The bathtub fills with warm water, but the drain is open. Each El Niño wave adds a pulse of heat, and only part of it escapes back to space. Over decades, this tub can heat up noticeably more than from just the slow turning of the greenhouse gas faucet.

Climate scientists have debated for decades how much natural ocean variability, like El Niño, can explain the observed warming. Earlier studies found a link between the Southern Oscillation Index (the pressure difference between Tahiti and Darwin) and air temperature, but they couldn't tie it to the long-term trend because they removed the trend itself from the data. The new work does it differently: it sums all index values since 1960 into a single cumulative total, like an accountant tallying the drops that fell into the tub.

When this cumulative index was added to a statistical model alongside solar activity, volcanic dust, and anthropogenic factors, it explained half of the tropospheric temperature changes and 54% of surface temperature changes. A simple model with only the accumulated El Niño turned out to be almost as strong as a model with only anthropogenic forcing. This doesn't mean greenhouse gases aren't warming the planet, but their contribution over the past half century may have been overestimated.

The accumulation mechanism is not as mysterious as it seems. The warm pool of water after El Niño doesn't vanish immediately: part of it sinks and returns the following winter, like a half-finished cup of tea topped up with boiling water. In this way, the ocean acts not just as a seesaw but as a slow heat battery. If this inertia is indeed large, climate models that underestimate it will systematically attribute natural warming to human activity.

The bathtub from the first sentence turns out to be not a metaphor but a diagnosis. The water in it rises not only from our faucet but also from the ocean's own warm waves. And until we learn to separate these two sources, any promise to predict climate precisely will remain nothing but reading tea leaves.