The growing volumes of fresh water held behind dams in the world's artificial reservoirs have had an appreciable mitigating effect on rising sea levels, according to a surprising study published today in the journal Science. Researchers at Taiwan's National Central University carried out the first comprehensive global assessment of water held in reservoirs. They concluded that global warming would have raised sea levels by an additional 3 cm over the past 50 years, as a result of melting ice, if 10,800 cubic kilometres of water had not be impounded in reservoirs. The study may help to sort out some of the current scientific uncertainties over the causes of rising sea level. The two main factors are thermal expansion of the oceans as they warm up and the addition of water from melting glaciers and ice sheets - but their relative contributions have not been clear. The Taiwanese researchers say the impounding effect of dams and reservoirs has concealed the real extent of ice melt. Sea levels would have risen at least 20 per cent faster since the 1950s, if no new reservoirs had been built. Looking to the future, they say reservoir building is likely to slow down for environmental and financial reasons - leading to an acceleration of sea level rise. Copyright The Financial Times Limited 2008