WASHINGTON -- One of the most striking climate changes of the past 11,000 years caused the abrupt desertification of the Saharan and Arabia regions midway through that period. The resulting loss of the Sahara to agricultural pursuits may be an important reason that civilizations were founded along the valleys of the Nile, the Tigris, and the Euphrates. German scientists, employing a new climate system model, have concluded that this desertification was initiated by subtle changes in the Earth's orbit and strongly amplified by resulting atmospheric and vegetation feedbacks in the subtropics. The timing of this transition was, they report, mainly governed by a global interplay among atmosphere, ocean, sea ice, and vegetation. Their research is published in the July 15 issue of Geophysical Research Letters.

The researchers, headed by Martin Claussen of the Potsdam-Institut fuer Klimafolgenforschung (Potsdam Institute for Climate Impact Research) employed a model of intermediate complexity to analyze climate feedbacks during the past several thousand years of the current, or Holocene, era. Called CLIMBER-2 (for CLIMate and BiosphERe, version 2.1), the model led to the conclusion that the desertification of North Africa began abruptly 5,440 years ago (+/- 30 years). Before that time, the Sahara was covered by annual grasses and low shrubs, as evidenced by fossilized pollen.

The transition to today's arid climate 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 was brutal, lasted from 4,000 to 3,600 years ago. Summer temperatures increased sharply, and precipitation decreased, according to carbon-14 dating. This event devastated ancient civilizations and their socio-economic systems.

The change from the mid-Holocene climate to that of today was initiated by changes in the Earth's orbit and the tilt of Earth's axis. Some 9,000 years ago, Earth's tilt was 24.14 degrees, as compared with the current 23.45 degrees, and perihelion, the point in the Earth's orbit that is closest to the Sun, occurred at the end of July, as compared with early January now. At that time, the Northern Hemisphere received more summer sunlight, which amplified the African and Indian summer monsoon.

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

The CLIMBER-2 models showed that feedbacks within the climate and vegetation systems were the major cause of Saharan desertification, building rapidly upon the effects of the initial orbital changes. The model suggests that land use practices of humans who lived in and cultivated the Sahara, were not significant causes of the 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.