Few of us think more than a few hundred years into the future when imagining the effects of carbon dioxide emissions, or the benefits and dangers of burying the greenhouse gas underground. That kind of timescale is but a blink in the eye for Gary Shaffer, a professor of oceanography and climate at the University of Copenhagen, Denmark. In a paper published on Sunday in Nature Geoscience (doi:10.1038/ngeo896), he considers the effects of imperfect carbon dioxide storage, thousands of years into the future.

In Shaffer’s (theoretical) projections, humankind chooses to run amok, busily burning up all its fossil fuels until reserves run out around the year 2200. But it compensates for this most prudently by burying the carbon dioxide component underground. The total effect, in terms of carbon dioxide emissions, is in theory supposed to equal an emissions scenario for avoiding strong global warming. After all, the released carbon dioxide has been locked away.

The scenario is just a game: no-one pretends that carbon storage will lock away 100% of fossil fuel carbon dioxide, and in any case, other nasties like black carbon and methane are left in the atmosphere through fossil fuel burning. Sticking with the concept, however, if the carbon dioxide store is perfect, then strong global warming will be avoided.

But, Shaffer calculates, if the leakage rate is 1% every 10 years, by 5000 AD mean atmosphere warming will be as bad as if no storage had been attempted. Dial that leakage rate down to 1% every 100 years, and we get to 20,000 AD before atmospheric warming is as bad as no storage at all.

20,000 AD? By this point humankind will surely be on other planets or, more likely, extinct. But Shaffer is concerned about future generations in 20,000 AD. He points out that nuclear waste management works on these timescales – tens of thousands of years. He effectively says that once we have chosen to release the locked-up carbon dioxide in fossil fuels such as oil and coal, even stuffing almost all of the gas underground merely postpones its eventual deleterious effect on global temperatures, ocean acidification, and other problems. Only a leakage rate below 1% over 1000 years could avert this burden on future generations, he calculates. At that rate, they get to live well past 50,000 AD before the extra heat bothers them. (The IPCC’s special report on carbon capture and storage, incidentally, concludes that the fraction of carbon dioxide retained in appropriately selected and managed geological reservoirs is ‘likely’ to exceed 99% over 1000 years. ‘Appropriately selected and managed’ is the tricky clause in that conclusion – how many stores will be inappropriately selected and managed?).

I feel that this calculation adds little to the question of whether we should use carbon capture and storage right now as a way of cutting down on carbon dioxide emissions from coal. Yes, as Shaffer says, a leaky store will create delayed warming in the future. But what a comfortingly long way away that future is, in his projections. Moreover, he considers that future humans make no attempt at re-sequestering escaped carbon dioxide. And he does not suggest that today we might try both to capture carbon dioxide underground and reduce our fossil fuel emissions.

And aren’t there many more pressing practical concerns about carbon capture and storage to consider right now – such as whether it works over a hundred years, not whether it works over tens of thousands of years?

Nevertheless, Shaffer is right to be worried that carbon capture and storage can be seen as a ‘get-out’ that allows us not to lower fossil fuel emissions. “Given the present situation, CCS may still be a valuable tool – certainly much better than geoengineering via short-lived aerosols,” he tells me. “We should just have a better picture of what we may be getting future society into on the short and on the long term.”

In The Register, Lewis Page (who covers this paper, as does the press agency AFP) points out that Shaffer has also suggested saving up the planet’s fossil fuels to as to be able to burn them thousands of years later to avert a possible ice age. Now that’s future-gazing on a geological timescale.