Robust decrease in El Niño/Southern Oscillation amplitude under long-term warming

El Niño/Southern Oscillation (ENSO) is the primary mode of interannual climate variability, and understanding its response to climate change is critical, but research remains divided on the direction and magnitude of that response. Some twenty-first-century simulations suggest that increased CO 2 strengthens ENSO, but studies suggest that on palaeoclimate timescales higher temperatures are associated with a reduced ENSO amplitude and a weaker Pacific zonal temperature gradient, sometimes termed a ‘permanent El Niño’. Internal variability complicates this debate by masking the response of ENSO to forcing in centennial-length projections. Here we exploit millennial-length climate model simulations to disentangle forced changes to ENSO under transient and equilibrated conditions. On transient timescales, models show a wide spread in ENSO responses but, on millennial timescales, nearly all of them show decreased ENSO amplitude and a weakened Pacific zonal temperature gradient. Our results reconcile differences among twenty-first-century simulations and suggest that CO 2 forcing dampens ENSO over the long term.