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At least twice in Earth's history, nearly the entire planet was encased in a sheet of snow and ice. These dramatic "Snowball Earth" events occurred in quick succession, somewhere around 700 million years ago, and evidence suggests that the consecutive global ice ages set the stage for the subsequent explosion of complex, multicellular life on Earth.

Scientists have considered multiple scenarios for what may have tipped the planet into each ice age. While no single driving process has been identified, it's assumed that whatever triggered the temporary freeze-overs must have done so in a way that pushed the planet past a critical threshold, such as reducing incoming sunlight or atmospheric carbon dioxide to levels low enough to set off a global expansion of ice.

But MIT scientists now say that Snowball Earths were likely the product of "rate-induced glaciations." That is, they found the Earth can be tipped into a global ice age when the level of solar radiation it receives changes quickly over a geologically short period of time. The amount of solar radiation doesn't have to drop to a particular threshold point; as long as the decrease in incoming sunlight occurs faster than a critical rate, a temporary glaciation, or Snowball Earth, will follow.

These findings, published in the Proceedings of the Royal Society A, suggest that whatever triggered the Earth's ice ages most likely involved processes that quickly reduced the amount of solar radiation coming to the surface, such as widespread volcanic eruptions or biologically induced cloud formation that could have significantly blocked out the sun's rays.