The ocean less effective at absorbing CO2 emitted by human activity

The increase in the amount of CO₂ in the atmosphere, which is the cause of climate warming, is the result of human activity (use of fossil fuels and deforestation). However, warming is mitigated by the oceans and by terrestrial ecosystems, which are able to absorb a large part of CO₂ emissions. The oceans are the planet's main carbon sink(4), but in the last ten years they have become increasingly unable to play this role, in both the northern and southern hemispheres.  

This is what has been discovered by Nicolas Metzl and his team at IPSL's LOCEAN(5) laboratory. Their conclusion is based on data collected by the OISO(6) Indian Ocean observation service, which was set up ten years ago with the backing of INSU, CNRS, IPEV and IPSL in order to better evaluate variations in the oceanic carbon cycle on seasonal to decadal scales. From 1998 to 2008, the OISO observatory carried out repeated campaigns of CO₂ measurements in the Southern Indian Ocean between 20 and 60°S, on board the ship Marion Dufresne(7). The data collected, together with earlier data (1991-1995), show that the quantity of CO2 is increasing faster in surface waters than in the atmosphere (by 2.1 microatmospheres/year in water as opposed to just 1.7 in air). So although the CO2 content in the atmosphere remains higher than that in surface waters, the difference is decreasing.

According to Metzl, who is leader of the OISO program, this increase is the result of climate change at high latitudes, which has led to an increase in the relative difference of atmospheric pressure at latitudes between 40 and 60°S, and therefore to higher wind speeds, leading in turn to increased ocean mixing, with surface waters mixing with deep waters. Surface waters contain less CO₂ than deep waters, since CO₂ is taken up by the photosynthetic activity of marine phytoplankton. In addition, when these organisms die, they are deposited in deep water where they are broken down by bacteria, thus enriching the deep water in CO₂. Thus when there is increased wind mixing of the ocean, greater amounts of CO₂ are carried from the deep layers to the surface and, as a result, the ocean's ability to absorb atmospheric CO₂ is diminished._.  This is the first time that field measurements have confirmed the role of climate change in the oceanic carbon cycle in the southern hemisphere.

The IPSL researchers used their data to estimate the efficiency of the oceanic carbon sink on a larger geographical scale,  that of the whole Southern Ocean. They combined the data from the OISO program with other international oceanic CO₂ data. Together with their colleagues, they revised downwards the amount of CO₂ absorbed by the carbon sink in the Southern Ocean, in particular with the help of observations carried out during the southern winter. The carbon sink probably absorbs ten times less carbon than previously estimated: 0.05 gigatonnes of carbon/year (GtC/year) as opposed to 0.5 GtC/year.

Metzl and his colleagues also took part in analysing the oceanic CO2 sink in the North Atlantic(8), combining data collected in the region since 1993(9) with other international data. The conclusion is that the CO2 sink diminished by 50% between 1996 and 2005 in the North Atlantic. The mechanism proposed by the researchers appears for the moment to be connected more to fluctuating weather conditions than to climate change(10).

Routes of the OISO observation service's oceanographic campaigns to measure CO2, carried out on board the Marion Dufresne between 1991 and 2007

Both in the northern and southern hemispheres, the last ten years have seen a weakening of oceanic carbon sinks, which means an increase in atmospheric CO₂ content and thus in climate warming. Just how far can this go, and what will the consequences be for the future climate? To find out, researchers will need to continue these observations and take into account these new results to validate models, especially the coupled climate/carbon models that include marine biology, like those used in the reports of the IPCC(11).

This is because the models currently used for climate prediction do not correctly simulate the changes in oceanic CO₂ observed over the last two decades at high northern and southern latitudes.