If historical patterns repeat themselves, within about 2,000 years. But that's an extremely big "if." Over the past several million years, Earth has spent most of its time sheathed in ice. But about every 100,000 years, the planet thaws. These warm spells, called interglacial periods, usually last between 15,000 and 20,000 years. We've been enjoying our current interglacial period for about 18,000 years-giving us roughly 2,000 to go before the next deep freeze.