The airborne fraction is the share of anthropogenic carbon dioxide emissions that remains in the atmosphere and is a key indicator of carbon-cycle response and remaining carbon budgets under continued emissions. Whether this share is rising remains debated because inference is sensitive to uncertainty in land-use and land-cover change (LULC) emissions. Here we use all available LULC measurement series from Global Carbon Budget 2025 and estimate airborne-fraction trends with a mixed-effects model with random intercepts and slopes by LULC series. We find that the airborne fraction increased over 1959-2024, from about 0.40 to about 0.47, and that this conclusion is robust to excluding the final year and to alternative specifications that explicitly propagate denominator uncertainty. These results clarify why earlier studies reported weak or inconclusive trend evidence and strengthen support for the view that an increasing share of emitted carbon dioxide is accumulating in the atmosphere rather than being taken up by land and ocean sinks, with implications for carbon-budget assessment and near-term mitigation requirements.
