Life on a planet is a fragile thing, particularly if the life is at all complex. Naturally, astrobiologists like to catalog the ways that life can be decimated or snuffed out entirely. Asteroids, comets, and bursts of high-energy gamma radiation are commonly cited catastrophes, but a planet can also run into trouble without any cosmic interference. One of these more self-contained scenarios is “runaway ice-albedo feedback.”
Albedo, if you aren’t familiar with the term, is just the amount of light that’s reflected from a surface relative to the total amount of light hitting it. You can measure it using a device that collects all the light coming down from the sky, then flips over to collect all the light being reflected back from the surface below it. Divide the latter by the former and you have your albedo. Dark-colored things have a low albedo–for instance, the open ocean has an albedo of around 0.06, meaning that it reflects only 6% of the light that hits it and absorbs the rest, which is mostly converted to heat. Light-colored things have a high albedo–for instance, sea ice has an albedo between 0.5 and 0.7, which goes up to 0.9 or higher when the sea ice is covered by snow. The relationship between albedo and absorbed heat will probably be obvious to anyone who has ever worn a black shirt on a hot, sunny summer day. Summer clothes tend to be light-colored because it maximizes your personal albedo and keeps you pleasantly cool.
Ice-albedo feedback occurs because ice and snow are highly reflective. Suppose a planet cools down a little, perhaps because of a reduction in solar output or the amount of CO2 in the air. The ice caps will grow a bit, which will reflect more sunlight back into space. With less sunlight being absorbed, the planet cools a bit more, the ice caps grow a bit more, and so on. (This works in the other direction, as well–the ice at the North Pole is melting faster because it exposes low-albedo ocean as it retreats.) Usually this feedback loop runs into some limit and stops. Theoretically, however, it can go into a “runaway” state. Computer models of this process suggest that once ice reaches 30 degrees latitude, the feedback becomes unstoppable and the entire planet freezes over very rapidly.
This fully-frozen state is fairly stable; the ice reflects most of the sunlight back into space, keeping the planet cold. In fact, when Mikhail Budyko first modeled this phenomenon in the 1960s, he concluded that it could never have happened on the Earth because there would have been no way to escape from it.
However, in recent decades an increasing amount of geological evidence seems to suggest that the entire Earth did freeze over, around 600 million years ago (and perhaps at other times before that.) How did the Earth transform from its frigid Snowball Earth alter ego back into the warm, pleasant planet we know today? Tune in next time to find out!