SnowballEarthModel

http://www.cs.caltech.edu/~westside/ge/snowball.html

Class notes

What led to this theory

Finding things at wrong temperature compared to what you'd expect from a given latitude. For example, evidence of icebergs close to the equator, iron rich rocks, and so on. People had thought the earth's climate was much more stable, so this was a small scientific revolution.

Theory

• As the earth grew colder, there was more snow, so more light was reflected out (albedo), which made the earth colder still. (Freezing stage)

• Carbon dioxide from volcanoes remained trapped inside the atmosphere since the surface was covered with snow/ice. This led to a greenhouse effect and brought the earth out of snowball mode.

• Happened about 4 times in earth's history.

• Continents were amassed near the equator, which gets the most radiation, so there was more albedo. Therefore this also has to do with the arrangement of landmass.

• 5 variables for whole globe.

• More albedo => lower surface temperature (more reflection of heat)

• Higher surface temperature => less albedo (less snow)

• Optical depth = transparence, how much energy is absorbed as radiation moves.

Jake West's program:

Defines initial conditions. In loop, updates Albedo, carbon dioxide, water, optical depth, temperature.

With initial temperature and carbon dioxide at approximately earth's current level, earth reaches equilibrium at today's temperature.

With initially cold temperature and current carbon dioxide level, the snowball earth lasts for 16 million years (close to the 10 million years that it is believed to have lasted). This is followed by rapid temperature and rise and melting ice, and then stabilization.

With initially warm temperature and high carbon dioxide levels, there is no snowball effect, and the temperature stabilizes.

Model in article versus J.W.'s model

• The article accounts for differences between land and ocean, while JW treats the whole earth as uniform.

• JW treats the dynamics of carbon dioxide vaguely, as having an exponential relationship with temperature, ignoring the relationship between carbon dioxide and water. Sacrifices realism for "easier math."

• JW does not deal with volcanoes. The article assumes that volcanoes were the only major (continuous) source of carbon dioxide 800 million years ago, and also does not account for possible peaks or changes in volcanic activity (but some other models do).

• JW's model does not include the feedback loop that led to the snowball earth. But that may not be relevant to the purpose of his model.

Is JW's model useless?

• His equations still work, and "kind of" model what happened.

• It shows that you can have a stable earth, even without going into the details. (But how do you know that entering into the details would not change the outcome?)

• Some points that the model does not detail can be accounted for by other models. For example, optical depth can be explained/understood using Bier's Law (based on photons, model for how light moves through media, with the amount of light absorbed being proportional to the concentration of matter in the medium).

• In general, models can break a phenomenon into smaller variables that can be understood or modeled more easily, or that we already know about. This helps them turn theories into something complete.

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