Flood of floods: here comes the rain

IT WAS a monstrous monsoon. Over just a few days in late July last year, more than 300 millimetres of rain fell on northern Pakistan. As the water swept down the river Indus, it killed close to 2000 people and affected 20 million more.

Pakistan was not the only place to suffer. Australia, China, Thailand, Brazil, the Balkans, Bangladesh, Indonesia, Colombia, the Philippines, Sri Lanka and Tennessee all experienced devastating floods in the past year. What’s more, there were unusually heavy snowfalls in many regions, severely disrupting transport systems. Globally, 2010 was the wettest year ever recorded (see chart).

Inevitably after such freakish weather, people ask whether climate change is to blame. Is this flood of floods our fault? And is there worse to come?

There is no doubt that temperatures are rising, and basic physics suggests that warmer means wetter, because warmer air can hold more moisture. Observations confirm that the lower atmosphere holds about 5 per cent more water than a century ago, giving it that much more ammunition to unleash in a downpour or blizzard.

So does this explain the recent floods? “I don’t think it is legitimate to assume that climate change played a role in these events until we’ve done the work,” says Myles Allen of the University of Oxford. There are many types of floods, he says, and while climate change is making some extreme events more likely, others – such as floods caused by melting snow – may become less likely.

Others think we can say more. “The pervasive increase in water vapour changes the intensity of precipitation events with no doubt whatsoever,” Kevin Trenberth of the US National Center for Atmospheric Research told a meeting in January. “Yes, all events. Even if temperatures or sea surface temperatures are below normal, they are still higher than they would have been, and so too is the atmospheric water vapour amount and

thus the moisture available for storms.”

For instance, El Niño’s sister phenomenon, La Niña, which brings warmer surface waters – and thus moister air – to the western Pacific, has been blamed for some of the recent floods in Asia and Australia. Trenberth thinks high sea surface temperatures due to global warming also played a role. “The La Niña in place favours these sorts of things but the extra high sea surface temperatures make them record breaking,” he says.

It is clear is that rainfall patterns are changing. In the US, for instance, Kenneth Kunkel of the National Climatic Data Center in Asheville, North Carolina, is analysing data from more than 1000 rain gauges across the country. Over the past century, the intensity of extreme rainstorms that occur once per year on average has risen 1.4 per cent per decade, he has found.

Put another way, in 2000, a “once a year” storm was dumping 14 per cent more rain on average than it would have done in 1900. “It would be wise for society to take this into account when building a dam to last 50 or 75 years, or a new housing development,” says Kunkel. “That’s not really done right now.”

At the top end of the scale, the intensity of “once in 20 years” extreme rainstorms is increasing even faster, by about 3 per cent per decade, although for these rare events the statistics are more patchy. The overall increase also seems to be accelerating. Most of it happened in the last three decades, and Kunkel hasn’t even included 2010 in his analysis yet. “I’m guessing that when the data come in, they are going to be high,” he says.

Bucketing down

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Flood of floods: here comes the rain