It has long been believed that Earth went through a long freezing period lasting around 56 million years, during which it turned into a ball of ice. But how could this have happened and how did life on Earth's surface survive millions of years of ice? A recent study provides an answer.
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Imagine Earth covered in ice for extremely long stretches of time, and yet life persisted throughout. This is what scientists believe happened during the Neoproterozoic era, particularly the Cryogenian period. This freezing era is known as the Sturtian glaciation, and although Earth experienced periods of intense freezing that may have covered most of the planet, these were interspersed with periods of partial melting as well.
All of this lasted around 56 million years, and the idea that Earth remained frozen for such a long stretch while life persisted on its surface has remained a puzzling mystery for scientists. But a recent study conducted by Harvard University, published in the journal Proceedings of the National Academy of Sciences (PNAS), has explained and scientifically clarified the causes of this phenomenon.
This freezing period is considered one of the most extreme climatic episodes in Earth's geological history, as ice crept across the continents, reached tropical regions, and locked a large portion of the ocean beneath a frozen crust.
This prolonged freeze occurred during Earth's Cryogenian period, around 717 to 660 million years ago — before the dinosaurs and before complex plant life existed.
Scientists call this state "Snowball Earth." They believe it was driven by shifts in atmospheric carbon dioxide levels and chemical weathering processes affecting volcanic rocks.
Earth Between Freezing and Thawing
In their recent study, the scientists used a coupled model of ancient climate and the global carbon cycle — the system through which carbon moves between the atmosphere, oceans, rocks, and living organisms.
They suggest that Earth was not continuously frozen throughout that entire period — meaning the planet did not remain completely frozen for the full 56 million years but rather oscillated between fully ice-covered "frozen Earth" conditions and ice-free "hot Earth" periods. According to the researchers, the Franklin volcanic province in northern Canada erupted shortly before the onset of the Sturtian glaciation, depositing enormous amounts of basalt rock across Earth's surface.
Basalt rock is chemically reactive: as it undergoes weathering from exposure to rain and air, it absorbs carbon dioxide from the atmosphere and locks the carbon inside minerals and ocean sediments.
Simulations run by the Harvard research team indicate that this weathering process reduced carbon dioxide levels enough to trigger a global, full-scale freeze.
Once ice covered most of the planet, weathering processes slowed down, since rain and air could no longer reach large areas of basalt rock. At the same time, volcanoes continued releasing carbon dioxide gas, and with weathering absent, this gas began accumulating in the atmosphere. This buildup intensified the greenhouse effect, raising temperatures and causing the ice to begin melting.
As the ice retreated, volcanic rocks were exposed once again, allowing weathering to resume and begin absorbing carbon dioxide from the atmosphere again — causing temperatures to drop and the ice to return once more. This cycle continued repeating over millions of years.
How Did Life Survive During That Period?
The scientists' finding that Earth alternated between freezing and thawing periods during the Sturtian glaciation is what explains how life survived on the planet. During that time, Earth was almost entirely inhabited by single-celled microorganisms that needed oxygen to stay alive.
A completely frozen Earth throughout that period would have placed enormous strain on the atmospheric and oceanic systems responsible for maintaining oxygen availability, which would have caused life on Earth's surface to go extinct.
This supports the validity of the researchers' findings that Earth went through both icy periods and warmer periods, which helped living organisms survive and endure.
