The IEA said pilot projects for capturing gases -- emitted mainly by burning fossil fuels in power plants, refineries, and factories -- now accounted for just 0.05 percent of the potential total by mid-century.
"There is a need...for a worldwide agreement on the legal challenges," Claude Mandil, executive director of the IEA, which advises 26 industrialised nations, told a news briefing in Oslo during a conference on carbon capture and storage.
He said most IEA estimates showed that carbon capture would be the top contributor to curbing greenhouse gases by 2050, behind only energy efficiency savings and ahead of renewable energies and nuclear power.
Until now, most work has focused on ways of cutting the prohibitively high costs of capturing, piping away and entombing the gases under a broader UN goal of averting climate change such as more droughts, floods and rising sea levels.
But a legal framework was also urgently needed, in tandem with development of cheaper technologies, Mandil said.
He noted that most projects so far were small, such the Sleipner gas field off Norway where Statoil has been filtering out and reinjecting about a million tonnes of carbon dioxide a year since 1996 to avoid a carbon emissions tax.
"Carbon capture and storage potential is 6 billion tonnes a year by 2050," he told an audience of about 200 experts. "Pay attention to the scale. That means 6,000 Sleipner projects."
Six billion tonnes of carbon dioxide is roughly equivalent to current emissions by top emitters the United States and China. Carbon capture would help coal-fired power plants keep operating, he noted.
Mandil that there were unsolved legal questions about who would be liable in the event of leaks from carbon stores, for instance in 20, 50 or 100 years' time, and about issues such as property rights, waste and technology transfers.
The IEA issued a 140-page booklet about legal aspects of storing carbon dioxide outlining the problem but stopping short of giving clear recommendations, for instance about whether governments should assume liability for leaks in the long term.
The booklet also said that uncertainty about the rules should not block approval of new pilot projects in coming years.
"We need about 10 full-scale carbon capture and storage demonstration projects by 2015," Mandil said. "That's challenging but possible."
He praised Norway, the world's number five oil exporter, for technological advances at Sleipner and for planning to fit future gas-fired power plants with carbon capture and storage.
"Carbon capture will undoubtedly be an important part of the answer" to climate change, Norway's Oil and Energy Minister Odd Roger Enoksen said. "But the cost of carbon capture and storage has to be brought down."