Oceanic Subduction of CO2
Carbon dioxide is a by product of animal metabolism and the activities of man such as combustion products. Where does or will it go? One of the places where it goes are the cold southern oceans of the world. A few regions around the world are key in overturning deep and shallow layers of the ocean, and allow carbon to be locked away from the atmosphere for centuries. The Southern Ocean in particular is known to be a significant oceanic carbon sink, and accounts for 40% of all carbon entering the deep oceans. But how does it get sucked down into the ocean. A new paper from the National Environment Research Council resolves this matter, and shows the importance of 1000 kilometer-wide plunging funnels.
The carbon dioxide solubility pump is driven by two processes in the ocean:
The solubility of carbon dioxide is a strong inverse function of seawater temperature (i.e. solubility is greater in cooler water).
The thermohaline circulation is driven by the formation of deep water at high latitudes where seawater is usually cooler and denser
Since deep water (that is, seawater in the ocean's interior) is formed under the same surface conditions that promote carbon dioxide solubility, it contains a higher concentration of dissolved inorganic carbon than one might otherwise expect. Consequently, these two processes act together to pump carbon from the atmosphere into the ocean's interior.
One consequence of this is that when deep water upwells in warmer, equatorial latitudes, it strongly outgasses carbon dioxide to the atmosphere because of the reduced solubility of the gas.
The current southern ocean analysis is based around a decade’s worth of data from thousands of robotic floats, and hundreds of ships spread across the southern hemisphere oceans. It has shown that carbon capture process occur in well-defined regions of the Southern Ocean. Five such zones were pinpointed in the Southern Ocean, including one off the southern tip of Chile and another to the south-west of New Zealand.
It was found that certain combinations of winds and currents are required to pump carbon dioxide into the deep ocean, where much of it stays locked away for thousands of years. Mesoscale ocean eddies have a primary role in this process.
The importance of ocean eddy processes in the global carbon cycle, demonstrated by these new results, raises the question of how well coarse resolution climate models represent the relatively fine-scale eddy processes. This work lays down some very significant challenges to the developers of climate and Earth System models, but also shows the way forward in evaluating and improving model performance.
For further information see Southern Ocean.
Ocean Map image via Wikipedia.