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%。
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Possible causes of the vegetation shift include declining rainfall, growing human population and decreasing soil fertility
可能造成植被转移的原因包括降雨量的下降、人口的增长以及土壤肥力的减弱。