A. Bershadskii

A swing builds up when you push it in time. But sometimes the person on the swing responds not to every push but to every third one, and the amplitude grows slowly yet inexorably.

A swing pushed once per second usually oscillates once per second. But if you push it harder and at the right rhythm, it can swing three times slower: one full swing for every three pushes. Physicists call this subharmonic resonance, and it seems to govern the most prominent climate and solar oscillations.

Every summer, the same performance unfolds over the Indian Ocean: moist air rushes from the sea onto the scorched land, bringing rains that billions of people depend on.