Ocean thermal inertia sustains a tropical Pacific mean state shift under net zero

The tropical Pacific plays a pivotal role in global climate, and changes in its mean state and in the El Niño Southern Oscillation can substantially alter regional hydroclimates with major economic and societal impacts. Yet, its response to different emission pathways remains poorly constrained. Here we assess changes in the tropical Pacific mean state and variability under high emission and net zero scenarios, using climate model simulations spanning centennial to millennial timescales. Under transient warming, the tropical Pacific shifts towards an El Niño-like mean state. Following emission cessation, continued warming of the Southern Ocean and deep ocean sustains this mean state for centuries, driving persistent El Niño-like rainfall anomalies and more frequent extreme El Niño events even as atmospheric carbon dioxide declines. Changes in ENSO amplitude, by contrast, are non-monotonic and more model dependent. Key aspects of tropical Pacific change are therefore effectively irreversible on multi-centennial timescales, and net zero emissions slow further change without restoring pre-industrial conditions. The tropical Pacific shifts towards El Niño-like conditions under ongoing warming and does not recover for centuries after emissions cease, according to climate model simulations spanning centennial to millennial timescales.