But Greenpeace issued a 44-page report about the technology entitled "False Hope".

"Carbon capture and storage is a scam. It is the ultimate coal industry pipe dream," said Emily Rochon, climate and energy campaigner at Greenpeace International and author of the report.

Greenpeace and 112 green groups from 21 nations said governments should invest in wind, solar and other renewable energies rather than in capture technologies that would allow coal-fired power plants to stay in operation.

In a statement linked to the report, Greenpeace and allies including Friends of the Earth International said the "false promise" of carbon capture and storage (CCS) "risks locking the world into an energy future that fails to save the climate".

But some other environmental groups accept carbon capture as a way to slow rising temperatures and avert more powerful storms, heatwaves, droughts, disrupted monsoon rains and raised world ocean levels.

"Carbon capture and storage is not an ideal solution, but it buys us time," said Stephan Singer, head of the WWF's European Climate and Energy Programme in Brussels. "We believe it is part of the solution -- an emergency exit."

The UN Climate Panel has said CCS could be one of the main ways for slowing climate change by 2100 -- contributing a bigger share of greenhouse gas cuts than energy efficiency, a shift to renewable energy or a push for nuclear power.

CHINA COAL

Singer said China was opening one or two coal-fired power plants a week and, with a lifetime of 40 years, the world needed ways to retrofit plants to capture emissions rather than expect Beijing to close them down.

Greenpeace said carbon capture technology was largely unproven, could not be deployed on a large scale before 2030, was expensive and brought risks of leaks. It said it would mean electricity price hikes of between 21 and 91 percent.

But Oslo-based environmental group Bellona said 34 CCS projects were being planned in Europe alone. "If you exclude CCS in the battle against climate change, you don't take global warming seriously," said Bellona head Frederic Hauge.

Several national branches of Friends of the Earth did not sign up for the statement criticising CCS.

"We believe that CCS will be an important tool to reduce emissions from existing coal and gas-fired power plants," said Lars Haltbrekken, head of Friends of the Earth Norway. "We don't support new coal-fired power plants, even with CCS."