Increased CO2 growth rate following the 2023/24 El Niño in a warming climate

Atmospheric CO2 growth is correlated with the El Niño–Southern Oscillation (ENSO), showing enhanced growth associated with strong El Niño events. Here, we combine GOSAT satellite observations with new ground-based total column measurements from Palau in the Tropical Western Pacific, with surface ocean pCO2 data, to analyze the 2015/16 and 2023/24 El Niño events. We show that the CO2 response to the 2023/24 event was strong and delayed, with the largest concentration increase occurring after the El Niño peak, and it remained elevated as El Niño conditions weakened. This led to a pronounced accumulation of atmospheric CO2 over the tropical Pacific during 2024–2025. A linear regression framework explains about one-third of the historical inter-annual variability in global CO2 growth, yet it underestimates the recent increase in 2025, which cannot be explained by early oceanic forcing and exceeds expectations based on linear ENSO scaling. After oceanic forcing weakens, terrestrial carbon-cycle processes persist, as drought and heat stress in a warming climate suppress photosynthesis and enhance respiration. Such delayed amplification is characteristic of terrestrial carbon-cycle processes that integrate climate anomalies over time in a warming regime, leading to more persistent CO2 accumulation. Our results indicate that post–El Niño terrestrial amplification has shifted from a passive response to a dominant driver of sustained CO2 growth, representing a critical but previously under-observed mechanism of the global carbon cycle in a changing climate.