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Snowball Earth (changes)

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Contents

Idea

Starting about 850 million years ago, something dramatic happened: episodes of runaway glaciation during which most or all the Earth was covered with ice. Advocates of the extreme version of this scenario call them Snowball Earth events, while others argue for a mere ‘Slushball’.

Since ice reflects sunlight, making the Earth even colder, it’s easy to guess how such runaway feedback might happen. The opposite sort of feedback is happening now, as melting ice makes the Earth darker and thus even warmer. The interesting questions are why this instability doesn’t keep driving the Earth to extreme temperatures one way or another, why the Snowball Earth events started when they did, and why the Earth didn’t stay frozen.

Here’s a currently popular answer to the last question. Ice sheets slow down the weathering of rock. This weathering is one of the main long-term processes that use up atmospheric carbon dioxide, by converting it into various carbonate minerals. On the other hand, even on an ice-covered Earth, volcanic activity would keep putting carbon dioxide into the atmosphere. So, eventually carbon dioxide would build up, and the greenhouse effect would warm things up again. When the ice melted, weathering would increase and the amount of carbon dioxide in the atmosphere would drop again. However, this feedback loop is very slow. Indeed, it has been suggested that in the hot phase, as much as 13% of the atmosphere could be carbon dioxide — 350 times what we see today!

By the end of these glacial cycles, it is believed that oxygen had increased from its early value of 2% of the atmosphere to 15%. (Now it’s 21%.) This may be why multi-celled oxygen-breathing organisms date back to this time. Others argue that the ‘freeze-fry cycle imposed tremendous evolutionary pressure on life and led to the rise of multicellular organisms. Both these theories could be true.

Details

The picture above is taken from:

The following paper describes a simple energy balance model with ice albedo feedback?, which exhibits a Snowball Earth phase:

Equations (11) and (12) in this paper have, for certain amounts of solar radiation falling on the Earth, three stationary solutions that correspond to possible climates for the Earth:

Here ‘insolation’ is jargon for the amount of solar radiation reaching the Earth, and ‘EBM’ is an acronym for energy balance model. The dot shows the Earth’s climate now, in this simplified model. If the insolation were lowered there would be a tipping point, and the only solution would be a much cooler Snowball Earth. At somewhat higher insolations we see three phases: a stable warm phase, a stable Snowball Earth phase, and an unstable phase with an intermediate temperature, drawn as a dashed red line. At even higher insolations only the warm phase exists.

This situation is an example of a ‘fold catastrophe’.

When plotted as a function of ice line latitude (boundary of the polar ice caps) vs. insolation, instead of temperature vs. insolation, the equilibrium curve in the above figure is an example of the slope-stability theorem. This theorem states that the stability of the equilibrium solution depicted by such a curve is determined by its slope, with positive slopes corresponding to stable solutions, and negative slopes corresponding to unstable solutions.

A general review of the evidence concerning the Snowball Earth hypothesis is

A variety of Snowball Earth energy balance models, and a feedback analysis of the Snowball Earth system, are discussed in

A semi-popularized account of Snowball Earth, and its relevance to the evolution of life on this and other planets, may be found in Chapter 5 of

References

category: climate

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