Ice, albedo, and the Snowball Earth theory

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.

A diagram of a planet undergoing runaway ice-albedo feedback.

A diagram of a planet undergoing runaway ice-albedo feedback.

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!


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11 Responses to “Ice, albedo, and the Snowball Earth theory”

  1. Jim Cans Says:

    You mention that there are factors that limit the feedback effect that can lead to snowball earth. What are these factors?

  2. Psychroteuthis Says:

    Well, I don’t fully understand the models, but I think anything that keeps the Earth warmer can act to halt the progress. Carbon dioxide would be a big one. Ocean currents might be another.

  3. Kathe Says:

    From my paleo class this year (so Gina please feel free to correct this!):

    Long term (25,000+ years) ice sheet patterns are are mainly governed by the earth’s tilt and orbit. How much summer insolation (ie energy from the sun) the Northern Hemisphere gets determines how much the ice sheet grows according to Milakovitch Theory, which is currently the most popular.

    Shorter ice sheet patterns are mainly the result of internal dynamics. Accumulation/depletion of greenhouse gases and changes in energy transport (ie atmospheric/ocean circulation) are the two biggies.

    Snowball Earth is kind of unique. The albedo of the snow means that insoluation can’t melt the snow by itself. But CO2 would have accumulated in the atmosphere, slowly. Volcanoes would have added it to the atmosphere and there would have been no way to remove it in an ice covered world.

  4. Psychroteuthis Says:

    Milankovitch cycles are awesome. I think The Two Mile Time Machine has a particularly good overview of how they study that sort of thing using ice cores.

    I’ve been holding back on my next Snowball post while I work out the details of how CO2 is removed from the atmosphere in non-ice-covered-planet situations. V. interesting stuff, particularly in relation to the debate between “slushball” and “hard snowball.”

  5. Salt, Sea Ice and Science « Psychroteuthis Says:

    […] may recall from earlier posts how increased albedo can lead to a Snowball and CO2 can end one. As the planet cooled off and the climate system change, new types of ice could […]

    • Kathe Says:

      I will have to look up that book!

      Understandable reason to hold back that post! I’d be interested in reading it, my understanding is that it’s mostly ocean chemistry (specifically carbonate reactions) with overall biomass playing a smaller role. Not my field though so I would be interested in your take on it!

  6. Whys and Wherefores « Squid on the Ice Says:

    […] how much light reflects off of these blue ice zones, because there would have been a lot of them on Snowball Earth. (Here’s a great article in the Antarctic Sun that talks about our research.) Many areas of […]

  7. Feeding life, feeding back « Squid on the Ice Says:

    […] climate dynamics is the role of feedbacks. There are two kinds of feedbacks, positive and negative. This earlier post about Snowball Earth describes a positive feedback, in which Effect One (the freezing of seawater) leads to Effect Two […]

  8. Healthy Skin Treatment Products Says:

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  9. Mr. Science Says:

    The ice and snow have high albedo which keeps temperatures cool, so if temperatures rise enough, global temperatures are thought to rise initially when the ice-albedo feedback is no longer there. But, I am more interested in knowing how the cold and non-saline water mixing with the warmer ocean effects the currents, and how, as time progresses, the global temperatures and circulation change. For instance, could the earth fluctuate between extreme warming and cooling periods? I know this is a lot to explain, but if you know anything about this (scientifically), please let me know! šŸ˜› THANKS

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