While CarbonTracker applies bottom-up estimates of fossil fuel and wildfire emissions directly, it attempts to estimate optimal fluxes from the ocean and land biosphere. This optimization starts with a first guess of the flux component and its uncertainty (left panel). The one standard-deviation level of uncertainty for the land flux is shown with a thin green vertical line, and the uncertainty for oceanic exchange is shown with a thin blue vertical line. The total uncertainty, extending to the end of the vertical bar, incorporates within-model and across-model uncertainties. This is due to the use of the 8 prior flux models used in CT2011; within-model error is the average posterior uncertainty from these eight simulations, and across-model error is the additional uncertainty from the scatter of the eight inversions. As shown in the legend, the within-model error, which usually dominates the total error, is indicated by the small horizontal cross bar in each vertical error bar. Fossil fuel uncertainty is shown by the darker tan bar inset into the main fossil bar. The two fossil fuel emissions estimates used in CT2011 are at the ends of the darker inset bar.
The first-guess fluxes are subsequently evaluated against atmospheric CO2 observations and modified to be consistent with those measurements. The optimized results (right panel) have different mean fluxes and reduced uncertainty ranges, indicating the influence of information brought by the observations. The uncertainty on the net flux (black vertical line) is the RMS sum of land biosphere and oceanic flux uncertainties.
Note that fossil fuel emissions can occur over regions characterized as ocean or non-optimized regions such as ice, polar deserts, and inland seas. This is partly due to real emissions from international shipping, and partly due to emissions occurring in coastal land regions that are assigned to the ocean in our coarse 1° x 1° division scheme. Similarly, land regions may have a small contribution from air-sea gas exchange, and vice versa.
Fossil fuel flux (bottom panel). As in the top panel, but for fossil fuel emissions. Note that fossil fuel emissions are not optimized in CarbonTracker.