Frozen Planet: Snow or Slush?

So far, we've mainly looked at the present day climate mechanisms, but for this blog post, I want to go back in time - and quite a bit. Between 800-550 million years ago, during the period in the Neoproterozoic that that geologists call the Cryogenian (from Greek cryos, "cold" and genesis, "birth"), some scientists believe that the Earth was covered largely by ice, in some places up to 5 km of it. This "Snowball Earth" hypothesis  by Hoffman et al. (1998) has been scrutinised, critisised (Allen and Etienne 2008) and praised - but how does it actually work?

There are several possible situations that can trigger a Snowball Earth, but we'll examine those later. For now, let's just assume it gets cold. Really, really cold. Ice starts creeping from the poles to lower latitudes, a bit like the more recent Ice Ages. But unlike those, they don't stop. The ice goes further towards the equator. Meanwhile, all that icy surface reflects a lot of sunlight back into space because of it's high albedo, causing the planet to cool even further. At some point, there is so much ice that a runaway effect takes place: the planet's albedo is so high, that the amount of sunlight that is absorbed is too small to keep the Earth warm, and the planet freezes over.

Geologists believe that this feedback scenario happened during the Cryogenian, and that it happen not just once; most likely, three times. The BBC has made an informative short film that explains how scientists found evidence for a Snowball Earth event.

There is much uncertainty as what triggered a Snowball Earth. It has been suggested that the evolution of photosynthetic organisms caused a reduction in methane (a potent greenhouse gas), as the oxygen they put into the atmosphere reacted with the methane to form the weaker greenhouse gas, CO2. This could have ultimately lead to a cooling effect (Kopp et al. 2005).

Another theory proposes that the break-up of the continent Rodinia as the onset of the cooling (Donnadieu et al. 2004). The researchers looked at increased runoff following the continental disintegration. Higher runoff would mean more weathering, which through chemical processes could lead to a reduction in CO2 and thus to cooling.

Reconstruction of the breakup of Rodinia, 750 million years ago, based on palaeomagnetic data (Torsvik 2003)

Despite these and other proposed mechanisms (lower solar input, changes in orbital forcing, etc., also see Hoffman et al. 1998), there has been no conclusive answer as how the world changed into an ice house.

Dropstones in formations in the Flinders Ranges, SA, Australia

Some scientists have further studied the dropstones, and in fact discovered evidence that seems to contradict a world fully covered by ice during some of the glaciations (Le Heron et al. 2011). Markings on the dropstones from the Flinders Ranges shows that they were deposited by ice, but have been affected by turbulent waters as well. If ice had covered all the oceans, the water underneath it would have been gentle and calm. The turbulence would have been caused by storms raging over ice free patches; instead of Snowball Earth, there would have been a "Slushball" Earth. They suggest that these patches are the places in which some organisms managed to survive the chilly conditions (Allen & Etienne 2008, Le Heron et al. 2011), and ultimately emerge when the ice disappeared again.

Artist's impression of "Slushball" Earth, with pockets of ocean

For the next blog entry, I'll look at how the planet came out of this icehouse situation - not going into a greenhouse, but a hothouse, and how that too was impacted by the albedo effect.

Further Reading

• Snowballearth.org, a website maintained by Hoffman and Schrag, pro-Snowball scientists
• 'Life may have survived 'Snowball Earth' in ocean pockets', BBC article on the work of Le Heron et al. 2011
• 'Snowball Earth was more like a slushball', Discovery News article

References

• Allen, P.A., and J.L. Etienne (2008), Sedimentary challenge to Snowball Earth, Nature Geoscience, 1, 817, doi:10.1038/ngeo355
• Donnadieu, Y., Y. Goddéris, G. Ramstein, A. Nédélec, and J. Meert (2004), A 'snowball Earth' climate triggered by continental break-up through changes in runoff, Nature, 428, 303, doi:10.1038/nature02408
• 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
• Kopp, R.E., J.L. Kirschvink, I.A. Hilburn, and C.Z. Nash (2005), The Paleoproterozoic snowball Earth: A climate disaster triggered by the evolution of oxygenic photosynthesis, PNAS, 102, 11131, doi:10.1073/pnas.0504878102
• Le Heron, D.P., G. Cox, A. Trundley, and A. Collins (2011), Sea ice-free conditions during the Sturtian glaciation (early Cryogenian), South Australia, Geology, 39, 31, doi:10.1130/G31547.1
• Torsvik, T.H. (2003), The Rodinia jigsaw puzzle, Science, 300, 1379, doi:10.1126/science.1083469