Climate Science

Holding the Line

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Science  04 Dec 2009:
Vol. 326, Issue 5958, pp. 1321
DOI: 10.1126/science.326.5958.1321-c

Roughly one-third of all the CO2 emitted by human activity is ultimately absorbed by the ocean, a process that has helped slow down the rate of increase of atmospheric CO2 concentrations. As the ocean continues to absorb CO2, however, the rate at which it does so is expected to decrease because of the changes in carbonate chemistry that CO2 uptake causes. Global warming should then accelerate, a frightening prospect considering how quickly temperatures are rising already. Several studies have shown that the uptake of atmospheric CO2 by some regions of the ocean has slowed already, but does that mean that the integrated world ocean has become a less effective CO2 sink? Knorr combines data from ice cores, direct atmospheric measurements, and emission inventories to show that the global fraction of emitted CO2 that remains in the atmosphere has stayed constant over the past 160 years, at least within the limits of uncertainty of the measurements. Khatiwala et al. also fail to detect a significant recent change in the fraction of CO2 that the ocean is absorbing, in an examination of both ocean and terrestrial CO2 sinks for the longer period of the past two and a half centuries. That is welcome news, but not reason to be complacent about the future, as sooner or later the capacity of the ocean to absorb CO2 will be reduced. The real question is why we have not seen evidence of that reduction already.

Geophys. Res. Lett. 36, L21710 (2009); Nature 462, 346 (2009).

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