Hundreds of deaths caused by volcanic leaks of carbon dioxide from Cameroon to California are worrying experts seeking ways to bury industrial emissions of the gas as part of an assault on global warming.
Governments and companies are researching how to trap carbon dioxide -- a greenhouse gas released by burning fossil fuels in power plants and factories -- and then entomb it safely in porous rocks deep below ground.
However, they have done little to explain the vast costs and the risk of leaks from projects that could end up burying billions of tonnes of gas and do more to slow global warming than a shift to renewable energies such as solar or wind power.
"There may be massive public resistance, as we've seen with nuclear power" if governments fail to convince voters that storage is safe, said Bert Metz, co-chair of a 2005 UN report on carbon sequestration.
"Public acceptance...is a possible show-stopper if things are not done properly," he told Reuters during a conference of 1,000 researchers into carbon dioxide technologies in Trondheim, Norway.
Carbon dioxide is a non-toxic gas produced by respiration by animals and plants, making up a tiny 0.04 percent of the air. Levels are up 30 percent since the Industrial Revolution and most scientists say the rise is the main spur of global warming.
In pure form the gas can asphyxiate because it is heavier than air and so displaces vital oxygen.
In the worst case in recent decades, 1,700 people died after a catastrophic 1986 release of 1.2 million tonnes of carbon dioxide from the depths of Lake Nyos in Cameroon, according to the International Energy Agency.
Thirty-seven people died from a similar seismic release from Lake Monoun in Cameroon in 1984. In 1979, an explosion at Dieng volcano in Indonesia released 200,000 tonnes of the gas, smothering 142 people on the plain below.
In April this year, three ski patrol workers died at Mammoth Mountain, California, when they were overcome by carbon dioxide while trying to fence off a dangerous volcanic vent.
"Carbon storage is not risk-free but we think the risks are manageable," said Philippe Lacour-Gayet, chief scientist for research and development at Schlumberger oil and gas services group, one of many companies involved in research.
He and other experts said any greenhouse gas stores would be in geologically stable regions far from earthquake zones -- commercial carbon dioxide stores are now safely in operation in places such as Norway, Canada and Algeria.
The risks of carbon storage pale when compared with the threats of catastrophic climate change. Many scientists say rising temperatures could spur floods, droughts, heatwaves and could spread diseases and raise world sea levels.
Even so, massive storage could mean pipelines and stores under the countryside from Austria to Australia. The public might object to concentrating a normally harmless gas into a more risky form at a likely cost of tens of billions of dollars.
A strong argument for public acceptance is that people accept a host of risks every day -- flammable petrol in the fuel tanks of their vehicles, toxic natural gas piped into their homes or electricity generated from nuclear power.
"All sorts of toxic liquids and gases are already stored underground," said David Reimer, a lecturer in technology policy at the University of Cambridge in England. "Carbon dioxide poses a far lesser risk than many accepted hazards."
Berlin has an underground store for explosive natural gas near the stadium where the World Cup soccer final will be played, he noted. And acid gas is stored underground near Edmonton, Canada.
Carbon dioxide storage sites would have to be carefully chosen, and monitored for centuries.
"I'm more worried about public acceptance of the costs than of the hazards of leaks," said Frederik Hauge, head of Norwegian environmental group Bellona which favors carbon storage.
Metz's UN report said that storage could provide 15-55 percent of all the cuts in greenhouse gas emissions needed until 2100 -- probably a bigger contribution than from renewable energies or from any revival of nuclear power.
It estimates that the costs of generating electricity from a coal-fired power plant would typically rise to $0.06-$0.10 per kilowatt hour with technology to capture and store carbon dioxide from $0.04-$0.05 on a power plant with no filters.
Many companies are researching carbon storage, including Schlumberger, Alstom and oil companies such as Shell and Statoil.
Governments need to work out liability rules in case of leaks. Most experts say companies should initially be responsible but governments would take over, perhaps between five and 20 years after burial.