Climate change feedback on the future oceanic CO₂ uptake

Output from a coupled atmosphere’ocean model forced by the IS92a greenhouse gas scenario was used to investigate the feedback between climate change and the oceanic uptake of CO₂. To improve the climate simulation, we used Gent and co-workers eddy parameterization in the ocean and a prognostic equation for export production from the upper ocean. For the period of 1850 to 2100, the change in the oceanic uptake of CO₂ with climate was separated into 3 feedbacks. (i) Climate change warmed the sea-surface temperature which increased the partial pressure of CO₂ in the surface ocean and reduced the accumulated ocean uptake by 48 Gt C. (ii) Climate change reduced meridional overturning and convective mixing and increased density stratification in high latitudes which slowed the transport of anthropogenic CO₂ into the ocean interior and reduced the cumulative ocean CO₂ uptake by 41 Gt C. (iii) Climate change altered “natural” cycling of carbon in the ocean which increased the cumulative ocean CO₂ uptake by 33 Gt C. The change in natural carbon cycling with climate change was dominated by 2 opposing factors. First, the supply of nutrients to the upper ocean decreased which reduced the export of organic matter (by 15% by year 2100) and produced a net CO₂ flux out of the ocean. However, associated with the reduced nutrient supply was the reduction in the supply of dissolved inorganic carbon to the upper ocean, which produced net CO₂ flux into the ocean. For our model, the latter effect dominated. By the year 2100, the combinations of these 3 climate change feedbacks resulted in a decrease in the cumulative oceanic CO₂ uptake of 56 Gt C or 14% of the 402 Gt C of oceanic CO₂ uptake predicted by a run with no climate change. Our total reduction in oceanic CO₂ uptake with climate change for the 1850 to 2100 period was similar to the 58 Gt C reduction in oceanic CO₂ uptake predicted by Sarmiento and Le Quéré. However, our consistency with this previous estimate is misleading. By including the Gent and co-workers eddy parameterization in the ocean, we reduced the positive feedback between climate change and the oceanic uptake of CO₂ from 169 to 89 Gt C (80 Gt C change). This reduction reflects a decrease in both sea surface warming and anthropogenic forcing feedbacks. By using a prognostic parameterization of export production, we reduced the negative feedback response of the natural carbon cycle to climate change from 111 to 33 Gt C (78 Gt C). These 2 large offsetting changes in the ocean response to climate change produced only a net change of 2 Gt C. This resulted in a net reduction in oceanic uptake of 2 Gt C from the previous study.