SLOW-DOWN OF OCEANIC CO2 UPTAKE IN RESPONSE TO GLOBAL WARMING
Authors: Traute Crueger, Erich Roeckner, Reiner Schnur, Thomas Raddatz, and Patrick Wetzel
Abstract: The continuously increasing emissions of greenhouse gases especially during the last century led to a rise of atmospheric CO2 from about 280 ppm in 1800 to 379 ppm in 2005. This increase however, accounts only for a portion of the total anthropogenic emissions, since the rise of atmospheric CO2 concentration is substantially damped by the ocean. The oceanic CO2 uptake is determined by the state of the ocean, which in turn is influenced by climate change. We investigate the mechanisms of the ocean’s carbon uptake within the feedback loop of atmospheric CO2 concentration, climate change and atmosphere/ocean CO2 flux. For that reason, we evaluate two transient simulations from 1860 until 2100, performed with a version of the Max Planck Institute Earth System Model (MPI-ESM) with the carbon cycle included. In both experiments observed anthropogenic CO2 emissions were prescribed until 2000, followed by the emissions according to the IPCC Scenario A2. In one simulation the radiative forcing of changing atmospheric CO2 is taken into account (coupled), in the other simulation it is artificially suppressed (uncoupled). In both simulations, the oceanic carbon uptake increases from 1 Pg C/yr in 1960 to 4.5 Pg C/yr in 2070. Afterwards, this trend weakens in the coupled simulation, leading to a reduced uptake rate of 10% in 2100 compared to the uncoupled simulation, thus representing a positive feedback. This includes a partial offset due to higher atmospheric CO2 concentration in the coupled simulation owing to reduced carbon uptake by the terrestrial biosphere. The positive feedback is primarily due to fluxes owing to partial pressure difference and secondary to those due to solubility changes. These contributions are widely offset by fluxes due to changes of gas transfer velocity following sea ice melting and the southward shift and intensification of the wind belt in the Southern Hemisphere. The major impact of climate change on oceanic CO2 uptake is found in the Southern Ocean (-45%) and in the North Atlantic (-30%), related to reduced vertical mixing and North Atlantic meridional overturning circulation, respectively. In the polar areas, sea ice melting induces additional CO2 uptake (+20%).
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