Sahara dried out slowly, not abruptly: study
Thu May 8, 2008 5:46pm EDT
By Alister Doyle, Environment Correspondent
OSLO (Reuters) – The once-green Sahara turned to desert over thousands of years rather than in an abrupt shift as previously believed, according to a study on Thursday that may help understanding of future climate changes.
And there are now signs of a tiny shift back towards greener conditions in parts of the Sahara, apparently because of global warming, said the lead author of the report about the desert’s history published in the journal Science.
The study of ancient pollen, spores and aquatic organisms in sediments in Lake Yoa in northern Chad showed the region gradually shifted from savannah 6,000 years ago towards the arid conditions that took over about 2,700 years ago.
The findings, about one of the biggest environmental shifts of the past 10,000 years, 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.
The scientists, studying the remote 3.5 sq km (1.4 sq mile) Lake Yoa, found the region had once had grasses and scattered acacia trees, ferns and herbs. The salty lake is renewed by groundwater welling up from beneath the desert.
A gradual drying, blamed on shifts in monsoon rains linked to shifts in the power of the sun, meant large amounts of dust started blowing in the region about 4,300 years ago. The Sahara now covers an area the size of the United States.
FORECASTS
Kropelin told Reuters that improved understanding of the formation of the Sahara might help climate modellers improve forecasts of what is in store from global warming, blamed by the U.N. Climate Panel on human emissions of greenhouse gases.
The panel says that some areas will be more vulnerable to drought, others to more storms or floods.
The Sahara got greener when temperatures rose around the end of the Ice Age about 12,000 years ago. 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,” he said. And he said there were already greener signs in a huge area with almost no reliable weather records.
“I see a clear trend to a new greening of the Sahara, a very slow one,” he said, based on visits to some of the remotest and uninhabited parts of the desert over the past two decades.
“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.
In Darfur in Sudan, where U.N. officials say 300,000 people may have died in five years of revolt, slightly higher rainfall was more than offset by a rise in the human population to 7 million from 1 million half a century ago. People and their animals quickly eradicated any greenery.
— For Reuters latest environment blogs click on:
http://blogs.reuters.com/environment/
(Editing by Alison Williams)
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National Geographic News: NATIONALGEOGRAPHIC.COM/NEWS
Once Lush Sahara Dried Up Over Millennia, Study Says
James Owen
for National Geographic News
May 8, 2008
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.
(See photos of the Sahara today.)
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.
(Related: “Exodus From Drying Sahara Gave Rise to
Pharaohs, Study Says” [July 20, 2006].)
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, Kröpelin 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.
(Related: “Sahara Farming Village Struggles for Survival” [January 8, 2008].)
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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