COMMON DESCENT

Listen to Episode 124 on PodBean, YouTube, Spotify, or anywhere you can find it!

An Earth covered in ice might sound far-fetched, but there is enough evidence in the geologic record and climate modeling for Earth scientists to suspect this has actually happened in our planet’s past – more than once. This episode, we discuss the origins, evidence, and much-debated questions surrounding the hypothesis of Snowball Earth.

In the news
A tardigrade in amber! The third fossil tardigrade known.
Chemical signals in fossil hair indicate meat-eating habits of giant sloths
Shark community evolution revealed by thousands of teeth over millions of years
A mangrove forest stranded far from the ocean when sea levels dropped.

The Iciest of Ice Ages

Numerous times throughout Earth history, the planet has grown particularly cold, with permanent ice forming at the poles and at high elevations (like today!). During certain pulses of glaciation, glacial ice has extended even further across continents and down mountain slopes. The Snowball Earth hypothesis proposes, based on certain geologic evidence, that multiple times in the past, ice ages have grown so extreme that nearly the entire planet was covered in ice.

There are three periods of the past that are generally suspected to have been Snowball Earth events: the Huronian glaciation (~2.4-2.1 billion years ago), the Sturtian glaciation (~720-660 million years ago), and the Marinoan glaciation (~645-635 million years ago). All three occurred during the Proterozoic Eon, and thus they predate the Cambrian Explosion and the rise of complex plant and animal life. The term “Snowball Earth” was coined in 1989, but geologists and climatologists had noted potential evidence for world-wide glaciation decades earlier.

Over the years, there has been a lot of debate over this hypothesis, especially regarding just how icy the Earth might have gotten. Some scientists propose a “hard snowball” or “iceball,” an Earth whose continents and oceans are completely covered in ice; others prefer a “slushball” model, where at least some (and perhaps quite a lot) of the oceans remain ice-free; and still others have proposed that the Snowball Earth hypothesis is misplaced, and that these particular ice ages might not have been so extreme after all.

How to Freeze a Planet

According to the hypothesis, Snowball Earth occurs as a consequence of the albedo effect: ice reflects solar radiation (including heat), which cools its surroundings, which allows more ice to form, which reflects more heat, etc. This phenomenon, noted by climatologists back in the 1960s, is part of what keeps the poles cold today. Some climate models show that if enough ice formed on Earth, the planet would experience a “runaway albedo effect” that would cool down and ice over the whole globe.

Various other factors have been suggested to have helped the Earth get so cold in the first place, most notably that the breakup of ancient supercontinents (specifically, Kenorland and Rodinia) would have exposed a lot of rock to weathering, which draws CO2 (a greenhouse gas that traps heat) out of the atmosphere, allowing the planet to cool. The Huronian glaciation occurred close in time to the Great Oxidation Event, which would have seen rising levels of atmospheric oxygen, but also falling levels of methane, another greenhouse gas. Some researchers have also identified possible “triggers” that could have pushed an already-cold Earth over the edge, such as massive volcanic activity.

As for how a Snowball Earth would end, the answer might lie with plate tectonics. Iced-over continents would eventually stop drawing CO2 out of the atmosphere via weathering, but tectonic activity would continue to release CO2 out of volcanoes, and the greenhouse gas would build up in the atmosphere until it was trapping enough solar heat to start warming up the planet. Once the ice began to melt, a “reverse runaway” would occur: less ice means less reflected heat, which helps the Earth warm, which melts more ice, and so on – a meltdown! This would result in an especially warm planet with especially high sea levels and strange ocean structure.

Life in the Snowball

Proterozoic life consisted almost entirely of microscopic organisms, and fossil evidence is scarce from this time. Even so, a number of studies have found evidence of surprisingly rich ecosystems of microbial life persisting before, during, and after these Snowball periods. Some researchers have pointed out that a completely frozen planet should have created a mass extinction, and thus the survival of these organisms is evidence that these “Snowballs” weren’t so snowy after all. On the other hand, others note that microbes do quite well in modern-day cold environments, even with limited sunlight, so perhaps it isn’t surprising that ancient microbes could survive the Snowball. And still others suggest that, all things considered, we really don’t have enough fossil evidence yet to say one way or another.

It has also been proposed that Snowball Earths might have actually helped boost the diversity of life. The Huronian glaciation occurs close in time to the rise of eukaryotes, and the latter two glaciations occur just before the rise of complex life in the Ediacaran and Cambrian. However, dating the exact timing of these geologic and biological events is very difficult, so it’s hard to say for sure if these events were related.

Learn More

Snowball Earth, an overview (2000)

Snowball vs. Slushball:
Snowball Earth Has Melted Back To a Profound Wintry Mix (2010)
“Snowball Earth” Might Have Been Slushy (2015)

Huronian Glaciation and Atmospheric Change (technical)

Life during Snowball Earth, 2015 (technical)
Life response to Snowball Earth, 2005 (technical, paywalled)

And in regards to this episode’s Patron question:
Magnetic Reversals

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 113 – Paleoclimate
• Episode 25 – The Pleistocene Megafaunal Extinction
• Episode 11 – Antarctica
• Episode 114 – Polar Life

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