The warm El Niño current arrives in the Pacific Ocean off schedule. It recurs every 3–4 years, but no one knows the exact year. Scientists built a computer model of the ocean and atmosphere with 930 variables, and it showed that noise is to blame.

A swing without pushes would soon stop. But if you push it randomly, it will keep swinging forever, only each time with a new delay. So it is with El Niño: noise keeps the ocean from settling down, but it also prevents predicting the next push.

That is why a forecast for a few months ahead still works, but for years ahead it no longer does. The longer we wait, the more the phase drifts, like a clock that sometimes runs fast and sometimes slow. Scientists can now measure this uncertainty and understand where the boundary of predictability lies.