Saturday, May 05, 2012

How Much Carbon Can The Oceans Store?

At first sight, a lot. The oceanic reservoir of carbon is about fifty times as large as the atmospheric store, and much larger (ten times) than our available store of carbon in fossil fuels. So is there any reason to think that the ocean can't just go on packing away the excess CO2 we pump into the atmosphere? Well, yes.
Most of that oceanic CO2 is stored in cold deep ocean water where it is largely isolated from the year to year or century to century exchanges with the atmosphere. The near surface waters, which do readily exchange CO2 with the atmosphere constitute a reservoir only 1/3 larger than the atmospheric reservoir (1000 Peta grams, AKA giga tonnes, vs 750 Pg.) It is this reservoir which largely controls the rate of exchange with the atmosphere. Other things being equal, we could expect new carbon contributions to the atmosphere to reach a quasi-equilibrium with this reservoir in which the proportions were similar.
Other things are not equal, of course. For one thing, the solubility of CO2 in water decreases with increasing temperature, so warmer oceans (near surface waters) will hold less CO2 at comparable vapor pressures, not more.
Some have argued (not to mention any Czech names) that because the ocean currently sucks up about 2 Pg of the excess CO2 we dump into the atmosphere, we can expect it to keep doing so. I think that this view ignores the underlying physics, which is that the ocean surface waters fairly rapidly (years or decades?) equilibrate with the atmosphere and are best seen as adjusting their concentration in response to changes in atmospheric concentration. The ocean will continue to absorb more CO2 as long as atmospheric concentration increases. Then, should atmospheric concentration start decreasing, the ocean will give back what it borrowed.
It's also true that there are certain inherent hazards in packing ones ocean full of CO2. The evidence now seems to show that it was CO2 driven ocean "acidification" that did the dirty Permian extinction deed.