Warm water in the Pacific Ocean sometimes shifts toward South America. This phenomenon is called El Niño, and it changes weather around the globe. Occasionally El Niño is very strong, while at other times it barely appears. Why this happens has long been a puzzle.
Scientists built a simple computer model of the ocean. It turns out that the model has two modes, much like a pendulum. In one mode, the pendulum swings high and regularly, corresponding to years with strong El Niño events. In the other mode, it jitters with small oscillations, and no strong events occur. The switch between modes is triggered by very small perturbations, such as seasonal changes.
This means that precisely predicting the strength of El Niño many years ahead is impossible. However, one can estimate probabilities, just like in weather forecasts: not "will it rain?" but "what is the chance of rain?" The Pacific pendulum swings on its own, and guessing its next big swing is beyond our reach.
In a 6000-year simulation of the model, an epoch without strong El Niño could last more than two centuries, then suddenly give way to a series of intense events, all without any external forcing other than a weak seasonal cycle.