Which one of the following continents is at a greater risk of desertification?

Concept — Desertification is land degradation in arid, semi-arid and dry sub-humid zones, produced jointly by climatic variability and human land use. Two quantities therefore fix how much of a landmass is at risk: exposure, meaning what share of its area is dryland, and vulnerability, meaning how heavily that dryland is leaned on for rain-fed cropping, grazing and fuelwood.

Applying it here — Africa stands at the top of both quantities. UNCCD places roughly two-thirds of the continent's surface in desert or dryland, and UNEP reports that about 45 per cent of Africa's land is already affected by desertification, with more than half of that area at high or very high risk of further degradation. Those drylands, the Sahel, the Horn of Africa and the margins of the Sahara and the Kalahari, also carry populations whose food and fuel come almost entirely from rain-fed farming and pastoralism, so a run of poor rainfall converts directly into loss of vegetation cover and soil.

Set against the other three continents:

• Asia holds the largest absolute dryland area, roughly 1.7 billion hectares, and the largest affected population, but those drylands are a smaller share of the continent's surface and its severely degraded belts sit as separated pockets, the Aral basin, the Central Asian steppe, north-west China and north-west India, rather than one continuous continent-wide zone of exposure.

• South America is predominantly humid tropical and temperate; its dry zones, the Atacama, the Caatinga, the Gran Chaco and the Patagonian steppe, are regionally confined rather than continent-wide.

• North America's drylands, the Great Basin, Mojave, Sonoran and Chihuahuan deserts and the southern Great Plains, are extensive, but they support a small rural population that does not depend on subsistence rain-fed farming, and they are managed with irrigation and rangeland controls.

Exposure and vulnerability together therefore put Africa at the greater risk of desertification. The official UGC NET Paper 1 (December 2013) key marks Africa for this item.