The Great Green Wall: Stopping the spread of the Sahara

One of the greatest human endeavors is underway right now in Africa - to intentionally grow the largest living structure on the planet. The goal is to reforest 1 million square kilometers covering an entire one percent of all arable land on the planet. It is the largest reforestation project ever undertaken and until recently I’d not appreciated the brilliance of the plan. This colossal regreening project is called The Great Green Wall and it aims to create a belt of vegetation just south of the Sahara - in the Sahel - a semi-arid scrubland / grassland with the occassional tree. The Great Green wall will stretch all the way across Africa, measuring 8000 km from east to west and averaging 125 kilometers north to south. Originally planned as a great tree planting exercise to combat the ongoing expansion of the Sahara, the Great Green Wall (GGW) has morphed into a visionary project bringing African nations together to combat climate change, provide jobs, build infrastructure, reduce drought and famine, stem migration, and reduce conflict and the development of extremism. It has all the ingredients of success, building on decades of local action and local know how, informed by ongoing peer reviewed research - which includes the recognition that the project must work with and for the local people. While the project is plagued by some pretty hefty obstacles like underfunding, missing planting targets, and even questions about its overall longterm feasibility, there have already been impressive accomplishments.

The project was initially a response to one of the largest climatic anomalies of the 20th century. From 1968 to 1993, rainfall across the Sahel averaged 15 to 25% below the long term mean. That’s a substantial loss of water for a semi-arid place and it resulted in large swathes of the Sahel becoming denuded of vegetation. The process of desertification then feed on itself as the drought crusted over the soils in vegetatively free areas preventing any rainfall from penetrating into the soil. So dry became drier - a theme that will recur in this essay. In the 20th century, the Sahara expanded by more than 10%, primarily into the Sahel. During the drought of the 70s and 80s, over 100,000 people in the Sahel died of starvation with an additional 750,000 made dependent on food aid. Out of this tragedy, the Green Green Wall (GGW) project was forged.

You might well wonder if planting trees along the edge of a desert is a waste of time. Will there be enough water there to support them long term? That is a vastly complicated question and we don’t know all the answers. One analysis of current rainfall patterns, found that 26% percent of the planned GGW planting area doesn’t currently get enough rainfall to support new plantings. Most of that drier area is east of 10 degrees latitude so includes areas in Chad and Sudan. But current rainfall patterns are not the whole story. The location of the boundary between the Sahara and the Sahel is, of course, not a sharp line and is ever changing. The rainfall regime changes from year to year, largely driven by nearby sea surface temperatures as in the case of the 1970s and 1980s drought. In turn, sea surface temperatures are driven by climate change. So that chain of impacts is evolving and there is a hint that the future global climate will support more vegetation in the Sahara - more on that later. But other factors also affect rainfall distribution. Most importantly for the Sahel-Sahara demarcation is the status of the vegetation and soil. For while the plants distribution is dependent on climate, more vegetation increases the amount of rainfall in a given area. And to further complicate matters, the plant distribution is largely impacted by humanity’s activities like farming, fuel gathering and pasturing livestock.

If you can understand how plants can create their own rainfall, you’ll be a long way to understanding global climate - which I’m sure is on your list of New Year’s resolutions. Basically, plant induced rainfall arises from two processes which we are familiar with - hot air rising and cool air condensing. An example of the first is the rising of a hot air balloon and of the later morning dew. Translated into plants’ climatic impact we start with the fact that the ground in vegetated areas absorbs more radiation than if there were only soil present. Desert soils reflect about 50% of incoming light while the vegetation in central Africa reflects only about 20%. Thus the ground and air near vegetated areas is oddly warmer than the desert. This means the air above the forest rises, remember, hot air rises. When air rises - it cools (for the most part) and as it cools water vapor in the air condenses into liquid water droplets. And if the water droplets get large enough - it rains. Thus having more veg on the boundary between desert and scrub land encourages rainfall over the vegetated area. Vegetation encourages more vegetation. This is a classic positive feedback loop.

A second parallel feedback loop also occurs in the Sahel and pivots around how much dust is in the air. Lands with vegetation have less dust in the air because the soil is held in place by roots. As dust is very reflective, less dust means more heating, more air rising, and more rainfall. Together the dust and vegetative feedbacks of vegetated Sahel will increase local rainfall by tens of percents, more than the reduction in rainfall seen during the great drought of 70s and 80s. As someone who worked on a very similar question, but for tropical cloud forests, I can tell you it is not easy to predict just where or how much enhanced moisture we can expect from changes to vegetation. However, it is certainly possible that planting along the Sahel-Sahara boundary will be self-sustaining.

