Changing perceptions

Experts say changes in the Earth's orbit may have led to the formation of the Sahara desert

Last Updated: Saturday 04 July 2015

  Sahara desertification: the< if the latest findings by scientists at the Potsdam Institute for Climate Impact Research are found to hold water, the various views on the desertification of the Sahara may soon be turned upside down.

In a research paper, published in the July 15 issue of Geophysical Research Letters, they concluded that changes in the Earth's orbit led to the formation of the desert. More importantly, scientists now argue that land use practices of humans, who lived and cultivated the Sahara, were not responsible for desertification. The researchers, headed by Martin Claussen, employed climber-2, a new research model for analysing data on climate changes that took place during the last several thousand years.

Using this model, scientists discovered that desertification of north Africa began abruptly 5,540 years ago (30 years).

Earlier, the Sahara was covered by grasses and shrubs but the researchers concluded that desertification was begun by subtle changes in the Earth's orbit. This was amplified by data on the atmospherics and vegetation in the subtropics. The timing, according to the scientists, was governed by a global interplay among the atmosphere, ocean, ice and vegetation.

The transition was not gradual but occurred in two specific episodes. The first, which was less severe, occurred between 6,700 and 5,500 years ago. The second, which can be termed brutal, took place 4,000 to 3,600 years ago. Interestingly, El Nio, which began around 5,800 years ago could have contributed to the process.

Summer temperatures increased sharply and precipitation decreased during this period. This event devastated ancient civilisations and their socio-economic systems. The resulting loss of agricultural land may have been the reason behind why civilisations were established along the valleys of the Nile, the Tigris and Euphrates.

Some 9,000 years ago, Earth's tilt was 24.14 degrees, as compared with the current 23.45 degrees. Perihelion, the point in the Earth's orbit when it is closest to the sun, occurred at the end of July, while today this phenomenon takes place in early January. During the mid-Holocene period, the Northern Hemisphere received more sunlight, which amplified the summer monsoons in Africa and India.

The changes in the Earth's orbit occurred gradually but evolution of climate and vegetation in north Africa were abrupt. Claussen and his team believe that various feedback mechanisms within the Earth's climate system amplified and modified the effects touched off by the orbital changes. By modeling the impact of climate, oceans and vegetation, separately and in various combinations, the researchers concluded that oceans played only a minor role in Sahara's desertification.

The models showed that feedbacks within the climate and vegetation systems were major causes of Saharan desertification. Further investigation is necessary, the researchers say, to determine more precisely the specific effects of latitude and oceanic feedback, as compared with biospheric feedback, on the timing of the event.

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