Understanding “Snowball Earth” Extreme Climates — When the World Is Covered in Ice

New research in Geology suggests an unexpected mechanism could have caused a global glaciation to last for 56 million years, revealing the inner workings of the carbon cycle

In the whole history of Earth’s climate, few events are as extreme as those that geologists call “Snowball Earth.”

“These are times when geologic evidence indicates the Earth froze over, essentially from pole to pole,” says Trent Thomas, a planetary scientist at the University of Washington and author of a new study on Snowball Earth published recently in Geology. “The entire surface of the Earth is basically below the freezing temperature of water. That is probably the coldest the Earth has ever been.”

But scientists still wonder why these events happen in the first place and what causes them to end. The answers revolve around Earth’s carbon cycle, which regulates our climate like a thermostat. Thomas says that during Snowball Earth events this thermostat breaks down. Understanding how the carbon cycle responds to extreme scenarios is critical to predicting future climate change.

Thomas and his colleagues were interested in two Snowball Earth events that occurred between 720 and 635 million years ago. Confusingly, one lasted 14 times longer than the other, with respective durations of 56 and 4 million years. The reason behind the discrepancy could provide important insights into the chemical systems that make Earth tick.

A Snowball Earth begins when ice sheets grow from the poles and reach toward the equator. As the reflective glaciers cover more of Earth’s surface, more sunlight is reflected back into space in a feedback loop that keeps the planet frigid. But, over millions of years, the build-up of carbon dioxide spewed from volcanoes crosses a threshold, and the cold spell suddenly and catastrophically breaks. Typically, Snowballs are followed by some of the warmest times in Earth history.

But, says Thomas, “Something should be very different in the carbon system between the two Snowball Earth events if you're going to have such a dramatic discrepancy” in their duration. It could be two things: a change in the rate volcanoes add CO₂ to the atmosphere or the rate that CO₂ is removed by a natural process called “weathering,” in which rocks break down and pull carbon out of the atmosphere.

There’s no evidence the amount of volcanism changed between the two Snowballs, says Thomas, leaving weathering as the most likely culprit. But weathering on land—the primary mechanism through which carbon is pulled out of the atmosphere—slows to a crawl during a Snowball event.

To solve this conundrum, Thomas and his team created a computer model of the Earth system to see if they could replicate a 14-times difference in Snowball duration. They found that with a constant input of CO₂ from volcanoes, the only way to create such different lengths of glaciations during the Cryogenian was to change the rate of seafloor weathering.

In modern times, seafloor weathering accounts for a small proportion of CO₂ removal. But 700 million years ago, it seems to have played an important role in how long Snowball events lasted. During the longer glaciation, seafloor weathering must have been faster, pulling more CO₂ out of the atmosphere.

“Here is a self-consistent way to generate one cohesive, long Snowball. That was a problem before—models couldn't do it,” says Thomas. “Now we show with seafloor weathering, here's how.”

Though more investigation is needed to confirm it, Thomas thinks the difference in seafloor weathering rates between the two Snowball Earth events might be related to changes in the porosity of the rocks that form the seafloor. Higher porosity allows more seawater to interact with and weather the rock, capturing CO₂. This porosity could be controlled by the amount of sulfate dissolved in seawater, which can react with calcium in the heat of hydrothermal vents to form minerals that clog pores and prevent weathering.

“This is just meant to get the conversation started,” says Thomas, “to really highlight that we don't know enough to really understand how the changing conditions of a Snowball Earth event would impact seafloor weathering.”

Geology: Seafloor weathering can explain the disparate durations of Snowball glaciations, https://doi.org/10.1130/G53722.1

About the Geological Society of America

The Geological Society of America (GSA) is a global professional society with more than 18,000 members across over 100 countries. As a leading voice for the geosciences, GSA advances the understanding of Earth's dynamic processes and fosters collaboration among scientists, educators, and policymakers. GSA publishes Geology, the top-ranked “geology” journal, along with a diverse portfolio of scholarly journals, books, and conference proceedings—several of which rank among Amazon’s top 100 best-selling geology titles.

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