Abstract
The rare stable carbon isotope, 13C, has been used previously to partition CO2 fluxes into land and ocean components. Net ocean and land fluxes impose distinctive and predictable fractionation patterns upon the stable isotope ratio, making it an excellent tool for distinguishing between them. Historically, isotope constrained inverse methods for calculating CO2 surface fluxes—the ‘double deconvolution’—have disagreed with bottom-up ocean flux estimates. In this study, we use the double deconvolution framework, but add, as a constraint, independent estimates of time histories of ocean fluxes to the atmospheric observations of CO2 and 13CO2. We calculate timeseries of net land flux, total disequilibrium flux and terrestrial disequilibrium flux from 1991 to 2008 that are consistent with bottom-up net ocean fluxes.We investigate possible drivers of interannual variability in terrestrial disequilibrium flux, including terrestrial discrimination, and test the sensitivity of our results to those mechanisms. We find that C3 plant discrimination and shifts in the global composition of C3 and C4 vegetation are likely drivers of interannual variability in terrestrial disequilibrium flux, while contributions from heterotrophic respiration and disturbance anomalies are also possible.
Highlights
The atmospheric growth rate of CO2 and global fossil fuel emissions are both well known, with low uncertainty
If ocean flux variability is as low as is suggested by bottom-up estimates, we find that global disequilibrium flux must have higher than expected interannual variability to satisfy atmospheric observations of CO2 and 13CO2
The high level of variability in disequilibrium flux is inconsistent with the 13C Suess Effect, we find that, taken together, variability in a range of terrestrial isotopic parameters can explain the disequilibrium changes (Fig. 10)
Summary
The atmospheric growth rate of CO2 and global fossil fuel emissions are both well known, with low uncertainty This means that the calculated net surface exchange history, the combined ocean and terrestrial biospheric fluxes, is well known. The bottom-up approach is more directly informative about processes, but its primary weakness is the scaling problem it entails; surface observations are often limited in space and time and scaling up from local observations to regional to global fluxes is a challenge. Bottom-up and top-down approaches are naturally complementary, and each provides a valuable check on the other
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