Abstract
<title>Abstract</title> <p>How long a climate-driven carbon imbalance persists determines how long atmospheric carbon dioxide (CO₂) continues to accumulate after the forcing peaks. Here, using a four-decade surface CO₂ record, we quantify the post-peak atmospheric response toacross 12 El Niño events since 1980, including six strong events with peak Oceanic Niño Index values ≥ 1.5°C. The 2023/24 event reached its peak response later than any other event studied here and had one of the longest durations, even though its peak forcing was significantly weaker than that of the super El Niño events of 1997/98 and 2015/16. Atmospheric CO₂ growth reached its maximum seven months after the El Niño peak and remained above half of that maximum for 10 months. The detrended cumulative anomaly in 2023/24 was approximately twice than that following the 2015/16 event. The prolonged late response was not matched by the observed oceanic or fire signals, both of which were weaker than in 2015/16. Independent land-flux estimates showed a persistent late-phase shift toward greater net carbon release, temporally consistent with the delayed atmospheric CO₂ response and reproduced across multiple inversion products. These results identify a prolonged post-peak atmospheric response that has not previously been quantified systematically and is temporally consistent with a persistent late-phase imbalance in net land–atmosphere carbon exchange. Because atmospheric CO₂ integrates net surface carbon exchange, the persistence and cumulative magnitude of its anomaly provide a system-level record of how long climate-driven carbon imbalances endure. This imbalance can prolong CO₂ accumulation and might amplify positive carbon–climate feedbacks.</p>