Abstract: Since the late 1960s, especially during the 1980s, the global climate has warmed up significantly. However, this warming mainly occurs in winter, but the monsoon rain belt over the east Asia/west Africa during high summer shows a trend moving southward. Based on strict statistics with the support of numerical modelling, it is found that such a southwardly shift of the summer monsoon rain belt with less rainfall in North China and Sahel is caused by the decrement of direct solar radiation in the clear skies over the Northern Hemisphere/China. And the radiation decreasing is the result brought out by the increment of volcanic eruptions as well as air pollutants in recent years