Feedback mechanisms and sensitivities of ocean carbon uptake under global warming

Gian-Kasper Plattner, Fortunat Joos, Thomas F. Stocker, and Olivier Marchal, Climate and Environmental Physics, Physics Insitute, University of Bern, Bern, Switzerland

Global warming simulations are performed with a coupled climate model of reduced complexity to investigate global warming-marine carbon cycle feedbacks. The model is forced by emissions of CO₂ and other greenhouse agents from scenarios recently developed by the Intergovernmental Panel on Climate Change and by CO₂ stabilization profiles. The uptake of atmospheric CO₂ by the ocean is reduced between 7 to 10% by year 2100 compared to simulations without global warming. The reduction is of similar size in the Southern Ocean and in low-latitude regions (32.5 S - 32.5 N) until 2100, whereas low-latitude regions dominate on longer time scales. In the North Atlantic the CO₂ uptake is enhanced, unless the Atlantic thermohaline circulation completely collapses. At high latitudes, biologically mediated changes enhance ocean CO₂ uptake, whereas in low-latitude regions the situation is reversed. Different implementations of the marine biosphere yield a range of 5 to 16% for the total reduction in oceanic CO₂ uptake until year 2100. Modeled oceanic O₂ inventories are significantly reduced in global warming simulations. This suggests that the terrestrial carbon sink deduced from atmospheric O₂/N₂ observations is potentially overestimated if the oceanic loss of O₂ to the atmosphere is not considered.