When trying to predict what will happen to the desert-scrubland line south of the Sahara it is very informative to look at the many past shifts of this boundary line. Sometimes the desert disappears altogether as it did during the Holocene, 12,000 to 5,000 years ago, when the Saharan region was fully vegetated. It is a challenge to explain this in full but we know it is largely driven by the changes in the earth’s orbital cycle. For example, the direction the earth’s axis points changes over the millennia and our distance from the sun at the peak of summer varies, ever so slightly. Because of these changes and other changes, the amount of radiation received during the summer changes. During the Holocene, when the Sahara was vegetated, the northern hemisphere got slightly more radiation during its summer than it does now. This means it was slightly hotter, and hopefully it now makes sense to you that the uplift which drives rainfall in this region was also stronger. This brought water into the Sahara. Indeed, there have been 230 African humid periods in the last 8 million years with consequent greening of the Sahara. One of these humid periods may have facilitated the migration of humans out of Africa some 120 thousand years ago.

Of great interest for how the modern day Sahel-Sahara boundary will evolve, the dust and pollen records show that the transitions between vegetated and desert states was very fast, like a century or two. This is ten times faster than the orbital forcing, suggesting that something else might be nudging along the transition. Supporting the need for nudging, climate models cannot reproduce the observed shifts in vegetation through out the paleo-record with orbital forcing alone. You may well have guessed that scientists believe that the positive vegetation and dust feedbacks described above may have helped hurry the transitions between desert and vegetated states. The short timescale for transitions and large number of these transitions in the past suggests that it is relatively easy for the Sahara to switch between these two relatively stable states - vegetative and desert.

But for the last two transitions, from vegetated to desert at the end of the Holocene (5,000 years ago) and the expansion of the desert in the past century, it seems likely that human activity also played a role. There is evidence that humans were farming in the Sahel 5,000 years ago and obviously people farm there now. These small scale devegatition events may well be responsible for nudging the system into its desert state. A final argument from the paleo record that supports the hypothesis that humanity has contributed to desertification recently comes from the Pliocene. The Pliocene is a warm period that occured about 3 million years ago and is largely viewed as a decent analog to our future climate. During the Pliocene, northern Africa was much wetter than it is today and supported dense vegetation. If the climate today is indeed similar to the Pliocene, we’d have expected the Sahara to be shrinking if climate were the only trigger. But the Sahara is expanding, suggesting humans may be causing this expansion through devegetating activites like farming, grazing cattle and foraging for wood.

If we are the trigger for the expansion of the Sahara, we can flip our behavior and shrink the Sahara. Enter the Great Green Wall. If humans can get vegetation started there, the ‘willingness’ of the Sahara to switch states may well make the Great Green Wall a raging success. Signs are good that it will be. Survival rates of planted trees range from 20% to 70%, but even 20% is a lot of trees in a deserted area. The project has all ingredients that we’ve learned are essential for environmental projects like investing in local education, local stewardship, learning about and sharing best farming practices and adaptive strategies for the planters. Iconic strategies, like forage banks, are helping the local farmers and the ecosystems. Forage banks are large areas which are protected from grazers and are harvested in the dry season to feed cattle - so they don’t go eat newly planted trees. Senegal alone has 27 forage banks and (as of 2019) has planted 18 million trees. This replanted area has seen the return of gazelles, jackels, tortoises and song birds. And people are thriving too. A study of socio-economic impacts found that between 2016 and 2020 perceived food insecurity dropped from 46% of the populatce to 15% in Senegal, with a smaller drop in Nigeria from 69% to 58%. Overall, the GGW has created over 350,000 jobs, generated 90 million US dollars in income, restored 20 million hectares and trained 10 million people in sustainable land and water management.

Of course there are still problems. Only 18% of the planned 100 million hectares have been planted, only half the pledged donations from foreign countries have been sent, and ecosystem destruction elsewhere in these nations still dwarfs the rate of regeneration along the GGW. But it is the still the most extensive and boldest climate, people and ecosystems solution we have seen. And it is up and running: improving lives, reducing migration and conflict, providing jobs, and promoting food security all while absorbing carbon and providing homes for wildlife and helping the locals to become the stewards of the land they live on. It is immensely satisfying, and hugely humbling, to see people just getting on with it. And I for one say “Well done.”