Sahara - from Green to Desert
Sunday, 11. May 2008, 08:41:44
Around 14,800 years ago, a strengthening of the summer monsoons - moist tropical Atlantic monsoons from south-west - led to a dramatic climatic change in North Africa and created a “green Sahara”. How did this North African humid period come to an end and lead to the the world’s largest warm desert today. Was it abruptly or gradually?
The drying of the Sahara in the Holocene, that is approximately the last 11,550 years, is widely believed to have been an abrupt event, completed within a few hundred years, but new research published in Science of 9 may 2008 indicates that it happened gradually over the last 6000 years.
The authors of Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years studied a sediment record from Lake Yoa in northern Chad. Lake Yoa is one of the very few Saharan lakes in which sediments have accumulated without a break during the Holocene. Despite its extremely arid location, the lake is fed by ancient groundwater and therefore does not dry up.
The vegetation history of the surroundings is reconstructed from pollen. The reconstructed salinity values provide a record of changing precipitation. The input of atmospheric dust to the lake reflects wind regimes and the extent of vegetation cover in the surrounding landscape. The results show that vegetation and dust flux changed gradually over the past 6000 years, accompanied by the slowly weakening monsoon. The pollen source area implies that average north-easterly wind strength must have increased during this time, either because wintertime trade-wind circulation intensified or because a change in the mean position of the Libyan high-pressure cell now channeled low-level northeasterly flow more effectively through the Tibesti-Ennedi corridor.
Tibesti Mountains is a volcanic region to the west of Lake Yoa and the Ennedi Plateau, which is located to the east of the lake, is a sandstone plateau surrounded on all sides by sands, that encroach the deep valleys of the Ennedi.
However fast the drying occurred, it pushed people out of north-central Africa, and that climatically forced migrations might have led to the rise of the pharaohs and Egyptian civilization.
According to the lead author there are now signs of a tiny shift back towards greener conditions in parts of the Sahara, apparently because of global warming.
* http://www.sciencemag.org/cgi/content/short/320/5877/752
* http://sciencenow.sciencemag.org/cgi/content/full/2008/508/2?rss=1
* http://www.abc.net.au/science/articles/2008/05/09/2240138.htm
* http://www.nytimes.com/2008/05/09/science/09sahara.html?_r=1&partner=rssnyt&emc=rss&oref=slogin
* http://www.iht.com/articles/2008/05/09/africa/09saha.php
* http://www.redorbit.com/news/science/1378928/sands_of_sahara_moved_slowly/index.html?source=r_science
The drying of the Sahara in the Holocene, that is approximately the last 11,550 years, is widely believed to have been an abrupt event, completed within a few hundred years, but new research published in Science of 9 may 2008 indicates that it happened gradually over the last 6000 years.
The authors of Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years studied a sediment record from Lake Yoa in northern Chad. Lake Yoa is one of the very few Saharan lakes in which sediments have accumulated without a break during the Holocene. Despite its extremely arid location, the lake is fed by ancient groundwater and therefore does not dry up.The vegetation history of the surroundings is reconstructed from pollen. The reconstructed salinity values provide a record of changing precipitation. The input of atmospheric dust to the lake reflects wind regimes and the extent of vegetation cover in the surrounding landscape. The results show that vegetation and dust flux changed gradually over the past 6000 years, accompanied by the slowly weakening monsoon. The pollen source area implies that average north-easterly wind strength must have increased during this time, either because wintertime trade-wind circulation intensified or because a change in the mean position of the Libyan high-pressure cell now channeled low-level northeasterly flow more effectively through the Tibesti-Ennedi corridor.
Tibesti Mountains is a volcanic region to the west of Lake Yoa and the Ennedi Plateau, which is located to the east of the lake, is a sandstone plateau surrounded on all sides by sands, that encroach the deep valleys of the Ennedi.
However fast the drying occurred, it pushed people out of north-central Africa, and that climatically forced migrations might have led to the rise of the pharaohs and Egyptian civilization.
According to the lead author there are now signs of a tiny shift back towards greener conditions in parts of the Sahara, apparently because of global warming.
* http://www.sciencemag.org/cgi/content/short/320/5877/752
* http://sciencenow.sciencemag.org/cgi/content/full/2008/508/2?rss=1
* http://www.abc.net.au/science/articles/2008/05/09/2240138.htm
* http://www.nytimes.com/2008/05/09/science/09sahara.html?_r=1&partner=rssnyt&emc=rss&oref=slogin
* http://www.iht.com/articles/2008/05/09/africa/09saha.php
* http://www.redorbit.com/news/science/1378928/sands_of_sahara_moved_slowly/index.html?source=r_science
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