A regression-based approach to the CO2 airborne fraction

The global fraction of anthropogenically emitted carbon dioxide (CO 2 ) that stays in the atmosphere, the CO 2 airborne fraction, has been fluctuating around a constant value over the period 1959 to 2022. The consensus estimate of the airborne fraction is around 44%. In this study, we show that the conventional estimator of the airborne fraction, based on a ratio of changes in atmospheric CO 2 concentrations and CO 2 emissions, suffers from a number of statistical deficiencies. We propose an alternative regression-based estimator of the airborne fraction that does not suffer from these deficiencies. Our empirical analysis leads to an estimate of the airborne fraction over 1959–2022 of 47.0% (± 1.1%; 1 σ ), implying a higher, and better constrained, estimate than the current consensus. Using climate model output, we show that a regression-based approach provides sensible estimates of the airborne fraction, also in future scenarios where emissions are at or near zero. A presented regression-based approach to the CO 2 airborne fraction is statistically superior to the conventional ratio-based approach. It reduces estimation uncertainty, requires fewer assumptions, and treats future scenarios with zero emissions.