BBC News

Will the Pakistan floods strike again?

By Howard Falcon-Lang
Science reporter

image captionMillions of people have been displaced by the floods

The recent floods in Pakistan's Indus Valley are of truly Biblical proportions.

The UN estimates that the humanitarian crisis is now larger than the combined effects of the three worst natural disasters to strike in the past decade.

These include the Asian tsunami and the major earthquakes that devastated Kashmir and Haiti.

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

As I write, the southern city of Hyderabad, with a population of 1.5 million, stands on the brink of inundation as peak floodwaters surge downstream.

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 "Great Mother"

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.

Humans have had long experience of Indus floods.

image captionSatellite images show flood waters swelling the Lower Indus river; flow through the Sukkur barrage has exceeded what it was built to withstand

Its floodplain was one of the early cradles of civilisation 9,000 years ago. Here people 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.

History lessons

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 ending about 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?

Perfect storm

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: "Although many factors are involved, monsoon intensity is especially 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."

So, will global warming have the reverse effect, returning the Indus to the monster river of 6,000 years ago?

"That is the million-dollar question", said Professor John Clague, from Simon Fraser University in Burnaby, Canada, an expert on the Asian monsoon.

"There is huge uncertainty… and this is a matter of heated debate amongst scientists at present."

However, Professor Rajiv Sinha, from the Indian Institute of Technology in Kanpur, who has had first hand experience of Asian river 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.

'When the levee breaks'

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.

image captionPakistan has experienced one of the most intense monsoons in decades

But Pakistan's rivers are different.

UK rivers carry very little sand and mud. In contrast, the Indus is choked with sediment eroding off the Himalayas. Building levees causes the river channel to silt up.

This has the unexpected effect of making Pakistan's rivers prone to even bigger floods when the levees eventually break.

"What we've done is apply a system from the West that just doesn't work [in South Asia]," said Professor Sinha.

That problem has been made worse by deforestation. Trees protect the headwaters from erosion. But over the past half century, more sediment has been flushed down the rivers as forests have been cut.

However, Dr James Dalton, water management advisor to the IUCN, said that "building levees also brings huge benefits and is essential for managing agriculture, but such systems cannot cope well with extreme events."

Unpredictable future

Our understanding of why the Indus Valley is prone to catastrophic floods is steadily improving.

However, this will be of no consolation for those displaced by the worst humanitarian crisis in a decade.

And it is likely to become increasingly difficult to predict the future flood patterns of the Indus. Climate change will probably mean that monsoon rains are increasingly erratic.

History tells us that the "Great Mother" is fickle. For the 100 million people who call the floodplain home, the future is uncertain.