Two years ago, our family fulfilled a long-held dream by purchasing a home. In hindsight, we might have been overly optimistic, as we envisioned a semi-renovated space that we could customize to our tastes. Yes, the allure of the DIY movement had us convinced, although it conveniently omitted the challenges of juggling renovations with two small children and our full-time jobs.

Fortunately, despite its unappealing appearance, the house was livable.

It was the height of summer, and we had grown accustomed to having ice available thanks to the built-in ice maker in our rental. To our dismay, our new fridge lacked this feature, and buying a replacement was not financially feasible.

Nevertheless, we wanted ice to keep our drinks cool during this particularly sweltering summer, especially as I was recovering from postpartum. My husband purchased several ice cube trays, one of which produced perfectly round ice spheres, approximately 2.5 inches in size. As a paleontologist, I affectionately dubbed them "Snowball Earths."

This past weekend, we hosted friends who were eager for their seltzer water served over ice. When I mentioned the special Snowball Earthed drink, they looked at me in confusion.

“Snowball Earth? Why?”

Well, let me explain.

Imagine a world so frigid that glaciers enveloped nearly the entire planet, stretching from the poles to the equator. This isn’t a distant icy moon; it’s our Earth… around 700 million years ago. Yes, our beloved planet was once a massive ice sphere, devoid of fire… at least for a while. But don’t fret, fire will soon make its entrance.

Numerous pieces of evidence lead scientists to believe that Earth experienced two significant freezing events during a time known as the Cryogenian period. These events, identified as the Sturtian and Marinoan glaciations, are collectively referred to as “Snowball Earth” occurrences. During these phases, our planet might have appeared as a colossal ice ball for millions of years.

The Snowball Earth hypothesis posits that Earth was either nearly or entirely encased in ice on two separate occasions, each lasting tens of millions of years. These glacial periods drastically altered the climate, oceans, and even the biosphere. But what led scientists to arrive at this conclusion, and how do we know it truly happened?

The concept of Snowball Earth has circulated within scientific circles for several decades. Geologist Joe Kirschvink first introduced it in 1989, proposing that geological evidence indicated a planet almost entirely frozen.

Initially, this notion seemed far-fetched. How could Earth—especially the equatorial region—be covered in ice? However, as time passed, more evidence emerged supporting the theory, prompting scientists to take it more seriously.

One crucial piece of evidence comes from glacial deposits discovered in areas that were near the equator during the Cryogenian. These include glacial dropstones—rocks transported by icebergs that settled into ocean sediments upon melting.

Geologists have identified these dropstones in locations like Namibia, a place where it’s difficult to picture glaciers once floated. Yet, the evidence does not end there.

Recent research led by Dr. Elias Rugen and his team from University College London (UCL) has fortified the Snowball Earth theory even further.

They examined over 2,000 zircon grains from rock formations in Scotland and Ireland. Zircon grains serve as tiny time capsules, aiding scientists in dating rocks.

Rugen’s team discovered that these rocks were formed between 720 and 662 million years ago, right during the Sturtian glaciation. As Dr. Rugen noted, “These layers document a tropical marine environment rich in cyanobacterial life that gradually transitioned to cooler conditions, marking the end of a billion-year temperate climate on Earth.”

This research not only verifies the timing of the Sturtian glaciation but also provides a clear geological record of Earth's shift from a warm, tropical landscape to a frozen one.

So, how did Earth end up encased in ice? Scientists believe the answer lies in a blend of factors.

One major contributor appears to be the disintegration of a supercontinent known as Rodinia. As Rodinia fragmented, it resulted in increased rainfall and rock weathering.

When rocks weather, they extract carbon dioxide (CO2) from the atmosphere, which can lower the planet’s temperature by diminishing the greenhouse effect. Over time, this reduction in CO2 may have initiated the cooling that led to the Sturtian glaciation.

Now, I promised we would discuss fire too, so here it is.

Volcanic activity played a significant role as well. Approximately 717 million years ago, massive volcanic eruptions in present-day Canada likely released sulfur aerosols into the atmosphere. These aerosols may have reflected sunlight away from Earth, further cooling the planet.

According to UCL geochemist Dr. Graham Shields, “These sulfur gas particles reflect incoming solar radiation and exert a strong cooling effect.” Consequently, this cooling may have been the critical factor that initiated the first Snowball Earth.

Once the ice began to form, a feedback loop took effect. Ice reflects sunlight more efficiently than open water; therefore, as more ice accumulated, less sunlight was absorbed, causing the planet to cool even further. This positive feedback, known as the ice-albedo effect, likely trapped Earth in a deep freeze.

You might be wondering: What happened to life on Earth during all this? Well, as usual, life finds a way…

Despite the frozen surface, life beneath the ice somehow managed to endure. Before Snowball Earth, simple organisms such as bacteria and algae thrived in the oceans.

However, during the ice age, it is believed that some of these organisms took refuge in pockets of open water or beneath the ice in shallow seas. There’s also evidence that multicellular life, including primitive sponges, may have evolved during or shortly after the Snowball Earth events.

Eventually, as the glaciers receded and temperatures rose, life not only survived but flourished!

Dr. Shields emphasizes that “all complex, multicellular life, including animals, originated from this deep freeze, with the first evidence appearing in the fossil record shortly after the planet thawed.” Once Earth warmed up, life diversified explosively, culminating in the Cambrian explosion, a period marked by the emergence of most major animal groups.

Dr. Rugen and his team’s new research provides essential data that bolsters the Snowball Earth hypothesis. Their findings confirm that the rocks studied in Scotland and Ireland are a part of this dramatic chapter in Earth’s history.

As Rugen articulates, “Most regions of the world lack this remarkable transition because ancient glaciers scraped and eroded away the underlying rocks, but in Scotland, by some miracle, this transition is visible.”

Snowball Earth was a period of extreme cold, but it also laid the groundwork for the incredible variety of life that followed. Ongoing research continues to unveil more about this captivating and icy era in Earth’s history. So, remember to support science and scientific exploration, as it is key to better understanding and adapting to a changing world.

On a lighter note, don’t forget to refer to those spherical ice cubes as “Snowball Earths” from now on; I assure you, your friends will be impressed when you share this story. A perfect conversation starter! (Pun intended)

Published in Fossils et al. Follow to learn more about Paleontology and Evolution.