* Risk of power cuts rises in coming decades as world warms
* Falling river flows, higher water temps main threats
* Shift to gas can curb water use
By David Fogarty
SINGAPORE, June 4 (Reuters) - Warmer water and reduced river flows will cause more power disruptions for nuclear and coal-fired power plants in the United States and Europe in future, scientists say, and lead to a rethink on how best to cool power stations in a hotter world.
In a study published on Monday, a team of European and U.S. scientists focused on projections of rising temperatures and lower river levels in summer and how these impacts would affect power plants dependent on river water for cooling.
The authors predict that coal and nuclear power generating capacity between 2031 and 2060 will decrease by between 4 and 16 percent in the United States and a 6 to 19 percent decline in Europe due to lack of cooling water.
The likelihood of extreme drops in power generation, either complete or almost-total shutdowns, was projected to almost triple.
"This study suggests that our reliance on thermal cooling is something that we're going to have to revisit," co-author Dennis Lettenmaier, a professor of civil and environmental engineering at the University of Washington in Seattle, said in a statement.
Thermoelectric power plants supply more than 90 percent of electricity in the United States and account for 40 percent of the nation's freshwater usage, says the study published in the journal Nature Climate Change.
In Europe, such plants supply three-quarters of the electricity and account for about half of the freshwater use.
Coal, nuclear and gas plants turn large amounts of water into steam to spin a turbine. They also rely on water at consistent temperatures to cool the turbines and any spike in river water temperatures can affect a plant's operation.
Disruptions to power supplies were already occurring, the authors noted.
During warm, dry summers in 2003, 2006 and 2009 several power plants in Europe cut production because of restricted availability of cooling water, driving up power prices.
A similar event in 2007-2008 in the United States caused several power plants to reduce production, or shut down for several days because of a lack of water for cooling and environmental restrictions on warm water discharges back into rivers, the study said.
In the past few months, large parts of the United States have suffered record heat, with March being the warmest on record for the contiguous 48 states, according to the National Oceanic and Atmospheric Administration.
The study projects the most significant U.S. impacts at power plants inland along major rivers in the Southeast.
"Considering the increase in future electricity demand, there is a strong need for improved climate adaptation strategies in the thermoelectric power sector to assure future energy security," the authors say in the study.
They also point to U.S and European laws enshrining strict environmental standards for the volume of water withdrawn by plants and the temperature of the water discharged.
Adaptation strategies include placing new plants near the sea or building more gas-fired power plants, which are more efficient and use less water.
(Editing by Ed Davies)