The research shows that during the 20th Century rainfall in the Sahel dropped between 20 and 30 percent.研究显示在20世纪期间,撒海尔的降雨下降了20%到30%。 "One in six trees died in the last half of the 20th Century and, second, one in five tree species disappeared locally. And then third, together these changes shifted vegetation zones southward toward areas of more rainfall."他说:“20世纪后半期,六棵树中就有一棵树死掉,当地五个树种中就有一种消失。这些变化一起将南方植被区向更多降雨量的区域移动。” ___________________________________.