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.

FIZZY RELEASE

In natural lakes much of the gas is broken down to less-insulating carbon dioxide as it drifts to the surface.

But when the methane-rich water from the bottom of reservoirs is fed into power-generating turbines, the pressure drops, so the gas fizzes out like the bubbles when a soft drink can is opened.

But Brazil's Lima said there is already a solid body of research, and a link with significant methane emissions is borne out by a levelling off of emissions around the turn of the century, after a slowdown in dam construction.

"If hydroelectric dams are really important to this, then the atmospheric methane would respond to the dams," he told Reuters in a telephone interview.

Other scientists say the levelling off in emissions may have been a result of the destruction of wetlands -- which are major methane emitters -- offsetting a rise in emissions from natural gas fields.

China can at least rest easy that no one will be pointing a finger directly at its prestige project, the world's single-largest hydropower project, centred around a massive dam that flooded one of the country's most famous natural sites and threatens to cause erosions and landslides. [ID:nPEK73224]

"With the Three Gorges, the amount of power produced means compared to coal it looks good," said IRN's McCully, adding it is still not an ideal answer to climate change concerns.

"Because it is still very large, there could be quite sizeable emissions," McCully added. "At the top end, where the reservoir is shallower, the problems are exacerbated."

NEW SOURCE OF GAS?

Lima said his research was not intended to demonise dams, and instead he would like to see governments change reservoir design to minimise emissions and trap the rest for power. The technology exists to do both, he says.

A question mark hangs over how willing Beijing or its power firms will be to invest time, money and expertise in tackling a problem so new it is not even really on the U.N. agenda.

But Beijing is concerned about the growing financial and diplomatic burden generated by its reliance on overseas oil and gas, currently meeting around half the country's needs.

Methane is the main component of natural gas and he estimates that in China alone around 2.6 million tonnes could be collected from dams for additional power generation, or the equivalent of over seven months of natural gas imports.

"We are wasting an important source of energy -- we might be able to extract it and produce power, and it is renewable," Lima said.