A regression-based approach to the CO₂ airborne fraction

The fraction of anthropogenic CO₂ emissions that remains in the atmosphere—the airborne fraction (AF)—has fluctuated around a near-constant value since 1959, with a consensus estimate near 44%. This study shows that the conventional AF estimator, based on the ratio of yearly atmospheric CO₂ increases to emissions, has statistical deficiencies, including problematic behavior under trends and when emissions approach zero. The authors propose a regression-based estimator that avoids these issues. Using 1959–2022 data, they estimate the AF at 47.0% (±1.1%; 1σ), higher and more tightly constrained than the consensus. Using climate model output, they further show that the regression-based method yields sensible AF estimates in future scenarios, including when emissions are near or at zero.

Mikkel Bennedsen, Eric Hillebrand, Siem Jan Koopman