El Niño and La Niña events control much of Earth’s interannual variability, impacting surface temperatures and precipitation worldwide. Despite the importance of these events, how they will change as the climate warms is not well understood. Here, we use idealized atmosphere-ocean simulations to study ENSO under steady-state greenhouse warming. We find that the mean Pacific zonal temperature gradient weakens in warmer climates, which decreases ENSO growth rates and reduces the magnitude of extreme ENSO events. Additionally, weakening of ENSO leads to a decrease in ENSO asymmetry, i.e., the strength of El Niño and La Niña events become more similar in warmer climates. We interpret these results with a simple energy budget of the central-eastern equatorial Pacific.