Science —

Natural CO2 seeps suggest carbon storage is low risk

Is it safe to pump CO2 from the burning of fossil fuels underground for …

A volcanic carbon seep in Iceland.

Progress toward limiting carbon emissions continues to be slow, which has helped maintain interest in carbon capture and storage (CCS). This approach involves separating out carbon dioxide from other gasses, often directly from the exhaust stream of a fossil fuel power plant. This gas can then be pumped under ground for storage on time scales that can exceed thousands of years. In some cases, chemical reactions make the sequestration essentially permanent, but in others, geological features will be counted on to trap the CO2.

The potential for leaks from this storage has raised safety concerns, with some pointing to the Lake Nyos disaster in which about 1,700 people were killed by an eruption of CO2 from a volcanic source. How large a risk could carbon capture and storage pose? To get a greater feel for the typical situation, a group of Scottish researchers looked at Italy, where naturally occurring carbon dioxide seeps are common.

Italy makes a great test case because sites of natural CO2 release have been very well characterized; there's even a Web interface called Googas that shows the location of each, along with some information about its type (watery or dry, etc.) and rate of release. The authors correlated this information with cases of reported fatalities, both human and animal. These include 19 human deaths over the past 50 years, as well as a large number of dead animals, ranging from entire herds of cows to an individual fox.

Carbon dioxide can lead to asphyxiation because it's more dense than the atmosphere, and it will pool in low lying areas unless it's mixed by the wind. And, in keeping with that, the authors found that local topography played a large role in determining whether a site was likely to trigger fatalities. This included both natural depressions and man-made ones; the combination of a local seep and work on a building's foundation turned out to be a bad one. Similarly, low-temperature seeps tended to be more lethal than warm ones, as higher temperature promoted rapid mixing of the gas with the rest of the atmosphere.

Overall, the authors calculate that the risk per individual from carbon leakage is very low; only a touch more likely than winning the UK lottery, and far less likely than being killed by carbon monoxide in a UK home, or being struck by lightning in the US. It's several orders of magnitude safer than getting behind the wheel in Italy. Since the rates of release are consistent with the planned integrity of long-term storage—capable of keeping most of the sequestered carbon out of the atmosphere for over a thousand years—so the authors conclude that the risk posed by CCS is much lower than many other dangers we consider socially acceptable.

That said, there are a couple of notable caveats to this work. The first is that the volcanic seeps in Italy tend to also contain hydrogen sulphide. Although this gas is toxic on its own, the levels are typically well below those that pose a risk to human health. They are, however, well above the levels at which humans detect it as a horrific stench, meaning that many of Italy's CO2 leaks come with a natural warning system that sequestered carbon won't have.

PNAS, 2011. DOI: 10.1073/pnas.1018590108  (About DOIs).

Ars Science Video >

Apollo: The Greatest Leap


You must to comment.