Future Weakening of the ENSO Ocean Carbon Buffer Under Anthropogenic Forcing

The El Niño Southern Oscillation (ENSO) is the largest driver of atmospheric CO₂ interannual variability. The equatorial Pacific Ocean, which is a natural source of CO₂ to the atmosphere, acts as a buffer of the terrestrial variations by releasing less CO₂ during El Niño events and releasing more CO₂ during La Niña events. Here, we examine how this ocean carbon buffer will respond to high anthropogenic emissions using Earth system models from Coupled Model Intercomparison Project Phase 6. The models project weaker ocean CO₂ flux anomalies in the future due to two main factors: a stronger compensation between thermal and nonthermal changes in ocean biogeochemistry (55% of signal) and a weaker influence of ENSO on the CO₂ transfer across the air-sea interface (10%–25% of signal). The ocean would no longer buffer the land response to ENSO, and could even reinforce its impact on atmospheric CO₂, amplifying the atmospheric carbon variability on interannual timescales.