Pakistan floods "may be caused by climate change"

13 August 2010

Source: BBC News

The UN estimates that the humanitarian crisis caused by the recent floods in Pakistan's Indus Valley is larger than the combined effects of the three worst natural disasters of the past decade.  Some experts attribute the severity of the floods to climate change.

A once-in-a-century flood?

The headline figure of 1,700 killed masks the real scale of the disaster that has displaced 14 million people.

Scientists have described this catastrophe as a once-in-a-century flood.  But could climate change mean that floods of this magnitude, or even bigger, become a more regular occurrence?

The Indus is one of the world's great rivers. From its headwaters in the Himalayas of Tibet, it flows north-west through India before turning sharply south across Pakistan. It finally discharges into the Arabian Sea, a journey of some 3,200km (2,000 miles).  Although some of its water comes from melting Himalayan glaciers, the vast majority is dumped by the summer monsoon. As torrential rain sweeps in from the Indian Ocean, floods are triggered almost annually.

The Indus: friend and foe

Humans have had long experience of Indus floods. Its floodplain was an early cradle of civilisation 9,000 years ago. Here people first gave up their nomadic ways to farm livestock and cultivate crops. Today, the Indus Valley is home to 100 million people, who rely on it completely for drinking water and irrigation. To many, it is "the Great Mother".  Yet, as the catastrophic floods of August 2010 demonstrate, the Indus is both friend and foe.

Ancient history of floods

Geologists are working round the clock to better understand the ancient flood history of the Indus River.  "Such history lessons will help to better predict its erratic behaviour and "plan for our own uncertain future", said Professor Peter Clift of Aberdeen University, an expert on the Indus River. His team recently used makeshift "rigs" to drill down into the sands and mud of the Indus floodplain. By precisely dating layers of flood-deposited sand, they were able to work out past changes in river flow. Their results were startling.

During a warm period 6,000 years ago, the Indus was a monster river, more powerful and more prone to flooding than today.  Then, 4,000 years ago, as the climate cooled, a large part of it simply dried up. Deserts appeared whether mighty torrents once flowed.

Professor Clift believes that this failure of the Indus may have triggered the collapse of the great Harappan civilisation.  The city ruins of Mohenjo-daro, a relict of this lost culture, date from the time when the rivers ran dry.

But what caused these thousand-year cycles of Indus drought and flood?

Monsoon changes

Professor Martin Gibling of Dalhousie University in Halifax, Canada, a river expert who has worked in the region, thinks that changes in the strength of the monsoon caused by climate change may be to blame.

He explained: "Monsoon intensity is somewhat sensitive to the surface temperature of the Indian Ocean. During times of cooler climate, less moisture is picked up from the ocean, the monsoon weakens, and the Indus river flow is reduced."

Professor Rajiv Sinha, from the Indian Institute of Technology in Kanpur, who has had first hand experience of the floods, takes a more strident position.

"What all the climate models predict is that the distribution of monsoon rains will become more uneven in the future," he told BBC News. "Total rainfall stays the same, but it comes in shorter more intense bursts."

In August 2010, more than half of the normal monsoon rain fell in only one week. Typically it is spread over three months.

Professor Sinha remarked: "Rivers just can't cope with all that water in such a short time. It was five times, maybe 10 times, more than normal."

So, if the unusually intense 2010 monsoon is the shape of things to come - and that is uncertain - the future may hold more flood misery for the people of Pakistan.

River management may also have played a part

Climate change may not be the only cause of Pakistan's woes. There is also a sense that the current floods have been exacerbated by the way the Indus has been managed.

In the UK, flood risk is reduced by building levees (embankments) along vulnerable part of rivers. These barriers prevent them from bursting their banks in extreme floods. It is a system that has served well for generations.