The Great Green Wall of the Sahel and its unexpected effects on the climate

The Great Wall of China. Everyone knows her. But the Great Green Wall. Have you heard of it ? This is how a project launched in 2007 was baptized. The idea: to plant millions oftrees by 2030 in the semi-arid zone of the north of the Sahel, on a strip of land 8,000 kilometers long. With the objective of greening the region and restoring land to prevent the Sahara from spreading south. At present, the project is only about 15% complete. But $ 20 billion was recently pledged internationally to push it forward.

Store carbon, restore the biodiversity and bringing socio-economic benefits to the populations of this part of Africa are among the positive side effects expected from this Great Green Wall. But like all the projects of climate geoengineering, it could also have some negative effects, in the region or beyond. Possible negative impacts which have not, for the time being, been sufficiently studied, estimate researchers from the University of Quebec (Canada).

To remedy this, they worked on computer simulations to high resolution. These suggest that the effects of the Great Green Wall on the weather local, regional and global could be deeper than those previously considered. So in a context of anthropogenic global warming Strong, the researchers’ work shows a rather welcome reduction of 1.5 ° C in average summer temperatures across much of the Sahel. But regions that are already warmer should become even warmer. With average temperatures increasing by 1.5 ° C.

A disrupted climate on a large scale

Precipitation would also increase sharply. Even to double in certain areas. In question, an intensification and a spreading of the monsoon West African. Let us recall that until then, scientists saw in the orbital cycles of our Earth, the main cause of variation of this monsoon. With the consequence, for example, a few millennia ago, during a period between 11,000 and 5,000 years ago, a Sahara much greener than today. But researchers from the University of Quebec finally show that the vegetation cover of the region also plays a major role.

Their simulations indeed suggest that with more vegetation in the region, a pool of humidity will be created. Because the water cycles will go on increasing. And the precipitation will increase. In addition, the presence of plant tends to obscure the surface – compared to sands from desert – and reduce the amount of dust present in theatmosphere. All this allowing more than heat reach and be absorbed into the surface. Result: a warmer and more humid land compared to the neighboring ocean and therefore, a difference of atmospheric pressure increased and winds stronger and more intense monsoons.

A stronger West African monsoon could displace some patterns of atmospheric circulation important to the west. And influence for example the El Niño phenomena. Or, to modify the trajectories of cyclones tropical. To get to the bottom of it, researchers will have to work to better understand what led to changes in the Sahara in the past. They will also have to test their simulations on scenarios ofemissions more moderate or even weak. Which, hopefully, should better fit our future …