Seepage of carbon dioxide from long-term carbon capture and storage projects may lead to delayed global warming unless the gas can be tightly controlled, according to a new study.

Unless the seepage rate of sequestered carbon dioxide can be held to 1 percent every 1,000 years, overall temperature rise could still reach dangerous levels that cause sea level rise and ocean acidification, concludes the research published yesterday in Nature Geoscience.

The delayed warming resulting from escapes of gas would occur gradually for hundreds of years, but could be problematic and expensive for future generations who would have to figure out how to recapture the C02 from the atmosphere, said study author Gary Shaffer, a professor at the University of Copenhagen and the University of Concepción in Chile.

"It may be a useful thing to carry out this carbon sequestration, but there are dangers, and the best thing would be to decrease emissions in other ways that make it unnecessary," said Shaffer.

Carbon capture and sequestration, or CCS, has never been proved at scale. It envisions grabbing C02 from an industrial process or a power plant stack, piping the gas to a suitable storage spot and injecting it deep beneath the Earth's surface.

The technology is considered critical for the survival of coal in a carbon-constrained world, considering that the fossil fuel produces almost 50 percent of U.S. electricity and about a third of the nation's carbon dioxide emissions.

What if 1 percent leaked?
Billions of dollars are committed to research and deployment of CCS globally, but the commercial operation of equipment on all the nation's coal plants could be decades away. Engineers also are considering deploying the technology on other sources of emissions, such as natural gas plants.

Shaffer examined several leakage scenarios of roughly 3,000 gigatons of stored C02, which would take two centuries globally to accumulate after multiple underground injections. He then examined how the world's temperatures would fare if that amount of gas leaked at rates of 1 percent every 10, 100, and 1,000 years.

"The whole reason for doing this study is because C02 in the atmosphere has a very long lifetime," Shaffer said.

A leakage rate of 1 percent every decade could be "very serious," he said, and would eventually lead to temperature spikes of about 3 degrees Celsius in the next century and a rise close to 4 degrees Celsius over the following 2,000 years. Many scientists have argued that warming needs to stay within the range of 2 degrees Celsius above pre-industrial temperatures.

Stored gas leaking at 1 percent every 100 years would keep the temperature rise close to 2 degrees Celsius, Shaffer said. Storing gas underneath the ocean floor is a "poor choice," he said, since gas leaking at the same rate could spur acidification and harm corals and other sea life.

A 1 percent leak rate every 1,000 years would keep warming levels "manageable," Shaffer said. The study predicts, for example, that warming would hover below the 2 degrees Celsius threshold for the next 20,000 years at that rate.

Experts want more research, more demonstrations
Because CCS has never been tried at a commercial scale, it is impossible to pinpoint the exact leakage rate that would occur with 3,000 gigatons of gas, analysts say. A 2005 Intergovernmental Panel on Climate Change report on carbon capture and storage, which guided Shaffer's modeling, said it was "very likely" that 99 percent of stored C02 would stay in place over the first 100 years and "likely" that the same percentage would stay in place over 1,000 years.

On Friday, several CCS experts said that the results underscore why there needs to be more research on CCS, rather than less. The U.S. Department of Energy publishes a national atlas of storage capacity by state, but it is critical to "get wells in the ground" to get more data about exact leakage speeds and optimal storage spots for C02, said Sarah Forbes, an analyst at the World Resources Institute.

State and federal rules governing who would be responsible in case of leaks would speed the research process, she said. As an example, she said it needs to be clear that an operator overseeing a well is held responsible for any gas leak.

Sean McCoy, an engineer at Carnegie Mellon University, said that the stability of stored gas increases over time because C02 dissolves in briny water and eventually forms solid carbon formations, making it less likely to creep back up the surface. Shaffer's discussion of C02 seeping into the ocean is less of a concern, he said, because much of the CCS research focus is now on storage spots in land formations.

"The bottom line is that if you select the right storage sites, then you have very little chance of leakage," McCoy said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500