May 8, 2008
New Study Shows the Sahara Dried Out Slowly
A recent study on the future of climate changes suggests that the once-green Sahara turned to desert over thousands of years rather than in an abrupt shift as previously believed.
The study's lead author said that parts of the Sahara now show signs of a tiny shift back towards greener conditions, apparently due to global warming.
The researchers looked at ancient pollen, spores and aquatic organisms in sediments in Lake Yoa in northern Chad that show that the region gradually shifted from savannah 6,000 years ago towards the arid conditions that took over about 2,700 years ago.
This has been considered one of the biggest environmental shifts of the past 10,000 years. These findings now challenge past belief based on evidence in marine sediments that a far quicker change created the world's biggest hot desert.
"The hypothesis of a sudden shift was astonishing but it was still taken up," said Stefan Kropelin of the University of Cologne in Germany, lead author of the study with scientists in Belgium, Canada, the United States, Sweden and France.
Researchers studied Lake Yoa, a remote salty lake only 1.4 sq miles. They found that the region was once scattered with grasses, acacia trees, ferns and herbs. The lake is renewed by groundwater that wells up from beneath the desert.
About 4,300 years ago, large amounts of dust started blowing into the region"”caused by gradual drying due to shifts in monsoon rains linked to shifts in solar power. The Sahara now covers an area roughly the size of the United States.
Kropelin said that improved understanding of the formation of the Sahara might help climate modelers improve forecasts of what is in store from global warming.
The U.N. Climate Panel blames global warming on human emissions of greenhouse gases and says that some areas will be more vulnerable to drought, others to more storms or floods.
Around 12,000 years ago, the Sahara got greener when temperatures rose around the end of the Ice Age. Warmer air can absorb more moisture from the oceans and it fell as rain far inland.
"Today I think we have the same thing going on, a global warming," said Kropelin. He said there were already greener signs in a huge area with almost no reliable weather records.
Kropelin has visited some of the remotest and uninhabited parts of the desert over the past two decades. "I see a clear trend to a new greening of the Sahara, a very slow one," he said.
"You go to unoccupied areas over a long time and you know there was pure sand there without a single snake or scorpion. Now you see tens of kilometers covered by grass," he said.
Slightly higher rainfall was more than offset by a rise in the human population to 7 million from 1 million half a century ago in Darfur in Sudan, where U.N. officials say 300,000 people may have died in five years of revolt. People and their animals quickly eradicated any greenery.
Kropelin's report on the Sahara desert's history was published in the journal Science.
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