Snowball Earth was a severe ice age, covering Earth entirely in kilometres of thick ice and snow around 635 million years ago.

Ice on our planet is more or less uncommon, but once in a while, temperatures on Earth fall low enough, and kilometres of thick glaciers begin to form on its poles. Sometimes, glaciers spread even further and cover more and more of the surface. These periods are called ice ages.

There are many reasons behind ice ages, and whether alone or together, they may begin to cool the Earth. For example, the proportion of warming greenhouse gases, such as carbon dioxide, may decrease in the atmosphere, sometimes withdrawn from the air by heavy rains and stored in the ground for millions upon millions of years. Warm and cold deep ocean currents may shift as continents slowly change their positions. Besides, every now and then, Earth travels a bit farther from the Sun, wobbles and tilts back and forth around its axis (Milankovitch cycles), each altering the amount of sunlight reaching our planet and especially its poles. And once glaciers begin to spread, and the whiter our planet becomes, the more sunrays are reflected back to space, making our world ever colder (albedo effect).

The first ice age on Earth appeared more than two billion years ago when more and more oxygen began to spread to the atmosphere and replace greenhouse gases. The young Sun was once much dimmer than today, too. However, the most severe ice age occurred hundreds of millions of years later, an episode threatening the birth of all complex life forms.

Less than a billion years ago, all landmasses had joined together in the tropics, forming one vast continent – Rodinia. The ground was still barren and desolate – no plants or animals existed. Microscopic organisms, however, were teeming in the oceans, but little did they know about the catastrophe looming.

Around 750 million years ago, the supercontinent began to tear apart, and vast amounts of molten rock deep within Earth found a way onto the surface – for millions of years. Meanwhile, heavy tropical rains captured ever more carbon dioxide from the air. In the ground, carbon dioxide combined with the volcanic rock and carbonates (minerals that contain carbon dioxide) came into being, flowed into the oceans with the rainwater, and sank into the seabeds. Carbon dioxide in the air began to decrease.

As more and more carbon dioxide was drawn out from the air, Earth began to cool. Ice formed at the poles and started to spread. The whiter our world became, the more sun rays were reflected back to space, lowering the temperatures further.

Perhaps also coupled with meteoroids hitting our planet – throwing a lot of debris into the sky and blocking sunlight – glaciers spread more and more until Earth was entirely covered in kilometres of thick ice and snow, temperatures plummeting to freezing. By 635 million years ago, if not multiple times before, our planet had transformed into a snowball.

Life survived in the hostile environment beneath the ice, adapting to radical temperature changes, perhaps becoming more complex because of them.¹ And when glaciers finally melted before erupting volcanoes and breathing microscopic organisms (carbon dioxide and other greenhouse gases in the air began to increase again), the first animals and plants evolved, ready to take over the world.

¹ To explain how the life we are made of (eukaryotic cells) survived through this extremely icy period, some suggest there may have been a small ice-free area in the tropics, and others propose Earth was more of a slush ball than a snowball. Unlike prokaryotes (bacteria and archaea), which can survive in extreme environments such as in boiling temperatures, in radioactive waste, or under enormous pressure, eukaryotes (cells all complex life forms are made of) are more dependent on "normal conditions" such as sunlight and warmth. Alternatively, eukaryotes could have survived in hot springs or lakes in underground caves, where temperature remained warmer.

Bibliography

BBC Earth. 2023. Earth Fire. Available at: https://areena.yle.fi/1-66606586 (Accessed: 30 March 2024).

Panciroli, E. 2022. The Earth a Biography of Life The Story of Life On Our Planet Through 47 Incredible Organisms. London, United Kingdom: An Hachette UK company. 255 p. ISBN 9781529413984. Page 23.

Lane, N. 2016. Oxygen the Molecule That Made the World. Oxford, United Kingdom: Oxford University Press. 374 p. ISBN 9780198784937. Pages 60-64.

Frankopan, P. 2023. The Earth Transformed an Untold History. London, United Kingdom: Bloomsbury Publishing. 695 p. ISBN 9781526622563. Pages 27-28.

Dartnell, L. 2019. Origins How the Earth Shaped Human History. London, United Kingdom: Penguin Random House. 346 p. ISBN 9781784705435. Pages 8-9, 153.

Bryson, B. 2003. A Short History of Nearly Everything. London, United Kingdom: Transworld Publishers. 668 p. ISBN 9781784161859. Pages 370-372.

Kilpua, E. & Koskinen, H. 2023. Aurinko Tuttu ja Tuntematon Tähtemme. Helsinki, Finland: Gaudeamus. 318 p. ISBN 9789523452329. Pages 267-272.

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