China’s got ample space underground to store the carbon dioxide it pumps into the sky, according to a recently publicised study by Chinese and US researchers (Dahowski et al, Energy Procedia 1, 2009; doi:10.1016/j.egypro.2009.02.058). It has 2,300 Gt/CO2 of theoretical capacity spread generally across the country, and not too far from the powerplants that are large point sources of the greenhouse gas – meaning that transport and storage costs can be kept down to less than $10 per ton of CO¬2, the study finds.

Lump this in with other capacity estimates, such as the US Department of Energy’s Carbon Sequestration Atlas, and it’s clear that the world is generally not short of space, and could likely store hundreds of years of carbon dioxide output.

The IEA has more capacity estimates in its CCS roadmap [pdf], released on Tuesday to coincide with political support for carbon capture and storage (see Nature’s news story, ‘Urgency’ needed on carbon capture).

But (there’s always a but).

It’s likely that only a tiny proportion of the estimated capacity ends up being usable in practice – as engineers realize that water already in saline aquifers means less carbon dioxide can be forced in than thought, that it cannot sweep through all the space supposedly there, or that the rate at which it can be injected is slower than hoped for.

Edinburgh geologist Stuart Haszeldine notes that our original giant estimates of capacity have been scaled back recently, and we are now entering a phase of more pessimistic but guaranteed estimates – something like 70-100 years of the world’s power output. “Only after dynamic demonstrations in aquifers will true capacity values become apparent,” he says (Science, doi: 10.1126/science.1172246).

What’s, more some countries won’t be as fortunate as China or the US when it comes to their storage areas – Japan and Korea’s ability to burn fossil fuels may be compromised by small domestic storage capacities, and they may find they have to ship carbon dioxide in tankers to space-rich countries.

And knowing that there’s enough space in the world is reassuring but doesn’t help much at the local project level. If you’re an individual company that’s signed a contract to store 20Mt of carbon dioxide in an aquifer, you don’t want to find out that in practice the space will only take 10Mt; or that it will only take 20Mt when injected over 80 years rather than 20 years. This uncertainty is something that engineers in the oil and natural gas industry are used to dealing with, but it can still give pause for thought.