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 a nd the oceanic uptake of CO2. To improve the climate simulation, we used Ge nt and co-workers eddy parameterization in the ocean and a prognostic equat ion for export production from the upper ocean. For the period of 1850 to 2 100, the change in the oceanic uptake of CO2 with climate was separated int o 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 oce an interior and reduced the cumulative ocean CO2 uptake by 41 Gt C, (iii) C limate change altered "natural" cycling of carbon in the ocean which increa sed the cumulative ocean CO2 uptake by 33 Gt C. The change in natural carbo n cycling with climate change was dominated by 2 opposing factors. First, t he supply of nutrients to the upper ocean decreased which reduced the expor t of organic matter (by 15% by year 2100) and produced a net CO2 flux out o f the ocean. However, associated with the reduced nutrient supply was the r eduction in the supply of dissolved inorganic carbon to the upper ocean, wh ich 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 fee dbacks 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 chan ge for the 1850 to 2100 period was similar to the 58 Gt C reduction in ocea nic CO2 uptake predicted by Sarmiento and Le Quere. However, our consistenc y with this previous estimate is misleading. By including the Cent and co-w orkers 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 war ming and anthropogenic forcing feedbacks. By using a prognostic parameteriz ation of export production, we reduced the negative feedback response of th e natural carbon cycle to climate change from 111 to 33 Gt C (78 Gt C). The se 2 large offsetting changes in the ocean response to climate change produ ced only a net change of 2 Gt C, This resulted in a net reduction in oceani c uptake of 2 Ct C from the previous study.