A frozen planet - II

Planet vitals:

- Average temperature: -20 to -50°C

- Pack ice thickness: 500-1500 m
(Kirschvink, et al., 2000)

Source: BBC

How did the Earth fall into such extreme conditions?

First of all, the young Sun was weaker than in the present day (Tziperman et al., 2011; Young, 2012). Perhaps the only reason the Earth was not under permanent icehouse conditions in the past was the high concentration of greenhouse gases (mainly CO2 and CH4) in the atmosphere. This means that with a reduction in greenhouse gases, the Earth would cool down (Young, 2012 and Tang and Chen, 2013).

CH4 levels fell with the rise of oxygen during the Great Oxidation Event, eliminating one of the greenhouse gases. CO2 reduction was also related with this event, since microorganisms would consume it during photosynthesis (Tang and Chen, 2013). However, weathering is also an important mechanism to drawdown CO2. This is where the supercontinents come in. Young (2012) notes that the Snowball events at the beginning and end of the Proterozoic coincided with the presence of supercontinents. These are very thick and rise higher with respect to sea level than smaller continents, exposing more surface subject to subaerial weathering, so more CO2 can be "buried", effectively reducing atmospheric CO2. Fig. A is a simplified diagram that shows the conditions that made the Snowball Earth events (marked with red arrows) possible. For the first event there was low solar luminosity and a drop in CO2, caused by increased weathering due to the Kenorland supercontinent. Once it broke up, the CO2 drawdown was reduced, but maintained a downward trend (the atmospheric O2 was still rising). Solar luminosity also progressively increased, preventing icehouse conditions despite the lowering CO2. However, once Rodinia was formed, the CO2 weathering increased once again, breaking the balance and plunging the Earth into another glaciation.

Fig. A: Relation between glaciations and supercontinental cycle. Young, 2012

Once the ice began to expand across the planet, there was no stopping it; a runaway ice-albedo effect would be triggered when sea ice crossed between 30° to 40° latitude, causing the whole planet to be covered in ice (Tziperman et al., 2011).

How did it end?

It seems like there is no escape. With the runaway ice-albedo effect, the cooling effect is reinforced endlessly. However, as shown in fig. B, weathering is prevented by the ice cover, allowing CO2 levels to build up again, warming the planet and allowing the ice to melt (Young, 2012). Volcanoes would be the source of CO2 (Hoffman et al., 2002) and Kirschvink et al. (2000) estimated it would take roughly 35 million years to end a Snowball event.

Fig. B: Feedback loop causing alternation between icehouse and greenhouse conditions. Young, 2012.