and infrastructure will have to meet this challenge. As great as it hopefully were for the dry Sahel to have so much additional rain,” concluded Levermann, “the dimension of the change calls for urgent attention.”

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