Snowball Earth

The boring billion was followed by Snowball Earth. The equatorial position of the continents and rise of a mountain belt that straddled the equator created a “perfect storm” for global cooling. In a world before plants and animals, a barren rocky supercontinent would have reflected energy received from the Sun back out into space. 

This is called the albedo effect. If this supercontinent was at the equator it would reflect more energy because Earth receives more energy at the equator. The result is global cooling.  The rise of mountain belts, powered by the collision of tectonic plates, brings fresh rocks in contact with the atmosphere. These rocks react with carbon dioxide and water and slowly dissolve to form clay. After this reaction, carbon dioxide remains in a soluble state (HCO₃^–) and is carried by rivers to the oceans where it reacts with calcium to form limestone. In this way, the slow dissolving of rocks which is called weathering, removes carbon dioxide from the atmosphere which, in turn, weakens the greenhouse effect. The result is global cooling. 

According to the proponents of the Snowball Earth hypothesis, a combination of an albedo effect strengthened by the equatorial position of the continents and a greenhouse effect weakened by mountain building was just enough to trigger glaciation in the tropics. Because ice has high albedo, even more solar energy would be reflected back into space and Earth would be cooled even more, causing further glaciation. This “runaway” effect led to extensive (or, according to proponents of the Snowball Earth hypothesis, worldwide) glaciation.

Artist’s impression of Snowball Earth by Susanne Hjerp

In an entirely glaciated world, rocks would not weather because they would not be in direct contact with the atmosphere. This would allow carbon dioxide emitted by volcanic eruptions to accumulate in the atmosphere. This would strengthen the greenhouse effect until it became sufficiently powerful for its warming effect to compensate for cooling due to the albedo effect. At that point, glaciers would retreat and Snowball Earth would come to an end. The amount of carbon dioxide needed to make this happen would result in extreme greenhouse conditions. If Snowball Earth happened, Earth’s average temperature may have plummeted to as low as -50°C and risen beyond +30°C in the greenhouse world that followed, making Snowball Earth a worthy candidate for one of the most extreme climate change events in Earth’s history.

The lessons we can learn from Snowball Earth are that Earth’s climate really can flip from one state to another (sometimes referred to as “passing a tipping point”) and that, although Earth will recover from an extreme climate change event (such as the one we are presently causing), recovery will take a long time (thousands of years).