You Asked: Can the Great Green Wall Stop the Sahara From Expanding?

Sunlight (solar radiation) coming to Earth is on the left, in yellow. Longwave (terrestrial) radiation going away from Earth is on the right, in beige. (Image: K. Trenberth, J. Fasullo, and J. Kiehl)
FROM THE FIELD
You Asked
BY PHEBE PIERSON|SEPTEMBER 18, 2019
https://blogs.ei.columbia.edu/2019/09/18/great-green-wall-sahara-desertification/

You Asked” is a series where Earth Institute experts tackle reader questions on science and sustainability. In honor of Climate Week NYC and the Covering Climate Now initiative, we’ll spend the next few weeks focusing on your questions about climate change.

The following questions were submitted through our Instagram page by two of our followers. Answers are provided by Alessandra Giannini.

Is Africa’s Great Green Wall the only natural way to prevent further expansion of the Sahara into the rest of Africa?

First of all, the Sahara is not expanding into the rest of Africa. Drought in the Sahel in the 1970s and 1980s made it look like the desert was expanding, because the reduction of rainfall at the desert margin (the Sahel) caused a reduction in vegetation. However, since the advent of satellite observations of land cover in the early 1980s, we know that how far north into the desert vegetation grows depends on how much it rains at the edge of the desert. How much it rains in the Sahel over decades to centuries is primarily controlled by very large-scale influences, such as the temperatures of the global oceans, not by how much vegetation there is.

The Great Green Wall — a project that aims to plant a vast wall of trees across North Africa — cannot prevent an expansion of the Sahara. Nor can planting trees in semi-arid regions in general increase rainfall.

The Sahara is a desert because it receives negligible rainfall. It receives little rainfall because of where it’s located. Climatologically, deserts are where they are — around 30 degrees north and south in both hemispheres — because of circulation patterns in the atmosphere. Warm, moist air rises near the equator, then cools and condenses its moisture, which falls as rain or snow. Thus the equatorial regions are characterized by very wet ecosystems, like rainforests. This same air later descends over the Sahara, but unfortunately, sinking air cannot lead to rain, and most of its moisture is depleted anyway. The latitude of sinking motion is largely determined by the earth’s rotation rate around its axis.

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Author: Willem Van Cotthem

Honorary Professor of Botany, University of Ghent (Belgium). Scientific Consultant for Desertification and Sustainable Development.