Poor India! It seems like only yesterday that the severe droughts in the country provided heart-rending footage for our television screens. That was three months ago; now it must suffer from the opposite extreme. The monsoon floods in Andhra Pradesh are the worst experienced in decades. Swollen rivers have left thousands homeless, destroyed crops and killed more than 160 people. This follows earlier catastrophic flooding in other parts of India, Bhutan, Nepal and Bangladesh.
But just as we could not blame global warming for the pre-monsoon droughts earlier this year, neither can we blame it for the floods. Extremes might be said to be the norm in India; the summer monsoon rainfall, averaged over the whole country, has been stable for 120 years since it was first measured and shows no signs of systematic change. It is characterised, however, by a high degree of variability, both from year to year and place to place. Droughts and floods are virtually certain in India nearly every year.
The Indian monsoon cycle is familiar. Every spring, the arid soil is baked lifeless by the sun and desiccating winds. As the heat of the summer sun intensifies, the air over the Asian landscape expands and becomes lighter, causing low pressure to develop over the continent. The anti-clockwise swirl of winds around this low draws warm, moist air from the Indian Ocean over India from the southwest. In June or early July, just as heat becomes unbearable, clouds that have been piling up in the distance for some time burst open to disgorge torrential rains.
The monsoon rains continue intermittently until late September, delivering about 90 per cent of the sub-continent's annual rainfall in a few months. They are a vital life-giving force, but sometimes, as we have seen, there can be too much of a good thing. With the coming of autumn, the continental anticyclone over Siberia re-asserts itself and brings back the dry north-easterly winds of the winter season. And so the cycle begins again.
The extreme character of Indian rainfall is illustrated by the pattern at Cherrapungee, a little town south of the Himalayas on the Shillong Plateau in the far north-east. It holds two longstanding records: between August 1860 and July 1861, it experienced 26,000 mm of rain, the most anywhere in the world in a 12-month period; and the 9,300 mm measured in July 1861 is a global record for a calendar month. Cherrapungee's average yearly total of 11,430 millimetres is about 10 times that which we experience in our allegedly very wet and soggy Ireland.