Oceans' response to climate change

Last week I showed a mechanism through which the oceans could absorb CO₂. Since it follows a cycle, however, carbon’s motion is circular. In that sense, the oceans in turn release CO₂ into the atmosphere. I will go through the major implication of global warming in relation to the solubility pump.

There exists an atmosphere/ocean pressure balance. If the partial pressure of CO₂ increases in the atmosphere, the gas will disperse into the water body below it. That is because the air-water boundary’s partial pressures must be in equilibrium (NOAA).

The more CO₂ in the atmosphere, the more oceans will uptake carbon dioxide to compensate. This induces us to think that yes, oceans are a reliable CO₂ sink.

Then, we can count on the oceans to absorb the CO₂ we release, can’t we?

The solubility pump is all very nice and comforting for future climate change.

Well, it’s not that easy. To sum up in Riebeek’s poetic words: “carbon dioxide leaks out of the ocean like a glass of root beer going flat on a warm day” as a result of rising temperatures (2008).

The portion of unabsorbed atmospheric CO₂ leads to global warming. Rising atmospheric temperatures warm the oceans over time. As we have seen last week, warmer temperature decreases CO₂ solubility in water. Conclusively, an increase in CO₂ released through fossil fuel combustion, decreases oceans' ability to uptake CO₂.

This is called a positive feedback effect where the consequence of CO₂ release into the atmosphere (rising ocean temperature) increases the cause.

Le Quere modelled and measured the change in CO₂ sink (in PgC/year) over the Southern Ocean between 1981 and 2004 (2008).

Figure 1 Expected CO₂ uptake variability (blue) plotted with the observed changes (in red) over time. The y axis represents the change in carbon dioxide sink ranging from -2 to 1.5 PgC/year. Source: Le Quere 2008

We can see that the modelled ocean uptake did not fit observations. What was expected was a steady increase in CO₂ uptake. Instead, there is annual variability but “no long term increase” (Riebeek 2008). We can even suspect a slight overall decrease in ocean ability to absorb CO₂.

We are not fully understanding ocean carbon mechanisms. Quantifying atmospheric, oceanic exchanges has proved even more difficult. Although it is hard to predict absorption variability, evidence suggests we cannot count on oceans’ uptake to increase. Scientists even suspect an imminent decrease (2008). Obviously, this decrease is very alarming. Oceans have been mitigating effects of fossil fuel combustions so far. If carbon cannot be transferred to the oceans, global warming is thought to intensify in the near future. For these reasons, a lot of debate is going on as to how oceans will respond to climate change.