What Makes Zones Shift

The slowest, biggest shifts come from Milankovitch cycles — tiny, regular wobbles in Earth's orbit and the tilt of its axis. They change how much sunlight reaches each latitude. Playing out over tens of thousands of years, these rhythms are the main pacemaker of the ice ages that have come and gone for the last few million years.

Faster shifts have faster causes. A big volcanic eruption can fling sulfur high into the sky and dim the sun for a year or two. Greenhouse gases can rise or fall. And the ocean itself can flip: the thermohaline circulation, a global conveyor belt of deep currents, can slow down or restart over just decades — fast enough to reshape weather across whole continents.