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.

翻译：大气CO2滞留分数是指人为二氧化碳排放中留存在大气中的比例，它是反映碳循环响应及持续排放下剩余碳预算的关键指标。由于该指标的推断对土地利用与土地覆盖变化（LULC）排放的不确定性高度敏感，因此其是否呈上升趋势仍存争议。本文利用全球碳计划2025年发布的所有可用的LULC测量序列，通过引入含随机截距和随机斜率的混合效应模型（按LULC序列分组），估算了大气CO2滞留分数的变化趋势。研究发现，1959-2024年间大气CO2滞留分数从约0.40上升至约0.47，且该结论在剔除末年度数据及采用能显式传导分母不确定性的替代模型时依然稳健。这些结果揭示了早期研究为何未能获得明确的趋势证据，并进一步支持"排放的二氧化碳中留存在大气中的比例持续增加，而非被陆地与海洋碳汇吸收"这一观点，对碳预算评估及近期减排需求具有重要启示。