Dreams of braking global warming by storing carbon emissions from power plants could be undermined by the risk of leakage, according to a study published on Sunday.
Rich countries have earmarked tens of billions of dollars of investment in carbon capture and storage (CCS), a technology that is still only at an experimental stage.
Under CCS, carbon dioxide (CO2) would be snared at source from plants that are big burners of oil, gas and coal.
Instead of being released into the atmosphere, where it would contribute to global warming, the gas would be buried in the deep ocean or piped into underground chambers such as disused gas fields.
CCS supporters say the sequestered carbon would slow the pace of man-made warming. It would buy time for politicians to forge an effective treaty on greenhouse gases and wean the global economy off cheap but dirty fossil fuels.
Critics say CCS could be dangerous if the stored gas returns to the atmosphere. They also argue that its financial cost, still unknown, could be far greater than tackling the source of the problem itself.
The new research, published by the journal Nature Geoscience, wades into the debate with an estimate of capturing enough carbon to help limit warming to two degrees Celsius (3.6 degrees Fahrenheit), the figure set in last December's Copenhagen Accord.
Storing CO2 in the ocean will contribute to acidification of the sea, with dangers that reverberate up the food chain, says its author, Gary Shaffer, a professor at the Danish Centre for Earth System Science in Humlebaek, Denmark.
It also carries a higher risk of being returned to the atmosphere by ocean currents and storms.
Underground storage is a better option, but only if the geological chamber does not have a significant leak or is breached by an earthquake or some other movement, says the paper.
The gas will have to be stored for tens of thousands of years to avoid becoming a threat to future generations, a scenario similar to that for nuclear waste, it says.
This means less than one percent of the stored volume can be allowed to leak from the chamber per 1,000 years.
To offset any bigger leak, re-sequestration would be needed - in other words, grabbing an equivalent amount of CO2 from the air and storing it. But this would be a cost burden that could last for millennia.
"The dangers of carbon sequestration are real and the development of CCS should not be used as a way of justifying continued high fossil fuel emissions," said Shaffer.
"On the contrary, we should greatly limit CO2 emissions in our time to reduce the need for massive carbon sequestration and thus reduce unwanted consequences and burdens over many future generations from the leakage of sequestered CO2."
Until only recently, CCS was widely dismissed as fantasy or a last-ditch option. In the last five years, though, it has become progressively enshrined as a favourite policy among rich economies.
In 2008, the Group of Eight (G8) summit recommended launching 20 large-scale CCS demonstration projects by 2010.
This target "remains a challenge," according to the International Energy Agency (IEA) in Paris.
It estimates that over the past two years, countries have committed 26 billion dollars in CCS projects. Thanks to this funding, "between 19 and 43" large-scale demonstration projects would be launched by 2020.
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