阅读理解
A
new study suggests climate conditions in the Sahara Desert, one of hottest,
driest and most deserted areas in the world, have changed from wet to dry a
about every 20,000 years.
But
previous evidence has suggested the Sahara did not always experience such
extreme heat and dry conditions. At times, the Sahara Ranged to a very wet
climate. This permitted plants and animals to develop and grow and led to the
creation of human settlements. Now, they have discovered more evidence, which,
the lead researcher David Mc Gee says, supports the idea that the Sahara's
climate kept changing between wet and dry about every 20,000, years.
McGee
reported these climate changes were mainly driven by changes in the Earth's(地轴)as the planet travels round the sun
.This process affects the amount of sunlight between seasons. Every 20,000
years the Earth receives more strong summer sunlight. When the Earth's axis
changes again, the amount of sunlight is reduced. This season change happened
continuously every 20,000 years. The other part of the season produced rainy
conditions, resulting in a wetter, greener, plant-rich environment. When the
rainy activity weakens, the climate becomes hot and dry, like the Sahara
remains today.
The
scientists based their research on dust collected from ocean sediment(沉积物)in the Atlantic Ocean bottom off the
coast of West Africa. The dust contained layers (层)of ancient sediment built up over
millions of years. Each layer could contain traces of Saharan dust as well as
the remains of life forms. This information was used to assess over what time
period the dust had built up. This led to the overall finding that the Saharan
changes from wet to dry climates happened every 20,000 years.
David McGee said
that today we only see the Sahara Desert as an extremely deserted and "inhospitable"
place. The new study suggests that the area's climate has changes between
grasslands and a much wetter environment, and back to dry climates, even over
the last quarter million years. McGee says he thinks the latest research can be
valuable in studying the Sahara's history as it relates to human settlement.