Would Leaky Underground CO2 Storage Still Help Avoid Global Warming?
by Eric Larson
As carbon dioxide (CO2) from fossil fuel burning continues accumulating in the atmosphere, hundreds of scientists and engineers worldwide are trying to figure out how to put some of it back underground using a technology called CO2 capture and storage (CCS). Here is a nice summary of CCS activities.
The experience to date with large-scale CO2 storage projects is limited but encouraging, lending support to the conclusion of the UN Intergovernmental Panel on Climate Change (IPCC) in its 2005 special report on CCS that the probability is “very high” (90 to 99 percent) that more than 99 percent of any CO2 injected underground will stay there for at least 100 years, and “high” (66 to 90 percent) that it will stay there for at least 1,000 years.
Less than one percent leakage over 100 or 1,000 years sounds pretty small. But is it significant enough to limit the viability of CCS as a climate change mitigation option?
Gary Shaffer, a geoscientist at the University of Concepcion, Chile, in a recent paper in Nature Geosciences, presents results of climate modeling he did to help understand the long-term implications of such leakage rates.
What Shaffer’s work suggests is that underground CO2 storage — even it’s not 100 percent secure — would help keep the world cooler for at least the next century. That’s good news, because if, during this time period, we can increase energy efficiency and the use of non-carbon energy sources to the point where we can eliminate the use of fossil fuels altogether (rather than taking 300 years to do this, as in Shafer’s scenarios), the leak rate of the CO2 we store underground won’t matter nearly as much.
Study models different CO2 leakage scenarios
Shafer started by modeling how the earth’s temperature will change due to greenhouse warming under a “business as usual” (BAU) continuation of fossil fuel use through 2100, followed by tapering of fossil fuel use to zero by the year 2300. For disaster-movie aficionados, the kind of warming he predicts in that scenario (about five degrees Celsius within 100 years) will bring to mind images from The Day After Tomorrow.
At the other end of the spectrum he develops a scenario he calls “Avoiding Global Warming,” for which global warming is kept below two degrees Celsius compared to preindustrial levels. This is the stated goal of the world leaders — including President Obama — who signed onto the Copenhagen Accord last December. In the avoidance scenario, emissions of all greenhouse gases (including non-CO2 warming compounds such as methane and soot) fall by 60 percent by 2050 as a result of significant energy efficiency improvements and greater reliance on nuclear power and renewable energy sources such as wind and solar.
To look at the impact on warming from CO2 leaked from storage, Shaffer then creates a scenario in which most of the fossil fuel CO2 emissions of the BAU scenario are captured and injected underground. And then he allows some of this CO2 to leak back into the atmosphere.
He allows rapid leaking, moderate leaking, and weak leaking rates in three different scenarios, corresponding to one percent leakage over 10, 100, and 1,000 years, respectively.
Somewhat surprisingly, he finds that the warming pattern in all three cases is about the same through the early 2100s — in all cases the earth warms about 3 degrees C before starting to cool down. The warming is due in part to the “legacy CO2” — that which we have already put into the atmosphere — and, importantly, the continued non-CO2 greenhouse emissions (like methane and soot) that Shafer assumes would remain the same as in the BAU scenario. Cooling starts in the early 2100s because the oceans’ chemical absorption of CO2 (a slow process) is able to start drawing down CO2 accumulated in the atmosphere once its rate of release into the atmosphere is slowed enough via CCS. (CO2 is an acid, so a downside of increased ocean absorption is increased ocean acidification. Among other impacts, acidification is already tied to the accelerating demise of coral reefs).
For Shafer’s rapid leak scenario, by the mid-2100s the rate at which CO2 leaks overwhelms the rate at which the ocean can absorb it, and global temperature begins rising again, reaching nearly 4 degrees C at its peak in about 3,000 years. Meeting the two degree warming goal of the Copenhagen Accord would seem impossible in this case.
In the moderate leak case, the cooling that starts in the early 2100s lasts several hundred years longer than in the rapid leak case, after which the warming hovers in the 2 degree C range for several tens of thousands of years. Ocean acidification continues steadily, though not as far as in the rapid leak case.
In the weak leak scenario, the leakage is slow enough that the cooling that starts in the early 2100s continues long into the future, with warming leveling out at about 1 degree C within about 3,000 years. Ocean acidification continues, but far more slowly than in the other two cases.
So, would stored CO2 that leaks still help reduce the extent of global warming? Clearly the answer is yes, according to Shafer’s work. In my view, his work shows that the leakage rate is not all that important, within the range he examined, as long as the world pursues CCS aggressively starting within the next decade and if it completely weans itself from fossil fuels during this century.
Those are two big ifs.