Saturation Point

According to Real Climate's survey of the literature, the ocean's capacity for absorbing CO₂ is saturating. If that's true, the greenhouse effect is on the verge of serious acceleration.

The ocean has a tendency to take up more carbon as the CO₂ concentration in the air rises, because of Henry's Law, which states that in equilibrium, more in the air means more dissolved in the water. Stratification of the waters in the ocean, due to warming at the surface for example, tends to oppose CO2 invasion, by slowing the rate of replenishing surface waters by deep waters which haven't taken up fossil fuel CO₂ yet.

The Southern Ocean is an important avenue of carbon invasion into the ocean, because the deep ocean outcrops here. Le Quere et al. [2007] diagnosed the uptake of CO₂ into the Southern Ocean using atmospheric CO₂ concentration data from a dozen or so sites in the Southern hemisphere. They find that the Southern Ocean has begun to release carbon since about 1990, in contrast to the model predictions that Southern Ocean carbon uptake should be increasing because of the Henry's Law thing. We have to keep in mind that it is a tricky business to invert the atmospheric CO₂ concentration to get sources and sinks. The history of this type of study tells us to wait for independent replication before taking this result to the bank.

Le Quere et al propose that the sluggish Southern Ocean CO2 uptake could be due to a windier Southern Ocean. Here the literature gets complicated. The deep ocean contains high concentrations of CO₂, the product of organic carbon degradation (think exhaling fish). The effect of the winds is to open a ventilation channel between the atmosphere and the deep ocean. Stratification, especially some decades from now, would tend to shut down this ventilation channel. The ventilation channel could let the deep ocean carbon out, or it could let atmospheric carbon in, especially in a few decades as the CO₂ concentration gets ever higher (Henry's Law again). I guess it's fair to say that models are not decisive in their assessment about which of these two factors should be dominating at present. The atmospheric inversion method, once it passes the test of independent replication, would trump model predictions of what ought to be happening, in my book.

A decrease in ocean uptake is more clearly documented in the North Atlantic by Schuster and Watson [2007]. They show surface ocean CO₂ measurements from ships of opportunity from the period 1994-1995, and from 2002-2005. Their surface ocean chemistry data is expressed in terms of partial pressure of CO₂ that would be in equilibrium with the water. If the pCO₂ of the air is higher than the calculated pCO₂ of the water for example, then CO₂ will be dissolving into the water.

The pCO₂ of the air rose by about 15 microatmospheres in that decade. The strongest Henry's Law scenario would be for the ocean pCO₂ to remain constant through that time, so that the air/sea difference would increase by the 15 microatmospheres of the atmospheric rise. Instead what happened is that the pCO₂ of the water rose twice as fast as the atmosphere did, by about 30 microatmospheres. The air-sea difference in pCO₂ collapsed to zero in the high latitudes, meaning no CO₂ uptake at all in a place where the CO₂ uptake might be expected to be strongest.

One factor that might be changing the pressure of CO₂ coming from the sea surface might be the warming surface waters, because CO₂ becomes less soluble as the temperature rises. But that ain't it, as it turns out. The surface ocean is warming in their data, except for the two most tropical regions, but the amount of warming can only explain a small fraction of the CO₂ pressure change. The culprit is not in hand exactly, but is described as some change in ocean circulation, caused maybe by stratification or by the North Atlantic Oscillation, bringing a different crop of water to the surface. At any event, the decrease in ocean uptake in the North Atlantic is convincing. It's real, all right.

Canadell et al [2007] claim to see the recent sluggishness of natural CO₂ uptake in the rate of atmospheric CO₂ rise relative to the total rate of CO2 release (from fossil fuels plus land use changes). They construct records of the atmospheric fraction of the total carbon release, and find that it has increased from 0.4 back in about 1960, to 0.45 today. Carbon cycle models (13 of them, from the SRES A2 scenario) also predict that the atmospheric fraction should increase, but not yet. For the time period from 1960 to 2000, the models predict that we would find the opposite of what is observed: a slight decrease in the atmospheric fraction, driven by increasing carbon uptake into the natural world. Positive feedbacks in the real-world carbon cycle seem to be kicking in faster than anticipated, Canadell et al conclude...

References
Canadell, J.G., C.L. Quere, M.R. Raupach, C.B. Field, E.T. Buitehuis, P. Ciais, T.J. Conway, N.P. Gillett, R.A. Houghton, and G. Marland, Contributions to accelerating atmospheric CO₂ growth from economic activity, carbon intensity, and efficiency of natural sinks, Proc. Natl. Acad. Sci. USA, doi 10.1073, 2007.

Ciais, P., M. Reichstein, N. Viovy, A. Granier, J. Ogee, V. Allard, M. Aubinet, N. Buchmann, C. Bernhofer, A. Carrara, F. Chevallier, N. De Noblet, A.D. Friend, P. Friedlingstein, T. Grunwald, B. Heinesch, P. Keronen, A. Knohl, G. Krinner, D. Loustau, G. Manca, G. Matteucci, F. Miglietta, J.M. Ourcival, D. Papale, K. Pilegaard, S. Rambal, G. Seufert, J.F. Soussana, M.J. Sanz, E.D. Schulze, T. Vesala, and R. Valentini, Europe-wide reduction in primary productivity caused by the heat and drought in 2003, Nature, 437 (7058), 529-533, 2005.

Le Quere, C., C. Rodenbeck, E.T. Buitenhuis, T.J. Conway, R. Langenfelds, A. Gomez, C. Labuschagne, M. Ramonet, T. Nakazawa, N. Metzl, N. Gillett, and M. Heimann, Saturation of the Southern Ocean CO₂ sink due to recent climate change, Science, 316 (5832), 1735-1738, 2007.

Schuster, U., and A.J. Watson, A variable and decreasing sink for atmospheric CO₂ in the North Atlantic, J. Geophysical Res., in press, 2007.