The Cryosphere In The Distant Past

For decades, Earth scientists argued that the continuous persistence and success of life on Earth for more than 3 billion years is clear proof of a Goldilocks state: never too hot, never too cold for life to persevere. Earth somehow managed to avoid extreme states such as a runaway greenhouse effect where the oceans boiled away or, more germane to this chapter, a "snowball" state where the planet was completely ice-covered. The latter would be not only tough on life but also an extremely difficult state from which to escape. The positive cryospheric feedbacks discussed in chapter 8, in particular the effect of ice-covered tropical regions on the global albedo, would limit the available energy that could be harnessed to melt away the ice and break out of the snowball state.

This view has been challenged by an accumulating body of evidence indicating global or near-global ice cover on at least two occasions in the deep past: the Sturtian glaciation, ca. 720 Ma (million years before present), and the Marinoan glaciation, ca. 640 Ma. There is evidence from tillites (ancient glacial tills) and ocean sediments for tropical ice cover at this time. The Earth may have escaped an ice-covered state through buildup of CO2 in the atmosphere as a result of continued volcanic outgasing, combined with the absence of terrestrial and oceanic carbon sinks. Evidence from cap carbonates in the ocean supports this hypothesis. The meltdown process would have been aided by the gradual darkening (decreased albedo) of snow and ice as open-water moisture sources became cut off and fresh snowfall became limited in the hyperarid, snowball world. The climate conditions that initiated snowball Earth events are uncertain. The Sun was about 6% weaker during the Sturtian,1 which would have aided the process, but this does not explain what triggered the Neoproterozoic events.

The snowball Earth hypothesis remains controversial, and new ideas, geological evidence, and modeling studies are fueling ongoing debate. There are arguments for only partial ice cover during these two major events, with the tropical oceans remaining open. others argue for additional global glacial events throughout Earth's history. Although the story is still emerging, it is clear that major glaciations affected early Earth, probably assisted by the weaker Sun at this time, and there have been times when the cryosphere overwhelmed the entire planet for millions of years. Earth's oscillation between ice-covered and ice-free states is indicative of a delicately balanced climate. This climate sensitivity is largely a result of Earth's equilibrium temperature being so close to the triple point of water, along with cryosphere-cli-mate feedbacks that tend to amplify perturbations to our mean state.

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