Asian monsoon cycle disrupted by man-made climate change

A 1,800-year-old stalagmite from a Chinese cave has revealed that, in the past, warmer years were associated with stronger East Asian monsoons. That was true until just 50 years ago, when the relationship broke down because of man-made greenhouse gas emissions and pollution. The researchers also suggest that weaker monsoons may have led to the demise of some Chinese dynasties, as water shortages and poor harvests triggered public unrest.

As a monsoon advances, water vapour containing the heavy oxygen isotope 18O is more likely to condense and fall as rain. So the stronger the monsoon, the less 18O clouds contain and the lower the 18O/16O ratio. Stalagmites hold a record of this because they are formed from dissolved minerals and the rain water as it drips through cracks in caves. Scientists can also date monsoons by measuring the decay of uranium-234 to thorium-230 in the stalagmites.

In May 2003, Zhang Pingzhong, a geologist at Lanzhou University in Gansu province, and his colleagues collected an 11.8-centimeter-long stalagmite column in Wanxiang Cave, located between the Tibetan plateau and Loess plateau, a region dominated by the East Asia monsoon.

"The sample is absolutely amazing," enthuses Zhang. "It had been growing continuously since 190AD, and contained an unusually high uranium concentration and few contaminants." This allowed the team to make detailed measurements of rainfall and date monsoons far more accurately than previous studies - with an error range of decades rather than centuries.

Reporting in Science1, the researchers show that the intensity of monsoons oscillated over the past two millennia, with heavier monsoons accompanying warmer years – as determined by solar activity and proxy temperature records from, for example, tree rings.


View from inside Wanxiang CaveScience"However, this correlation has stopped since the 1960s," explains Zhang. "This may suggest that increasing levels of greenhouse gas emissions and atmospheric aerosols caused by human activities are making the monsoon weaker."
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We are the masters of our own fate. We have no one to blame but ourselves for the breakdown and demise of our life support systems. The question now is, do we have the moral courage and will to reverse the damage we have done? Our survival as a species may indeed depend on that.