The complexity of the phenomenon stems from the fact that several climatic and biogeochemical system responses influence carbon sequestration in the ocean. A first climatic feedback is linked to the effect of temperature onCO2 solubility and acid-base equilibria.
Following in the footsteps of the atmosphere, the ocean is warming from its surface, as shown by in situ oceanographic data since the end of the 19th century, and more recently by satellite infrared observations and measurements relayed by the thousands of Argo floats that automatically sample the first few kilometers of the water column. Since the beginning of the 20th century, the average temperature of surface waters has risen by around 1 °C. Over the century as a whole, virtually all regions of the ocean have warmed.
The solubility coefficient ofCO2 in water is a decreasing function of its temperature. In addition, the two acid-base dissociation constants are increasing functions of temperature. The combination of these effects leads to a decrease in oceanic pumping of excess atmosphericCO2 in response to warming. For an atmospheric pCO2 of 300 ppm, warming surface seawater by 1 °C to 20 °C leads to a new pCO2 equilibrium at 313 ppm between the atmosphere and the ocean. Warming therefore generates an apparent outgassing of around 13 ppm. To a lesser extent, an increase in salinity also leads to a rise in dissolvedCO2 and thus to outgassing to restore equilibrium at the atmosphere-ocean interface.
