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

The airborne fraction of atmospheric CO2 (AF), defined as the annual global CO2 growth rate (dCO2/dt) divided by the total emission of CO2 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 dCO2/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 dCO2/dt as a function of anthropogenic CO2 emissions, ENSO indices, and stratospheric aerosol optical depth (a proxy for major volcanic eruptions), for numerous time series of the emission of CO2 due to LUC (ELUC). For 20 out of 21 previously published ELUC time series, the trend in AF adjusted for natural variability (AFADJ) 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 ELUC 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 ELUC. Plain Language Summary The term airborne fraction (AF) refers to the annual rise in globally averaged atmospheric CO2 divided by the total annual anthropogenic emissions of CO2. There is considerable interest in determining whether AF is changing over time because a rise in AF would imply the existence of a feedback between climate change and the global carbon cycle. Such a feedback might inhibit society's ability to limit global warming to a particular threshold, such as the 1.5°C target and 2°C goal of the Paris Agreement. Here, we analyze trends in AF by accounting for variations in the annual growth rate of atmospheric CO2 induced by ENSO events and major volcanic eruptions. We examine trends in AF over various time periods and for 21 previously published estimates of the global, annual emission of CO2 due to land use change (ELUC). For 20 out of these 21 estimates of ELUC, we find that AF has not changed in a statistically significant manner over the past six decades. Our study also highlights the importanc