Why carbon emission rates matter for TCRE and oceanic heat and carbon uptake

Monday, 15 December 2014
John P Krasting, John P Dunne, Robert Hallberg and Ronald J Stouffer, NOAA / Geophysical Fluid Dynamics Laboratory, Princeton, NJ, United States
While the Transient Climate Response to cumulative carbon Emissions (TCRE) demonstrates that near-surface air temperature warming is approximately proportional to cumulative emissions, oceanic heat and carbon uptake and sea level rise are far less easily constrained by this metric. Past work using the GFDL-ESM2G Earth System Model explored the robustness of TCRE across a range of policy-relevant emission rates, including a very low emission rate (2 GtC/yr), a very high emission rate (25 GtC/yr), and the case of ceasing emissions. TCRE varies between 0.76 and 1.04 ºC/TtC over this range of emissions, but this range is small (~12%) compared to the range across CMIP5 models. Here we demonstrate that the timescales of oceanic heat and carbon uptake play an important role in the rate dependence of TCRE. Analysis of the zonal patterns in the air-sea surface fluxes of heat and carbon illustrate important differences between the two that are a function of emissions rate. We find the ultimate fate of anthropogenic heat and carbon in the ocean also depends on the rate of emissions, and particularly, the timescales of oceanic transport. Low emission rates afford the ocean more time to transport heat and carbon throughout the world oceans whereas high emission rates have distinct basin scale differences in their patterns of uptake and storage. These oceanic processes lead to basin scale differences in sea level rise under varying emission rates for the same cumulative emission levels and have important climate adaptation consequences.