Leveraging Multi-Model Land-Use Emissions Reveals a Rising Airborne Fraction of Atmospheric $CO_2$ Over the Past Six Decades
The airborne fraction, the share of anthropogenic $CO_2$ emissions that remains in the atmosphere rather than being taken up by land and ocean, is a compact diagnostic of sink efficiency and a direct input to the remaining carbon budgets behind climate targets. Here we show that it has risen over the observational record, from $0.32$-$0.40$ in $1959$ to $0.47$-$0.50$ by $2024$ (ranges across estimators), above the $0.44$ commonly reported for recent decades. Applied to IPCC AR6 assessments, this rise reduces the remaining carbon budget by $23$-$48$ $GtCO_2$ for a given temperature target, up to roughly one year of current global fossil emissions. The trend has been difficult to detect because two features of the standard analysis obscure it. First, three high-leverage years following the $1991$ Pinatubo eruption dominate the volcanic term of the natural-variability adjustment and attenuate the estimated trend; smoothing those years, or using a volcanically robust $ENSO$ index, removes the artifact. Second, collapsing land-use and land-cover change emissions to a single series discards the disagreement among models that carries the information needed to resolve a small trend. Propagating the dispersion of $69$ land-use series together with the reported fossil-emission uncertainty into delta-method weights, weighted and generalized least squares recover a positive and significant trend, corroborated by a mixed-effects model and robust across a grid of $49$ natural-variability specifications that maps which treatment choices reproduce which published result.