Climate change may be having an effect on one of the world’s most important weather systems. A study of the monsoon season over the last sixty years reveals that the intensity of extremes of precipitation and the number of very dry spells have both been increasing.
The Indian subcontinent gets more than four-fifths of its annual rainfall in that great, sustained seasonal cloudburst called the south Asian summer monsoon. Since 60 per cent of India’s people live by agriculture, and 70 per cent of the country’s exports are agricultural products, a lot rides on the monsoon.
Extremes of rainfall can bring catastrophic flooding, death, disease and homelessness. Dry seasons can bring a threat to India’s economy, and to national and even to global food security.
So a pattern of wetter wet spells and longer dry spells would – if sustained – create problems for the one-sixth of the world’s population that depends on the timing and strength of the monsoons.
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My grandfather grew up in a village that is primarily dependent on agriculture, and the farmers that live there say that the monsoon rainfall pattern has changed. They’ve noticed over the last decade that rainfall occurs in heavy bursts and comes earlier in the monsoon season, and that dry spells last longer
Deepti Singh, Stanford University in California
Deepti Singh of Stanford University in California and colleagues report in Nature Climate Change that they used statistical tools to compare rainfall patterns over two contrasting periods, from 1951 to 1980, and from 1981 to 2011.
No rush to judgement
The monsoon season is a complicated one: it generally begins in southern India in June, and is over by September. By mid-July, the rains cover the entire subcontinent. But what matters is how much rain falls, when it falls, and how long the intermittent dry periods last.
The researchers found that overall the total rainfall has declined, but the variability of precipitation during the peak monsoon months has increased. The wet spells are more intensely wet, and the frequency of the dry spells has increased. But it may be too early to be sure that global warming is behind the changes.
“The statistical techniques show that the changes in these characteristics are robust and that these changes are unlikely to happen purely by chance,” said Singh.
The finding matches anecdotal evidence. “My grandfather grew up in a village that is primarily dependent on agriculture, and the farmers that live there say that the monsoon rainfall pattern has changed. They’ve noticed over the last decade that rainfall occurs in heavy bursts and comes earlier in the monsoon season, and that dry spells last longer.”
And her co-author, Noah Diffenbaugh of Stanford, said: “There are many predictions that global warming should cause heavier downpours and more frequent dry spells. That’s what we’ve found here, but India is a complex region, so we want to be sure before we point the finger at global warming or any other cause.”
