Cultivation changed monsoon in Asia

Loss of forests in India, China during the 1700s led to a decline in monsoon precipitation

The dramatic expansion of agriculture in India and southeastern China during the 18th century — a sprawl that took place at the expense of forests — triggered a substantial drop in precipitation in those regions, a new study suggests.

Winds that blow northeast from the Indian Ocean into southern Asia each summer bring abundant rain to an area that’s home to more than half the world’s population. But those seasonal winds, known as monsoons, brought about 20 percent less rainfall each year to India and southeastern China in the 1850s than they did in the early 1700s, says Kazuyuki Saito, a climate scientist at the University of Alaska Fairbanks. That decline, he and his colleagues contend online June 1 in the Proceedings of the National Academy of Sciences, is the result of deforestation in the region.

In 1700, forests covered between 40 and 50 percent of India and China. But by 1850 that proportion had shrunk to between 5 and 10 percent, Saito says. The substantial decline in forests dramatically reduced the amount of moisture pulled from deep in the soil and sent skyward by trees — moisture that typically would have joined that present in the monsoon winds flowing from the ocean. The overall reduction in moisture, in turn, triggered a substantial slump in soil-dampening precipitation, the researchers note.

Western India, for example, received 20 centimeters (7.9 inches) less monsoon rainfall in 1850 than it did in 1700. The resulting drop in atmospheric humidity also led to a decline in cloud cover, which boosted heat at ground level and dried surface soil even further.

In their study, Saito and his colleagues used a global climate model to confirm the effect of deforestation in the region. One scenario depicted the forest coverage present in 1700, and the other included the reduced coverage present in 1850. Both simulations used month-to-month variations in sea-surface temperatures and sea ice coverage that reflect modern-day conditions. The decline in monsoon precipitation seen in the computer simulations mimicked the extent and pattern of those seen in the real world, the researchers report.

“What’s really exciting about this study is that you can rule out things such as greenhouse gases,” says Roger A. Pielke Sr., an atmospheric scientist at the University of Colorado at Boulder. The new findings “show that the climate system is more complex and less predictable than scientists had recognized.”

Many of the suspected causes of climate change probably had little to do with this drop in Asia’s monsoon precipitation, Saito and his colleagues contend.

Noteworthy changes in the atmospheric concentrations of greenhouse gases such as carbon dioxide didn’t begin to occur until after the 1850s. Solar activity varied on its normal 11-year cycle between 1713 and 1850, but there were no apparent long-term trends in activity over that period, they note. Several major volcanic eruptions occurred during the interval, but the cooling effects triggered by any individual volcano would have lasted only a few months or years. Sea-surface temperatures didn’t exhibit any unusually large variations, the researchers add.

Together, these observations tend to pin the blame on deforestation, the researchers say.

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