From Icehouse to Hothouse: Breaking Out of Global Glaciation

Last week's entry was about how a global onset of glaciation could take place, the so-called runaway albedo feedback. Quick recap: due to some sort of cooling event, ice sheets start forming on the poles and beyond, increasing the Earth's albedo. The vast expanses of ice reflect high amounts of the incoming sunlight back into space, cooling the surface even further, and a runaway effect takes place, covering the Earth completely in ice, even the tropics. These "Snowball Earth" events have taken place in the Cryogenian,  800-550 million years ago. Recent research suggests that the Earth may have actually been ice-free in some spots near the equator, a hypothesis promptly named "Slushball Earth".

What has puzzled scientists is not only how the Earth might have wound up covered in ice, but also how these icy conditions were reversed (which obviously happened at some point, considering that there are not meters of ice covering me right now as I'm sitting behind my desk in London). It would have been very difficult for a world with such a high albedo to warm up again.

Imagine a world covered in ice, like this icy plain in the Arctic 

(Source: Reuters)

CO2, carbon dioxide, is a greenhouse gas, which traps part of the outgoing radiation of the Earth. The higher the concentration in the atmosphere, the more heat from will be trapped (creating the well-known greenhouse effect). The original proponents of the Snowball Earth hypothesis, Hoffman et al. (1998), suggested that extreme amounts of CO2 could have defrosted the planet.

The mechanism is relatively simple. While the Earth is in a state of glaciation, vulcanoes continue to work, sticking out of the ice like pimples. Vulcanoes are notorious emitters of CO2. The carbon dioxide they emit builds up in the atmosphere during the millions of years of glaciation. At some point, there is so much CO2 in the atmosphere that temperatures start to rise and the ice begins to melt. This reduces the albedo, which means more solar heat is absorbed by the now open ocean waters and land masses, leading to higher temperatures, leading to more ice melt, reducing the albedo... Etc.

This video shows how vulcanoes might have broken through the snowball state of the planet. I do want to add as a side note that this is a rather dated video and so it does not take some of the "Slushball" hypotheses into account, nor does it examine the arguments of opponents of "Snowball Earth" theory (also see last week's post). Having said that, it does explain nicely in the first seven minutes what the relationship between vulcanoes, CO2 and deglaciation is, which is why I've added it to this post.

This feedback loop would have taken place very rapidly (on a geological timescale, that is). In a mere couple of millions of years, the Earth would have bounced from a complete icehouse to a hothouse (Hoffman et al. 1998).

To return the planet from a "Snowball" or "Slushball" state, a lot of CO2 is needed (Pierrehumbert 2004). Crowley et al. (2001) calculated that about 120,000 ppm (parts per million) of CO2 would be required - which is roughly 300 times of the amount of CO2 currently in the air.

In a research conducted by Bao et al. (2009), oxygen isotopes in sulphate deposits in Svalbard from 635 Million years ago, roughly the end of the Marinoan glaciation, and one of the supposed "Snowball Earth" events, were studied to find whether CO2 levels had been that high. They concluded that either the oxygen cycle must have been very, very different from what it is today, or CO2 levels were extremely high, supporting the case for a "Snowball Earth".

Eyjafjallajökull, an Icelandic vulcano, erupts in 2010 

(Source: National Geographic)

Nevertheless, that was not the end of the matter. An even more recent study by Sansjofre et al. (2011) reinterpreted the data, and found that CO2 levels were more likely around 3200 ppm, quite a bit less than the required 120,000 ppm. They suggest that glaciation was not as intense as thought before, or that perhaps there were different glaciation mechanisms at play.

To conclude; there is evidence that the world was once subject to intense glaciation, the extent of which remains under debate. Moreover, the end of this period and the processes behind the defrost are still uncertain and continue to be researched. Even so, it is interesting to see how important albedo can be, providing a feedback mechanism to enable glaciation, and to reverse it.

I wanted to add these two posts about the "Snowball Earth" hypothesis to give a bit of a framework when it comes to ice cover in the extremest sense of the word, even though "Snowball Earth" took place more than 550 million years ago. It is by studying these obscure details of the planet's natural history that we are able to better understand climate in general, and what that means for us today.

Next post: clouds - perhaps the most enigmatic (and important) component of the climate system.

References

• Bao, H., I.J. Fairchild, P.M. Wynn, and C. Spötl (2009), Surface environments: Neoproterozoic glacial lakes from Svalbard, Science, 323, 119, doi:10.1126/science.1165373
• Crowley, T.J., W.T. Hyde, and W.R. Peltier (2001), CO2 levels required for deglaciation of a "Near-Snowball" Earth, Geophys. Res. Let., 28, 283
• Hoffman, P.F., A.J. Kaufman, G.P. Halverson, and D.P. Schrag (1998), A Neoproterozoic Snowball Earth, Science, 281, 1342, doi:10.1126/science.281.5381.1342
• Pierrehumbert, R.T. (2004), High levels of atmospheric carbon dioxide necessary for the termination of global glaciation, Nature, 419, 646, doi:10.1038/nature02640
• Sansjofre, P., M. Ader, R.I.F. Trindade, M. Elie, J. Lyons, P. Cartigny, and A.C.R. Nogueira (2011), A carbon isotope challenge to the snowball Earth, Nature, 478, 93, doi:10.1038/nature10499