BEIJING - China is scrambling to build massive hydropower dams to curb pollution and slake its thirst for energy, but scientists warn that reservoirs can also worsen global warming by emitting a powerful greenhouse gas.
Methane, which traps heat much more efficiently than carbon dioxide, is produced by plants and animals rotting underwater and released when that water rushes through hydropower turbines.
In a country that is already the world's top hydropower generator and aims to more than double capacity, dams could raise methane emissions by around 8 percent, recent research shows.
The flammable gas could also be trapped and used for power generation if dam designs were adapted, providing Beijing with cheap and clean energy instead of a global warming burden.
But the data is so new that even United Nations rules on calculating national emissions do not require dams to be included, dimming the chances of fast action.
China is set to overtake the United States as top producer of carbon dioxide this year and is the leading emitter of acid-rain causing sulphur dioxide. As part of a bid to constrain emissions growth, it is promoting renewable energy.
A string of hydropower reservoirs are the centrepiece of this plan, and their methane emissions may offset many benefits.
Worldwide, dams could generate the equivalent of one-fifth of methane from all other sources, a study by Ivan Lima from Brazil's National Institute for Space Research showed.
Shallow tropical reservoirs pose the biggest problem, with the worst producing more methane per unit of power than some fossil-fuel burning options, scientists and activists say.
China's dams are mostly quite deep and in temperate zones, both emissions-mitigating factors. But Beijing's vast network and ambitious expansion plans mean it is still a serious concern.
"China has around half of all the world's large dams, so the chances are there are a lot of reservoirs in conditions to be high emitters," said Patrick McCully, executive director of International Rivers Network (IRN).
"If you have very polluted reservoirs, particularly with a lot of sewage entering reservoirs, you have the situation to create a lot of methane and obviously China has some very bad water quality problems," McCully added.
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