from AstroBio Website
the Sahara was once fertile grassland.
Ewan Robinson, NASA
This fact has been common
knowledge in the scientific community for some time, but scientists
are still grappling with historic data to determine whether that
transition took place abruptly or gradually.
In this latest study, researchers analyzed the sedimentation in Lake Yoa on a yearly basis and dated it to determine when and how the Sahara region dried-up.
Other studies have used
climate modeling to determine the time period that the Sahara went
dry and the prevailing climate conditions at that time.
In simpler terms,
insolation refers to the amount of sunlight shining down on a
particular area at a certain time, and depends on factors such as
the geographic location, time of day, season, landscape and local
Changes in the Earth's tilt cause changes in weather patterns.
Such a change is believed to have made the "Green Sahara" go dry.
Image credit: NASA
The tilt changed from around 24.1 degrees to the present-day 23.5 degrees.
The changes in the Earth's orbital tilt and precession (or the wobbling motion) occur because of gravitational forces emanating from other bodies in the solar system.
To understand exactly what happens, picture a spinning top when it is slightly disturbed.
Just like a top, the Earth too wobbles slightly about its rotational axis. This tilt changes between roughly 22 and 25 degrees about every 41,000 years, while the precession varies on about a 26,000-year period.
These cycles have been determined by astronomers and validated by geologists studying ocean sediment records.
located in southwest Egypt,
depicts a time when the Sahara was wetter.
However, recent research is suggesting that the effects of global warming - particularly the oceans - could cause a change in the Earth's axial tilt.
Scientists from NASA's Jet Propulsion Laboratory (JPL) say that the current melting of ice in Greenland is already causing the tilt to change at a rate of approximately 2.6 centimeters each year.
They predict that his
change could increase in the years ahead.
After the Earth's tilt changed, the monsoons decreased and the vegetation began to disappear. When there were no plants to retain water and release it back into the atmosphere, the rain progressively decreased.
The resulting feedback
loop between plant life and climate eventually created the current
The disagreement among
scientists is in part due to the lack of paleo-environmental records
from the region. Therefore, scientists must often resort to
They concluded that the climatic transition of the Sahara took place abruptly, within a possible span of about 300 years.
from the Atlantic Ocean some 3,000 miles to the Nile River
and the Red Sea, and southward from
the Atlas Mountains of Morocco and the Mediterranean shores
more than 1,000 miles to the savannah called the Sahel.
More than 16 times the size of France, the Sahara Desert blankets nearly all of
Mauritania, Western Sahara, Algeria, Libya, Egypt and Niger;
the southern half of Tunisia; and the northern parts of Mali, Chad and Sudan.
Image credit: NASA's MODIS instrument
(Moderate Resolution Imaging Spectroradiometer)
Since the Sahara spans a massive area - covering nearly a third of the African continent - it is quite possible that parts of it dried up abruptly while it took other regions a longer period of time to transform into a desert.
Pierre Francus explained that abrupt climate changes have been documented in many places on Earth at various times in the past.
One example he cited is the Younger Dryas, one of the most famous examples of abrupt climate change that occurred between (approximately) 12,800 and 11,500 years ago.
According to the National Oceanic and Atmospheric Administration (NOAA), the end of this period was particularly abrupt when, for example, in Greenland, temperatures increased 18 degrees Fahrenheit in about a decade.
Scientists from NASA's Jet Propulsion Laboratory
say that the current melting of ice in Greenland
is causing Earth's axial tilt to change at a rate
of approximately 2.6 centimeters each year.
Image credit: University of Colorado
Francus also noted that there are some models that cannot predict an abrupt climate shift at all.
Some scientists feel that
there is not enough knowledge to understand the processes driving
these changes primarily because it is difficult to model the soil
moisture and cover.
That will help climate researchers determine the precise role human behavior plays on current climate change.