Quantification of the Airborne Fraction of Atmospheric CO₂ Reveals Stability in Global Carbon Sinks Over the Past Six Decades

The airborne fraction of atmospheric CO 2 (AF), defined as the annual global CO 2 growth rate ( d CO 2 / dt ) divided by the total emission of CO 2 from combustion of fossil fuels and land use change (LUC), has a long‐term average of ∼0.44 over the past six decades. When quantifying trends in AF it is important to account for inter‐annual variability in d CO 2 / dt due to natural factors such as the El Niño Southern Oscillation (ENSO) and major volcanic eruptions, as well as assumptions regarding LUC. Here, a multiple linear regression model is used to compute d CO 2 / dt as a function of anthropogenic CO 2 emissions, ENSO indices, and stratospheric aerosol optical depth (a proxy for major volcanic eruptions), for numerous time series of the emission of CO 2 due to LUC ( E LUC ). For 20 out of 21 previously published E LUC time series, the trend in AF adjusted for natural variability (AF ADJ ) over 1959 to 2021 exhibits a trend that is statistically indistinguishable from zero and lacks statistical significance at the 95% confidence interval. Therefore, it is most likely that the relative efficacy of the combined global terrestrial biosphere and oceanic carbon sinks has been fairly constant on a global scale over the past six decades. Since the trend in AF exhibits considerable variability depending on which E LUC time series is used, more precise knowledge of the actual value of the AF trend will require resolving the current large differences in various estimates of E LUC .