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Monsoons may dry up

August 15, 2005 By Philip Ball This article courtesy of Nature News.

Land use changes in India could turn off the rain.

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The Indian monsoon, which waters India's agriculture, could run dry because of human impacts on the environment, a team of climate researchers has warned.

Kirsten Zickfeld and her colleagues at the Potsdam Institute for Climate Impact Research say that the monsoon has two major settings: on, as at present, and off, when it produces very little rainfall. A switch-off would be catastrophic for India's main crop, rice, which depends on heavy monsoon rains.

Even a minor change in monsoon timing or intensity can have a big impact. "If the rains are delayed by just a few days, that affects the agricultural yields," says Zickfeld. The monsoon's disappearance would wreak havoc, probably requiring Indian farmers to completely change their crops and methods.

Zickfeld and colleagues have shown that changes in land use and air pollution on the Indian continent are pushing conditions towards the off state. They don't know if or when it might happen, but they say there is cause for concern.

The pressure's on

The monsoon is driven by an air-pressure difference between the land and the Indian Ocean. Usually, the hot season creates low-pressure zones over the warm continent. Air rushes in from the higher-pressure zone over the water, bringing rain to the land.

Anything that reduces this pressure difference - such as cooler land temperatures - can weaken the monsoon. And once the weakening exceeds a certain threshold, the climate switches into a new state in which moist air over the ocean is no longer carried inland, they report in Geophysical Research Letters1.

In India and southeast Asia, several factors are causing less sunlight to warm the ground. There are more aerosols, because of industrial growth and greater vehicle use, which reflect light back into space. And clearing forests for farmland is replacing dark, light-absorbing treetops with lighter, more reflective soil.

"This raises a red flag", says Hans Joachim Schellnhuber, Zickfeld's co-worker and director of the Tyndall Centre for Climate Change Research in Norwich, UK. "If we continue to change land cover, and at the same time aerosols increase, we're moving towards the 'off' point."

Schellnhuber says there are signs that the Chinese monsoon is weakening, perhaps for the same reasons. "It's not science fiction," he says.

Roller coaster

Zickfeld and colleagues admit that their model is crude, so it can't predict when, or even if, current trends will trigger a change in the state of the monsoon.

But their work's simplicity also has its advantages. Complicated computer models have suggested that the monsoon might change, but it has been hard to pick out the crucial causes.

The researchers add that global warming, caused by rising levels of atmospheric carbon dioxide, might make the monsoon more intense, increasing rainfall. That could be equally bad for the area, as illustrated by the 1,000 people killed in Mumbai last week in floods due to abnormally heavy monsoon rains.

The worst case would be what Zickfeld's team calls a 'roller-coaster effect': drying of the monsoon, followed by the return of an even more intense wet monsoon as aerosol emissions are cleaned up but carbon dioxide goes on increasing. Such a series of changes "would seriously challenge the adaptive capabilities of India's rural society", the researchers say.

References

  1. Zickfeld K., et al. Geophysical Research Letters, 32. L15707 (2005).

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