D
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 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 McGee 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 axis(地轴)as the plan travels round the sun. This process affects the amount 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.
13.What does the new study find about the Sahara Desert?
A. It is one of the hottest,driest and most deserted.
B. It always experiences extreme conditions because of its position.
C. Its climate changes between wet and dry about every 20,000 years.
D. Its environmental conditions stop plants and animals from surviving.
14.What is the direct reason for Sahara's climate changes?
A. Rainy activity. B. Solar radiation.
C. Human activity. D. Global environment.
15.How did the researchers acquire the evidence?
A. By analyzing ocean life.
B. By studying Sahara's history.
C. By collecting dust on the coast.
D. By examining ocean sediment.
16.What does the underlined word"inhospitable"probably mean?
A. Unsuitable to live.
B. Insignificant to study.
C. Unable to explore.
D. Impossible to change.
答案 13-16 CBDA
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 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 McGee 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 axis(地轴)as the plan travels round the sun. This process affects the amount 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.
13.What does the new study find about the Sahara Desert?
A. It is one of the hottest,driest and most deserted.
B. It always experiences extreme conditions because of its position.
C. Its climate changes between wet and dry about every 20,000 years.
D. Its environmental conditions stop plants and animals from surviving.
14.What is the direct reason for Sahara's climate changes?
A. Rainy activity. B. Solar radiation.
C. Human activity. D. Global environment.
15.How did the researchers acquire the evidence?
A. By analyzing ocean life.
B. By studying Sahara's history.
C. By collecting dust on the coast.
D. By examining ocean sediment.
16.What does the underlined word"inhospitable"probably mean?
A. Unsuitable to live.
B. Insignificant to study.
C. Unable to explore.
D. Impossible to change.
答案 13-16 CBDA
