What is Snowball earth, when did it occur, and what impact did it have on life on earth?
Snowball Earth refers to the theory stating that, at multiple times, the entire surface of the planet was frozen. This theory has important implications for any existing life during those periods.
In brief, Snowball Earth is a theory stating that the entire planet was frozen, it may have occurred multiple times between 850-635 mya, and it would have made for very challenging living conditions for any organisms. Following Snowball Earth is the Cambrian Explosion.
Snowball Earth refers to the theory stating that, possibly at multiple periods in times, the entire surface of the planet was frozen. This includes both land surfaces but also oceans. Scientists have not agreed on what exactly caused Snowball Earth. It is likely that low levels of carbon dioxide in the atmosphere were at least partially responsible for cooling the planet.
These episodes are thought to have happened between 850-635 million years ago (mya), during the Cryogenian period.
With everything frozen, biological, geological, and chemical processes were disrupted, such as the carbon cycle and the rock cycle. Carbon dioxide was no longer being removed from the atmosphere and gradually levels increased until they were high enough to cause a greenhouse effect. Volcanic eruptions under the ocean may have sped this process along by adding more greenhouse gases to the atmosphere (see here).
Following Snowball Earth was a period of great change in the composition of the atmosphere. We see oxygen levels reach their current numbers. After this increase in atmospheric oxygen, the Cambrian Explosion occurred, and this is when we see a substantial number of multicellular organisms evolve.
Conditions made it very challenging for life to survive Snowball Earth. Life may have survived in small, unfrozen areas of the ocean (see here). There may also have been breaks of time where the entire world was not frozen, allowing microbial organisms to survive (see here).