Subsurface storage of carbon dioxide is a major option for mitigating climate change. On one account, much of the gas sequestered in this way would end up as carbonic acid in the pore waters of the host rock.rnAtmospheric concentrations of greenhouse gases, especially carbon dioxide, continue to rise at an alarming rate. We seem unable to tame our appetite for fossil fuels on a meaningful timescale, and the concept of carbon capture and storage has emerged as a serious option for reducing CO_2 emissions to the atmosphere. A clean coal' technology, in which CO_2 is collected from coal-fired power plants and stored safely below ground, might enable us to continue using this comparatively cheap and abundant energy source without climatic worries.rnHowever, little is known about the long-term fate of large quantities of CO_2 put into geological storage. Gilfillan et al. (page 614 of this issue) illuminate this crucial matter by showing that dissolution in groundwater is by far the most important trapping mechanism for CO_2 in the subsurface environment. In other words, sequestering CO_2 in geological formations would probably produce vast quantities of highly CO_2-enriched sparkling water.
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