Towards constraints on fossil fuel emissions from total column carbon dioxide

We assess the large-scale, top-down constraints on regional fossil fuel emissions provided by observations of atmospheric total column CO₂, X_CO2. Using an atmospheric general circulation model (GCM) with underlying fossil emissions, we determine the influence of regional fossil fuel emissions on global X_CO2 fields. We quantify the regional contrasts between source and upwind regions and probe the sensitivity of atmospheric X_CO2 to changes in fossil fuel emissions. Regional fossil fuel X_CO2 contrasts can exceed 0.7 ppm based on 2007 emission estimates, but have large seasonal variations due to biospheric fluxes. Contamination by clouds reduces the discernible fossil signatures. Nevertheless, our simulations show that atmospheric fossil X_CO2 can be tied to its source region and that changes in the regional X_CO2 contrasts scale linearly with emissions. We test the GCM results against X_CO2 data from the GOSAT satellite. Regional X_CO2 contrasts in GOSAT data generally scale with the predictions from the GCM, but the comparison is limited by the moderate precision of and relatively few observations from the satellite. We discuss how this approach may be useful as a policy tool to verify national fossil emissions, as it provides an independent, observational constraint.