A bathtub of warm water sits under an open tap, but the drain isn't closed. Each El Niño wave adds a bit of heat, and only part of it flows out. Over time, the water in the tub gets hotter, even if the greenhouse gas tap has barely been touched.

Scientists have long struggled to understand why Earth is warming so quickly. They knew that El Niño is a natural warming of the Pacific Ocean that occurs every few years. But all previous attempts to link it to global warming failed because researchers looked only at individual years, not at the cumulative total. The new study did something simple: they took all Southern Oscillation Index values since 1960 and added them up, like collecting raindrops in a barrel.

It turned out that this accumulated sum explains half of the temperature changes in the upper air and 54% at the surface. A simple model with only accumulated El Niño works almost as well as a model with only greenhouse gases. This doesn't mean emissions aren't warming the planet, but perhaps their contribution has been overestimated.

The bathtub from the first sentence hints at the answer: the ocean doesn't just slosh back and forth; it remembers heat. Warm water from El Niño partially sinks to depth and returns the following year, like tea that's been topped up with boiling water before being finished. If so, climate models that ignore this ocean memory will make wrong predictions.