What Is Desertification? Land That Stops Being Able to Grow Things

The processes involved are physical and chemical rather than geographical, and they overlap:

• •Wind erosion, which strips the fine fraction of topsoil once the vegetation cover falls below a threshold, removing exactly the particles that hold nutrients and water
• •Water erosion, which in dry regions is more damaging than in wet ones because rainfall arrives in intense bursts onto hard bare ground that cannot absorb it
• •Salinisation, where irrigation water evaporates and leaves its dissolved salts behind, accumulating in the root zone until nothing will grow, a process that ended agriculture in parts of ancient Mesopotamia and now affects a substantial share of the world's irrigated land
• •Compaction by livestock and machinery, which reduces infiltration and increases runoff
• •Loss of soil organic matter, which reduces the soil's capacity to hold water, making the same rainfall less effective
• •Falling water tables where groundwater is extracted faster than it recharges, which kills the deep-rooted vegetation that stabilised the surface

Two categories of driver operate at once and interact, which is why the subject generates so much argument. Climate supplies variability: drylands have always experienced multi-year droughts, and a dry decade will degrade land regardless of what people do. Human activity supplies pressure: overgrazing, cultivation of marginal land, removal of trees for fuel, and irrigation without drainage. The interaction is the key point, because land under heavy pressure that would recover after a drought in a normal decade may cross a threshold and fail to recover at all, at which point the change becomes self-reinforcing, since bare ground is hotter, reflects more light, absorbs less water and supports less growth. A famous hypothesis by Jule Charney in 1975 proposed that vegetation loss in the Sahel raised surface reflectivity enough to suppress rainfall directly, creating a feedback, and while the detail has been revised, the general principle that vegetation influences local climate is now well established.

The southern edge of the Sahara is the case that defined the subject, after severe droughts from the late 1960s to the 1980s killed hundreds of thousands of people and vast numbers of livestock, and produced the dominant image of an advancing desert. Later satellite analysis complicated it considerably. Measurements from the 1980s onward showed the vegetation boundary moving north as well as south, tracking rainfall rather than advancing steadily, and a substantial greening of the Sahel since the mid-1980s as rainfall partly recovered, a finding that surprised almost everyone. That does not mean the degradation was imaginary: local soil loss, tree cover decline and falling productivity were real and are documented, and greening measured by satellite can reflect a change in the mix of species rather than a recovery of useful land. What the episode taught the field is that the desert does not advance as a front, that dryland vegetation fluctuates enormously with rainfall, and that measuring degradation requires more than a vegetation index.

The interventions with the best record are small, local and cheap rather than large and engineered. In Niger and Burkina Faso, farmer-managed natural regeneration, in which farmers protect and prune the tree stumps and roots already present in their fields rather than planting seedlings, has restored tree cover across several million hectares, improving soil, providing fodder and fuel, and raising yields; the change was driven by a shift in who was legally entitled to the trees, since farmers had no incentive to protect trees the state owned. Simple water harvesting has comparable results: half-moon basins and the traditional zai planting pits, in which a farmer digs a small hole, adds manure and plants into it, capture enough runoff to re-establish crops on ground considered dead. Terracing, contour bunds, windbreaks and rotational grazing all have supporting evidence. Large-scale tree planting has a much poorer record, with survival rates often low, and the Great Green Wall across the Sahel, launched in 2007 as a continuous belt of trees, has been substantially reframed towards a mosaic of local land management practices after the original conception proved unworkable.

A United Nations convention on the subject was agreed in 1994 alongside those on climate and biodiversity, and it has always been the least funded and least prominent of the three, partly because the affected countries are poor and partly because the problem is diffuse and slow. The framing has shifted over time from combating desertification, which implies a fight against an advancing enemy, towards land degradation neutrality, a target of restoring as much land as is degraded each year, which is measurable and less dramatic. The stakes are large: degraded land pushes people to migrate, which has been argued as a contributing factor in several conflicts, though the causal claims are frequently overstated in political discussion. What is not disputed is the arithmetic, since a growing population needs more food from a land area that is not growing, and the fertile fraction of it is shrinking.

Desertification is the degradation of dryland soil and vegetation until productivity is lost, driven by wind and water erosion, salinisation from irrigation, compaction, loss of organic matter and falling water tables, and caused by the interaction of natural drought cycles with grazing, cultivation and tree removal. The Sahel showed that deserts do not advance as a front and that vegetation tracks rainfall closely. The interventions that work are local and cheap, chiefly protecting existing tree regrowth and harvesting runoff in planting pits, rather than large planting schemes.