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Human impact on the environment could drastically alter the state of the monsoon, the lifeblood of India, with disastrous results for the subcontinent, climate researchers have warned.

The monsoon could run dry, spelling disaster for its rain-fed agriculture, or become more intense due to global warming caused by rising levels of atmospheric carbon dioxide which would be equally bad, as illustrated by the recent deluge in Maharashtra where over 1,000 people were killed.

The worst case scenario, they say, would be 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”, Kirsten Zickfeld and her colleagues at the Potsdam Institute for Climate Impact Research were quoted by Nature magazine as saying.

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 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 letters.

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.

Hans Joachim Schellnhuber, Zickfeld’s co-worker and director of the Tyndall Centre for climate change research in Norwich, UK, were quoted as saying, “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 stresses.