The grassy prehistoric Sahara turned into Earth's largest hot desert more slowly than previously thought, a new report says—and some say global warming may turn the desert green once again.
The new research is based on deposits from a unique desert lake in remote northern Chad.
Lake Yoa, sustained by prehistoric groundwater, has survived for millennia despite constant drought and searing heat.
The body of water contains an unbroken climate record going back at least 6,000 years, said study lead author Stefan Kröpelin of the Institute of Prehistoric Archaeology at the University of Cologne in Germany.
Ancient pollen, insects, algae, and other fossil clues preserved in the lake's sediments point to a gradual transformation to a desert environment.
Sahara Myth
The study contradicts past research that suggested the region dried up within a few hundred years. That research was based on windblown Saharan dust found in Atlantic Ocean sediments.
"This was a hypothesis used by most of the modelers and many of the scientific community who were not working themselves in the Sahara," Kröpelin said.
"To a large degree we can now show that such an abrupt drying out of the Sahara was a myth," he said.
The new study, which appears tomorrow in the journal Science, instead found evidence for a slow decline in tropical plants, followed by the gradual loss of savanna-type grasslands, and then the eventual spread of desert species.
Pollen samples revealed, for example, that the decrease in tropical trees accelerated after 4,800 years ago, while desert plants took root between 3,900 and 3,100 years ago.
Sand particles in the lake show that fierce desert winds didn't start picking up until about 3,700 years ago, the study found.
The only rapid change noted was in the lake itself, which switched from a freshwater to a salt lake between 4,200 and 3,900 years ago.
The transformation happened exactly in the time period when monsoon rains began moving away to the south, Kröpelin said.
This meant there was no longer surface water flowing in to counter salinity caused by evaporating water.
The study supports previous archaeological findings that human populations in the Sahara moved south over several millennia, following the monsoon rains, Kröpelin said.
First Reliable Record
About 20 feet (6 meters) of water evaporate from the lake every year, which is equivalent to the annual water consumption of about a million people, Kropelin noted.
"No team had ever succeeded in getting geological and paleoclimate information for the past 4,000 years since practically all the lakes had dried up, so there were no more geological archives available," he said.
The Lake Yoa data represent the first "reliable and high-resolution record" in the Sahara for verifying climate models, he added.
Such checks are important, he argues, "because if climate computer models don't work for the past, they probably won't work for the future."
Understanding climatic effects in the Sahara are especially important, since the region covers an area larger than the United States, Kröpelin said.
"Climate evolution in the Sahara reflects to a very large extent climate evolution on the African continent and beyond," he added.
Jonathan Holmes, of the Environmental Change Centre at University College London, was not involved in the study.
He wrote an accompanying commentary on Kröpelin's research in the same issue of Science.
The latest findings fill "an important gap in our understanding of the past 6,000 years of North African climate," he wrote in the article.
The study provides a more accurate picture of climate change in the region since the last ice age, because the "record comes from one of the few Saharan lakes in which sediments have accumulated without a break."
Similar lakes "probably do not exist," according to Holmes.
"However, improving existing geological records and using these to refine climate models would go a long way toward furthering our understanding," he wrote.
Modern Climate Change
Future research at Lake Yoa should provide clues to a potential regreening of the Sahara, triggered by the current trend of global warming, according to Kröpelin.
"I'm expecting reliable information on this possible trend," he said.
The last green phase, which started some 12,000 years ago, may be due to increased water evaporation from oceans. This led to monsoon rains that penetrated the interiors of tropical continents, he said.
"Now, today, man is probably causing the same thing," he said.
Kröpelin, who has studied the region for almost 30 years, said that since 1988 "there [has been] a strong indication [of] a return of increasing rains" in the eastern Sahara.
Already in some areas "you can see slight changes in the vegetation," he said.
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