Ocean CO2 uptake update

The IPCC AR4 Scientific Basis report is a real goldmine of information, even if it isn’t perfect, as I recently pointed out.

As I discussed in a previous post, an idea I later developed a little, policies to address global warming must rely on an understanding of how natural systems will respond to the increase in atmospheric CO2. Will the oceans keep absorbing a couple of GtC worth of CO2 each year (as estimated since 1990 – AR4, p.26, as referenced previously) or more (as implicitly assumed by many) or less? And will land ecosystems manage to take up more or less carbon than in the past? Especially if we continue to reduce the area of ecosystems able to do this – since agricultural land clearly does not progressively store carbon.

I’ve been looking at a critical section in AR4 on ocean uptake of CO2. This is 5.4.2.2 on p.403-5 (though the main section on the carbon cycle is 7.3, p.511 ff). I quote:

“The fraction of net CO2 emissions taken up by the ocean (…) was possibly lower during 1980 to 2005 (37% +/- 7% [that is, 118 +/- 19 of 283 +/- 19GtC of emissions]) compared to 1750 to 1994 (42% +/- 7% [that is, 53 +/- 9 of 143 +/- 10 GtC of emissions)… The decrease in oceanic uptake fraction would be consistent with the understanding that the ocean CO2 sink is limited by the transport rate of anthropogenic carbon from the surface to the deep ocean, and also with the nonlinearity in carbon chemistry that reduces the CO2 uptake capacity of water as its CO2 concentration increases”. (my inserts in [ ]’s – based on Table 5.1, p.404).

And we also have to worry about “a decrease in CO2 uptake capacity” as the ocean warms.

On the other hand section 7.3.2.2.5 (p.521) notes that:

“The ocean uptake has increased by 22% between the 1980s and 1990s, but the fraction of emssions (fossil plus land use) taken up by the ocean has remained constant.”

though of course the ocean “knows” nothing about emissions – all it can possibly be affected by is the level of CO2 in the atmosphere.

We really need to get a handle on what the oceans are going to do in the future since it makes such a huge difference to the level of carbon emissions we can get away with. It’ll be the first section I turn to in AR5. As AR5 is due around 2012 (I suppose), maybe we should have a think about where we focus scientific resources now…