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01/10/99
Vegetation disappeared from the Sahara 6000
years ago
In the UK-based scientific magazine named "New Scientist"
No.2195 dated 17 July 1999, there was an extremely
interesting article written by Robert Adler about how and
when the desertification of the Sahara occurred.
According to this article, six thousand years ago, the
Sahara was covered in shrubs and grasses. Then why did it
suddenly turn into the driest region on Earth? The article
says that German climate modellers claim they have the
answer. The article goes on as follows:
"Today, bare sands and rock cover 9 million square
kilometers of northern Africa, but ancient pollen grains
show that as recently 9,000 to 6,000 years ago, the Sahara
was carpeted by low brush and annual grasses.
"Climatologists have known for some time that slow
changes in the Earth's orbit and the tilt of its axis
gradually cooled the northern hemisphere, starting about
9,000 years ago. But even the most sophisticated climate
models have been unable to explain how this gradual climate
change caused the Sahara's abrupt desetification.
"So, Martin Claussen of the Institute for Climate Impact
Research in Germany designed a model that can simulate the
interplay between climate and the planet's vegetation --- a
difficult task that isn't adequately tackled by most climate
models. To allow these computations to simulate the climate
over thousands of years, he sacrificed some of the fine
details present in other models. Claussen's model shows that
the transition from grass to sand could have occurred in
just 300 years.
"Starting with the warmer and wetter conditions of 9000
years ago, Claussen slowly reduced the solar heating of the
atmosphere to simulate the effect of the changes in the
Earth's tilt and orbit. This gradually weakened the monsoons
over India and North Africa, thinning the vegetation
cover.
"After a few thousand years of gradual change, the
decimated vegetation was no longer able to preserve soil
moisture and maintain the cycle of evaporation, atmospheric
circulation and precipitation that drove the African
monsoon. This triggered an abrupt switch to a desert climate
about 5,400 years ago.
The reason why the "Mere Old Man" is so much interested
in this article is very simple. It is because he, as he
wrote in his homepage No. 111
dated 13 September, flew over the Sahara countless times and
saw the bare sands and rock with his own eyes. The landscape
seen from 10,000 meters up there was desolate and was
actually beyond description. There was absolutely nothing
but bare sands and rock.
According to the "Journey from Eden", by Brian Fagan,
Eve, whose genes are supposed to be shared by all the 6
billion people living on the Earth now, was walking
somewhere in savanna about 200,000 years ago. Compared with
200,000 years, 6,000 or even 9,000 years is nothing. So, in
her days, the Sahara was definitely covered in shrubs and
green grasses.
Brian Fagan also says in the same book that the most
primitive humans using stone tools appeared at least 2.5
million years ago somewhere in East Africa.
Once again let's get back to the article of New
Scientist. It says that the German researcher is eager to
use his fast-running model to try and find out whether
future global warming could have the paradoxical effect of
turning the Sahara green once again.
No matter whether his model may be fast-running or
slow-running, the "Mere Old Man" feels that there are too
many factors that determine the future of the Earth. To
begin with, future global warming itself sounds like an
extremely complicated phenomenon, based on uncountable
numbers of different factors, including future industrial
activities, political activities, excessive human
population, environmental pollution, wars and local
conflicts, terrorism, shortage of food in developing
countries, and many other factors all inter-playing and
interrelated. So it must be a terribly difficult task!
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