The Simulated Influence of Anthropogenic Climate Warming on the Oceanic Carbon Cycle

Prediction of the redistribution of anthropogenic CO2 in the atmosphere-ocean-land biosphere system is complicated by our limited understanding of the mechanisms driving carbon sequestration by the ocean and the terrestrial biosphere. Furthermore, the rates of carbon sequestration are sensitive to climate change projected to result from increases in CO2 and other greenhouse gas and changes in aerosol concentrations. This sensitivity creates feedback loops; an understanding of these feedbacks is prerequisite to forecasting future atmospheric CO2 concentrations and climate change. For instance, climate affects the oceanic carbon reservoir through the temperature-dependent relationship between total dissolved carbon and the partial pressure of CO2, and through potential climate-induced changes in ocean transport and ocean biological activity. A two-dimensional (latitude-depth) coupled atmosphere-ocean climate-carbon cycle model is developed and used to examine possible feedbacks between global climate change and the ocean carbon cycle system. The model has surface-air, land-sea, and latitudinal resolution, and is forced with diurnally averaged but seasonally varying insolation. The ocean component consists of the zonal mean balance equations of horizontal momentum, mass, temperature and salinity. The momentum balance is diagnostic, and time dependence enters through the advection-diffusion equations for temperature and salinity. The density field is calculated using a nonlinear, pressure-dependent equation of state. In this study the model is used to project future atmospheric CO2 concentrations and temperature change due to greenhouse gas and aerosol scenarios recently developed by IPCC (Nakicenovic et al., 2000), and evaluated the climate change feedback contribution to spatial and temporal changes in ocean carbon uptake. Globally, projected warming over the next century can decrease the strength of oceanic uptake of CO2 resulting in a positive feedback on CO2 concentration. Including this feedback leads to greater projected warming than in simulations that neglect this effect. These results are consistent with recent 3-D AOGCM estimates.