Deserts resulted from land misuse

PUBLISHED : Friday, 23 October, 1998, 12:00am
UPDATED : Friday, 23 October, 1998, 12:00am

More than a third of the Earth's land surface is desert. Such areas are characterised by a low or almost non-existent precipitation and high evaporation rates. However, deserts are not all classified as hot deserts. The driest parts of the temperate dry grasslands are really a mid-latitude desert, and there are also cold or high-latitude deserts, such as Turkestan and the Gobi Desert in Asia, and the Great Basins in the US. These have hot summers but freezing winters.

Hot deserts constitute about a fifth of the total area of the world's deserts. Although the popular image is of vast areas of barren sand dunes, there are also large expanses of bare rock and gravel.

Daytime temperatures are very high and 58?C has been recorded as a shade temperature in the Sahara. However, due to the lack of cloud cover, temperatures at night fall rapidly, often dipping below freezing point. Deserts are located between 15? and 30? north and south of the equator as follows: On the western coasts of continents - the Atacama in South America, the Kalahari and Namib Deserts in southern Africa, the Sonora and Mojave in North America.

In the centre of Australia.

The majority of northern Africa, the vast Sahara Desert, extending eastwards into Arabia as the Rub's al Khali - the 'empty quarter' - and on into the Indian subcontinent as the Thar Desert.

Causes of deserts Deserts have several physical causes. The areas between 15? and 30? of latitude lie some 1,500 km north and south of the equator and are areas of constant high pressure, as they are beneath the descending air of the Hadley cells within the Earth's atmospheric circulation.

Subsiding air is increasingly compressed and becomes drier and warmer as it sinks. Thus it has a low relative humidity and clouds are seldom able to form.

Those deserts lying to the west of such mountain ranges as the Andes, Rockies and Great Dividing Range are rainshadow areas within the trade wind belts. These winds blow towards the southwest in North America and to the northwest in the Southern Hemisphere.

The west coast deserts have yet another cause. Cold ocean currents off-shore cool the winds blowing towards the land so that the air's ability to hold water vapour is greatly reduced. Thus little rain falls on the adjacent land, although there are often advection fogs in places like the Atacama and Kalahari Deserts.

The borders of the world's deserts are never static. In the semi-arid lands which lie on the desert margins rainfall is highly variable and unpredictable. Desert conditions advance in times of drought and retreat during wetter periods. But over the past 1,000 years more and more land has been lost to the desert.

There has been a slow but steady decline in land quality that turns forest into savanna and savanna into barren, unproductive scrub. This progressive downgrading is known as desertification. How far this is the result of a change in climate is not known. What is certain is that man has hastened the process by his misuse of the land.

Desertification happens most rapidly in times of drought, but goes on during more favourable climatic conditions as well.

The total area affected by some degree of desertification is about 50 million km2, which represents an enormous loss of agricultural land. Technically it would be possible to reclaim much of this and return it to full productivity. Moderate desertification can be solved fairly cheaply and simply, but land in the severe category needs much more time and investment to achieve a full recovery.

Causes of desertification For centuries nomadic herders have grazed their animals on the savanna grasslands. Farmers have burnt and cleared areas of forest to cultivate crops. Forested areas around settlements have been cut down to provide fuel.

On the whole the impact of these activities was relatively small until recently, when population pressure has greatly increased the demand for land. This has forced some farmers to cultivate marginal areas which are easily eroded once the natural vegetation is removed. Nomads have increased the size of their herds and pastures have been worn out through overgrazing. Irrigation schemes have dried out the land by lowering the water table in collection areas.