Extreme El Niño events amplify floods, droughts, and wildfire risk worldwide, yet models still disagree on how ENSO variability will evolve under anthropogenic warming. Using multi-century CESM1.2 simulations spanning the Last Glacial Maximum to future scenarios, together with individual foraminiferal analyses (including new central Pacific/Line Islands constraints), we show that ENSO variability weakens substantially in glacial climates and strengthens under greenhouse warming, primarily because extreme El Niño events are triggered and sustained more frequently. The key mechanism is state-dependent air–sea coupling: a stronger Walker circulation deepens the mixed layer and contracts the warm pool, weakening the Bjerknes feedback and suppressing extremes; the converse occurs in warmer climates. We contend that the identified mechanism points to an increased risk of intensifying El Niño events under future climate scenarios. 

Speaker-suggested reading: Thirumalai, K., DiNezio, P.N., Partin, J.W. et al. Future increase in extreme El Niño supported by past glacial changes. Nature 634, 374–380 (2024). https://doi.org/10.1038/s41586-024-07984-y