First proof ocean CO2 uptake has started to slow down

Warming oceans re-emit CO2From raw measurements we know that in recent years the oceans seem to take up a smaller percentage of the CO2 we emit. Analysing available data a group of three researchers finds in part of the North Atlantic this is not due to natural variability, but to warming waters, which means yet another positive feedback to climate change has set in.

The world’s oceans and atmosphere exchange CO2 through an equilibrium reaction: a certain percentage of the CO2 we emit to the air ends up in the water (where it causes ocean acidification). So far the oceans were able to absorb around a third of the extra CO2 human activity has added to the atmospheric carbon cycle.

Scientists however predict the ratio will not remain the same, because under the current climate change [as of course the atmosphere and oceans also exchange heat] the water too is gradually warming [which indeed it is] and warmer water cannot contain as much CO2*.

[*) Unfortunately this is no natural solution to the process of ocean acidification. Under a further increase in atmospheric CO2 the ‘pressure’ will remain high and the oceans will continue to absorb CO2, worsening acidification. The equilibrium in the exchange reaction will shift though, and the oceanic CO2 level rise will slow down, whereas the atmospheric rise will speed up.]

Oceanic & atmospheric CO2 deviate, but share trend

The study, which was published online in Nature Geoscience on Sunday, has investigated the role decadal temperature and thermohaline fluctuations play in the level of dissolved CO2 in the North Atlantic – and found in three different large gyres such natural variations are important. This means the oceanic CO2 graph often deviates from the atmospheric CO2 graph (although both have an increasing trend).

According to the researchers, from University of Wisconsin–Madison, Columbia University and Université Pierre et Marie Curie, it takes 25 years for the long-term trend to properly emerge from this natural variation. This made it difficult to distil hard statements, because the group only had access to data over the 29-year period from 1981-2009. But at least in the subtropical North Atlantic the analysis shows ocean warming is already slowing down carbon absorption of the water.

The global scale: oceans and carbon cycle

As ocean water measurements are usually carried out by commercial ships the busy lanes are best-examined – which means the lower latitudes. But we trust that what happens in the southern part of the North Atlantic will also have taken place in the far north, as all climate-related issues happen faster there, also ocean-CO2 trends.

We now wonder if the decline could already be showing in the atmospheric CO2 graph. We did have record-high emissions last year. And indeed there was that big Amazon drought. But does this fully explain a rise of 2.60 ppm – almost 25 percent faster than the average over the last decade? Or are we ignoring a couple of gigatonnes of krill population deflating?

It’s good to know the research never stops…

© Rolf Schuttenhelm |

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