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：大气碳留存率是指人为二氧化碳排放中滞留在大气中的比例，是反映碳循环响应及持续排放情景下剩余碳预算的关键指标。然而，由于土地利用与土地覆被变化（LULC）排放的不确定性对推断结果存在显著影响，该比例是否持续上升仍存在争议。本研究采用全球碳预算2025中所有可获取的LULC测量序列，通过构建含随机截距和随机斜率的混合效应模型估算大气碳留存率变化趋势。研究结果显示，1959-2024年间大气碳留存率从约0.40上升至约0.47，该结论对剔除末年份数据及明确传播分母不确定性的替代方案均保持稳健。这些发现厘清了早期研究为何仅呈现微弱或非结论性趋势证据，同时强化了以下观点：排放的二氧化碳中滞留在大气中的比例持续增加，而陆地与海洋碳汇的吸收比例相应减少，这对碳预算评估与近期减排需求具有重要启示。