Dams: Deep trouble

Every schoolchild is told how the water cycle works. Most of us can even remember the diagram from geography lessons, showing water evaporating from the ocean, forming clouds in the sky and then falling back to the ground as rain, where it runs into streams and rivers and, finally, back into the sea.

The reality is more complicated, with scientists arguing over the finer points of balancing the "water budget" – how much water should stay on land, and how much should be left to flow into the sea. Now, suddenly, this debate has become of more than academic interest.

The reason for the sudden interest is a new study that attempts to calculate the effect of man-made dams and reservoirs on how much water returns to the oceans. The results are startling: so much water is now stored in dams that it's having a profound influence on the rate at which sea levels are rising.

Predictions for sea-level rises due to global warming affect hundreds of millions of people at risk from coastal flooding. The new research suggests that, over the past 50 years, new dams and reservoirs have held back some 10,800 cubic kilometres of water, which would have been enough to raise global sea levels by about 30mm. In other words, the rises we have seen so far due to global warming might have been considerably larger if it were not for the huge numbers of dams and reservoirs built from the 1950s onwards.

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