FROM THE FIELD
You Asked

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

by |September 18, 2019

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.

Alessandra Giannini is a climate scientist at the International Research Institute for Climate and Society within the Earth Institute at Columbia University. She has done research on droughts in Africa.

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.

On paleoclimatic time scales (thousands to hundreds of thousands of years), the extent of deserts can vary depending on variations in the amount of solar radiation that reaches Earth. This depends, in part, on changes in the Earth’s orbit. The Sahara was “green” between 11,000 and 5,000 years ago. Variations in solar radiation can drive greater poleward penetration of monsoon systems, and consequently increase rainfall at the desert’s equator-ward edge.

All this said, there is evidence that vegetation cover — specifically farmers using agro-forestry and soil and water conservation techniques — may be a beneficial measure in adapting to climate change. By agro-forestry I mean not so much planting rows of trees, but rather integrating trees and shrubs into normal farming practices. Farmers choose what trees to allow to grow in their fields — typically trees that increase nutrient uptake in soils and/or provide additional nutritional input to people and animals.

Planting trees is beneficial because the indication from meteorological observations is that climate change in the Sahel may be taking the form of less frequent, but more intense rains. The techniques mentioned above can use rain more efficiently, by reducing run-off and increasing water infiltration into the soil, which recharges aquifers. But since deserts are defined by their rainfall amounts, not water storage, it ultimately can’t stop desertification.

I recently read that afforestation in the Sahara would lower the Earth’s albedo to an extent would make afforestation actually worsen the issue. Is this prediction accurate?

If I understand correctly, the concern here is that planting trees in the desert would cause a reduction in albedo (i.e., in the amount of solar radiation that is reflected back to space), which may result in more energy being absorbed by the Earth, thereby increasing warming. While that may be a well-founded concern per se, the climate system is more complicated than that. Considering this single effect in isolation is not sufficient to answer the question.

Ultimately, what matters is whether this additional amount of energy that is absorbed by Earth’s surface can find its way out of the Earth’s climate system. Not just out of the surface and into the atmosphere, but also out of the atmosphere and back to space.

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)

 

Once absorbed by the surface, energy can be emitted to the atmosphere through different processes, including:

  • As “terrestrial radiation” (labeled “surface radiation” in the image). The earth, like the human body and any body with a temperature above absolute zero, emits radiation.
  • Through conduction, or close contact with the atmosphere (labeled “thermals” in the image).
  • As “latent heat” or “evapo-transpiration” in the image — meaning the energy that’s used to evaporate water from the surface.

Once in the atmosphere, whether the energy is emitted to space will likely be mostly determined by clouds: thick or thin, near the surface or aloft, each will have a different effect. In addition, clouds depend on the global atmospheric circulation — that is, on the direction and strength of winds, the location of rains, etc. Therefore, ultimately, the fate of this additional energy absorbed at the surface due to a local albedo reduction will not depend solely on the local increase in tree cover, or local conditions, but on how these conditions may influence, and in turn be influenced by global phenomena.

The pictures below show some examples of soil and water conservation in the Sahel. I took them in the village of Abraha Atsbeha, in the Tigray region of northern Ethiopia, and they are testament to a remarkable environmental reconversion for a village that was nearly abandoned about 30 years ago!

Admittedly, these are small features, but if you squint, you will see rows in the fields: these are rows of rectangular pits, about 1 meter deep, where water collects during rains, and slowly infiltrates into the water table. Photo: Alessandra Giannini

 

A check dam to reduce downslope run-off. Photo: Alessandra Giannini

 

A well with a hand pump — the water table is so close to the surface that the water is trickling even without pumping! Photo: Alessandra Giannini

Got a question about climate change? Feeling curious about conservation? To submit a question, drop a comment below, message us on Instagram, or email us here.

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Oddmund DaleSteveian G UK Recent comment authors
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ian G UK
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ian G UK

PARTICULARLY wish to stress your point about carbon in soil which can be degraded organic matter (dug-in weeds, leaves, etc that could support worms (these draw the leaves in and DO THE JOB FOR YOU). On a visit to Western Australia’s Perth coastal plain the sandy surface has little potential to absorb water and suggest this should be a priority. After considerable rain the surface was not wetted more than 30 – 50 mm deep, a lot had evaporated. What chance has vegetation to grow when people are cocooned in cars and houses without getting out and doing organic gardening?… Read more »

Steve
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Steve

NASA satellite data shows the earth is becoming greener. They state it’s due to CO2 fertilization. Is this having any impact on African deserts?

Oddmund Dale
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More CO2 in the atmosphere will make Sahara greener. Less water evaporation from plants under photosynthesis and stronger growth. I spent some time in the center of Sahara 5-6 years ago. Plants are growing but camels eat every single one every year.