A New Approach to the CO2 Airborne Fraction: Enhancing Statistical Precision and Tackling Zero Emissions

The global fraction of anthropogenically emitted CO2 that stays in the atmosphere, the CO2 airborne fraction, has been fluctuating around a constant value over the recent period of 1959-2021. The consensus estimate of the airborne fraction is around $44\%$, indicating that approximately $44\%$ of emitted CO2 stays in the atmosphere and the remaining $56\%$ is absorbed by the oceanic and terrestrial biospheres. In this study, we show that the conventional estimator of the airborne fraction suffers from a number of statistical deficiencies, such as non-existence of moments and a non-Gaussian limiting distribution. We formulate a new estimator of the airborne fraction that does not suffer from these deficiencies. We show that the new estimator is superconsistent, has a Gaussian limiting distribution, and is able to reduce estimation uncertainty substantially. Our empirical analysis leads to an estimate of the airborne fraction over 1959-2021 of $47.6\%$ ($\pm 1.1\%$; $1 \sigma$). Hence, our empirical results imply a higher, and better constrained, airborne fraction than the current consensus. Our theoretical results indicate that a conventional analysis of the airborne fraction may be problematic, especially in future low emissions scenarios such as those implied by "net zero" international agreements. Our newly proposed estimator avoids these issues and thus provides a promising approach to future studies of the airborne fraction, especially in low emission scenarios.