deposits in Sanbao cave in Central China tell the story of the region's climate more than 200,000 years ago. They also reveal the working of the East Asian monsoon over timescales that range over thousands of years. The information was brought out in a paper in the February 28 issue of Nature.
But the significance of the paper extends beyond caves and monsoons. The rainfall patterns revealed by the study provide evidence for a bold hypothesis about climate change: wobbles in the earth passage round the sun are a prime mover of long-term monsoon variations.
As the Earth moves around the Sun, its orbital eccentricity--the deviation of its path from a perfect circle--and its obliquity--the tilt of its rotational axis--vary slowly over time. The monsoon rains that drench South and Southeast between May and September vary in duration and intensity in keeping with these wobbles in Earth's orbit, which follow a cyclic pattern. The rainfall patterns revealed by the Sanbao cave deposits confirm the association between the rainfall variations and the wobbles in the Earth's orbit.
The findings repudiate the theory that climate change is man made. It has some other implications as well. "The Asian summer monsoon is relatively weak in comparison to a few thousand years ago and going by past patterns it will stay at this level for a few centuries more," said Hai Cheng of Nanjing Normal University, one of the authors of the study.
The methods used to arrive at the findings are also significant. Cheng and his colleagues measured the oxygen isotope ratios locked in stalagmites in Sanbao Cave to determine changes in climate over millennia.
Compared to other commonly used indicators of long-term climate change such as tree-rings and ice cores, these isotopes provide a record over a much longer time scale. This technique "will quite likely replace the Greenland ice records as the chronological benchmark for correlating and calibrating climate variability," Cheng said.
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