Imagine the entire Earth locked in a deep freeze, covered in ice from the poles to the equator. This is the essence of the Snowball Earth hypothesis, a theory that suggests our planet may have been entirely or nearly entirely frozen over multiple times in its ancient past. It’s an idea that sounds like science fiction, but there is strong evidence to back it up.

The Snowball Earth hypothesis was first proposed in the 1960s but didn’t gain widespread attention until the 1990s when scientists found more compelling evidence. According to this theory, Earth experienced several global ice ages during the Proterozoic Eon, around 720 million to 635 million years ago. These ice ages were unlike any we’ve experienced in human history. Rather than just ice caps at the poles, ice sheets may have extended to the equator, turning the entire planet into a giant snowball.

The idea of a Snowball Earth is supported by geological evidence. In some regions, scientists have found glacial deposits that appear to have formed near the equator, a sign that ice may have reached even the warmest parts of the planet. Additionally, there are layers of cap carbonates — limestone and dolostone — that seem to have formed after these glaciations, indicating a sudden warming event. These cap carbonates are found all over the world, and their presence suggests that Earth may have undergone a massive, rapid shift from icehouse to greenhouse conditions.

The Snowball Earth hypothesis suggests that a combination of factors, including reduced solar radiation, changes in atmospheric composition, and tectonic activity, could have triggered these global ice ages. One of the key players was the amount of carbon dioxide in the atmosphere. Normally, CO2 acts as a greenhouse gas, trapping heat and keeping the planet warm. However, during the Proterozoic, the weathering of rocks — accelerated by the breakup of supercontinents — may have drawn down large amounts of CO2 from the atmosphere, reducing the greenhouse effect and allowing global temperatures to plummet.

Once the planet was frozen, it became even harder to escape the deep freeze. Ice reflects sunlight back into space, preventing the planet from warming. This process is known as the ice-albedo feedback loop. As Earth grew colder and more ice formed, even more sunlight was reflected, creating a self-reinforcing cycle of cooling.

But Earth didn’t stay frozen forever. Volcanic activity continued during this icy period, spewing CO2 into the atmosphere. Over millions of years, CO2 levels built up to such a degree that the greenhouse effect eventually overwhelmed the icy conditions, causing a dramatic warming event that melted the glaciers and brought Earth back to a more temperate state.

The Snowball Earth hypothesis is still debated among scientists. Some argue that the planet may not have been completely frozen and that there could have been ice-free regions, or “slushball Earth.”

Whether Earth was a snowball or a slushball, one thing is clear: Our planet has undergone extreme climate changes in its distant past. The Snowball Earth hypothesis helps us understand the delicate balance of forces that shape our climate and how life has managed to adapt to even the most hostile conditions. Understanding these ancient ice ages might also offer insights into the future of Earth’s climate in an era of human-caused change.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.