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"It is really quite amazing by what margins competent but conservative scientists and engineers can miss the mark, when they start with the preconceived idea that what they are investigating is impossible. When this happens, the most well-informed men become blinded by their prejudices and are unable to see what lies directly ahead of them. "

Arthur C. Clarke - Science Writer

While flows will increase overall, with some rivers becoming more swollen, many that provide water for the majority of the world's people will begin to dry up.

Some of these predicted changes are already happening. A second study shows temperature changes have affected the flow in many of the world's 200 largest rivers over the past century, with the flow of Africa's rivers declining over the past 10 years.

Veteran climate modeller Syukuro Manabe and colleagues at Princeton University modelled what effect a quadrupling of atmospheric carbon dioxide above pre-industrial levels would have on the global hydrological cycle over the next 300 years. That looks further ahead than most climate models, but the scenario is inevitable unless governments take drastic action to limit greenhouse gas emissions.

Evaporation and precipitation
 

Rising CO₂ levels will trigger higher temperatures not only at the Earth's surface, but also in the troposphere, the team says. By factoring this into the models, together with changes to levels of water vapour, cloud cover, solar radiation and ozone, the team predicted the effect that climate change would have on evaporation and precipitation.

Both would increase, the researchers found, causing the discharge of fresh water from rivers around the world to rise by almost 15 per cent. However, while water is going to be more plentiful in regions that already have plenty, the net effect will be to take the world's water further from where the people are.

"Water stresses will increase significantly in regions that are already relatively dry," Manabe reports in the journal Climate Change (vol 64, p 59).

Evaporation will reduce the moisture content of soils in many semi-arid parts of the world, including north-east China, the grasslands of Africa, the Mediterranean and the southern and western coasts of Australia. Soil moisture will fall by up to 40 per cent in southern states of the US, Manabe says.

Desert irrigation
 

The effects on the world's rivers will be just as dramatic. The biggest increases will be in the thinly populated tropics and the far north of Canada and Russia. For instance, the flow of the river Ob in Siberia is projected to increase by 42 per cent by the end of the 23rd century.

This prediction could encourage Russia's plans to divert Siberian rivers to irrigate the deserts around the Aral Sea (New Scientist, 9 February 2004).

Similar changes could increase pressure from the US for Canada to allow transfers from its giant Pacific rivers to water the American West. Manabe predicts a 47 per cent increase in the flow of the Yukon river.

By contrast, there will be lower flows in many mid-latitude rivers which run through heavily populated regions. Those that will start to decline include the Mississippi, Mekong and especially the Nile, one of the world's most heavily used and politically contested rivers, where his model predicts an 18 per cent fall in flow.

"Profound challenge"
 

The changes will present a "profound challenge" to the world's water managers, Manabe says. They are also likely to fuel calls for a new generation of super-dams and canals to move water round the planet, like China's current scheme to transfer water between north and south.

Some of the findings are controversial. The UK Met Office's climate model predicts that flows in the Amazon could fall this century, while Manabe's team predicts greater rainfall could increase its flow by 23 per cent.

And while Manabe foresees a 49 per cent increase in the flow of the Ganges and Brahmaputra rivers that drain the Himalayas, an international study reported that the Ganges would lose flow as the glaciers that feed it melt away (New Scientist print edition, 8 May 2004).

Time delay
 

Meanwhile, a team of researchers in France say that climate change is already affecting the world's rivers. David Labat and colleagues at the government's CNRS research agency in Toulouse reconstructed the monthly discharges of more than 200 of the world's largest rivers since 1875.

They took discharge data held by the Global Runoff Data Centre in Germany and the UNESCO River Discharge Database and used a statistical technique to fill in gaps left by missing data, or changes to run-off caused by dams and irrigation projects (Advances in Water Resources, DOI: 10.1016/j.advwatres.2004.02.020).

Their findings reveal that changing temperatures cause river flows to rise and fall after a delay of about 15 years, and the team predicts that global flows will increase by about 4 per cent for every 1 °C rise in global temperature.

However, climate change over the past few decades has already caused discharge from rivers in North and South America and Asia to increase. Run-off in Europe has remained stable, but the flow of water from Africa's rivers has fallen.

Fred Pearce