Half of the world's population is affected by Asian monsoons(季风), but monsoons are difficult to
predict. American researchers have put together a 700-year record of the rainy seasons, which is
expected to provide guidance for experts making weather predictions.
     Every year, moist (潮湿的)air masses,known as monsoon,produce large amounts of rainfall in India,
East Asia, Northern Australia and East Africa. All this wet air is pulled in by a high pressure area over the
Indian Ocean and a low pressure area to the south.
     According to Edward Cook , a weather expert at Columbia University in New York., the complex
nature of the climate systems across Asia makes monsoons hard to predict. In addition, climate records
for the area are too recent and not detailed enough to be of much use. Therefore, he and a team of
researchers spent more than fifteen years traveling across Asia, looking for trees old enough to provide
long-term records. They measured the rings, or circles, inside thousands of ancient trees in more than
300 places.
     Rainfall has a direct link to the growth and width of rings on some kinds of trees. The researchers
developed a document they are calling a Monsoon Asia Drought Atlas. It shows the effect of monsoons
over seven centuries, beginning in the 1300s.
     Professor Cook says the tree-ring records show periods of wet and dry weather. "If the monsoon
basically fails or is a very weak one, the trees affected by monsoons at that location might put on a very
narrow ring. But if the monsoon is very strong, the trees affected by that monsoon might put on a wide
ring for that year. So, the wide and narrow ring widths of the chronology that we developed in Asia
provide us with a measure of monsoon variability." With all this information, researchers say they can
begin to improve computer climate models for predicting the behavior of monsoons.
     "There has been widespread famine and starvation and human dying in the past in large droughts.
And on the other hand, if the monsoon is particularly heavy, it can cause extensive flooding." said Eugene Wahl, a scientist with America's National Oceanic and Atmospheric Administration. "So, to get a sense
of what the regional moisture patterns have been, dryness and wetness over such a long period of time
in great detail, I would call it a kind of victory for climate science."