Was "Snowball Earth" Frozen Solid or Slushy?
By Michael Schirber,
Astrobiology Magazine
August 2015
Imagine a world without liquid water — solid ice stretching in all directions. It’s not exactly a place where life thrives. Yet, Earth has experienced several such frozen periods, where runaway climate effects led to global or near-global ice cover.
The last of these so-called Snowball Earth events ended around 635 million years ago, just as complex life was beginning to emerge. But did ice completely blanket the planet, or did some regions remain ice-free, halting the deep freeze?
"Studying Snowball Earth can show us just how extreme climate change can get — conditions that might be too severe for life as we know it," explains geologist Linda Sohl of Columbia University's Center for Climate Systems Research and NASA's Goddard Institute for Space Studies.
Sohl and her team are using modified global climate models — the same types used to predict Earth's future — to understand its distant past. Their simulations of the Cryogenian Period (850 to 635 million years ago) suggest Earth’s global average temperature may have plunged to 12°C below freezing. Yet, surprisingly, the planet may never have fully frozen over. Their models indicate that even under extreme conditions, about half of Earth’s oceans could have remained ice-free.
This resistance to becoming a completely frozen "snowball" is a crucial insight into Earth's climate history. Sohl's team, funded by NASA’s Astrobiology Program, is exploring which factors — such as continental positions and ocean circulation — influenced glaciation, and which may have prevented a total freeze. Their findings could also inform our understanding of habitability on distant worlds. If planets like Earth have natural defenses against global freezing, liquid water — and potentially life — might be more common across the universe than previously thought.
Hard Snowball or Slushball?
Evidence suggests Earth experienced at least two major Snowball Earth events during the Cryogenian, around 710 and 640 million years ago, each lasting roughly 10 million years. Geological signs, such as evidence of glaciers near the equator, point to the severity of these events. If ice reached such low latitudes, the logic goes, it likely covered the entire planet.
This is due to ice's high albedo, or reflectivity. Ice reflects 55% to 80% of sunlight, sending much of the Sun’s energy back into space. In contrast, ocean water reflects only about 12%, and land areas between 10% and 40%, meaning they absorb more heat. During the Cryogenian, the Sun was also 6% dimmer than today, exacerbating the cooling.
Early climate models predicted that once ice advanced into the tropics, a positive feedback loop would set in: more ice lowers temperatures, which creates even more ice, perpetuating a vicious cycle until Earth became entirely frozen — a scenario dubbed the "hard snowball." This would result in a planet locked in an eternal deep freeze — no magical solutions, like in Frozen, to break the spell.
But this theory faces several challenges. For one, it's hard to explain how Earth could escape such a deep freeze. Some suggest volcanic eruptions could have released enough greenhouse gases to eventually warm the planet, but the amount of carbon dioxide required — hundreds of times today’s levels — lacks supporting geological evidence.
Another problem: a total freeze should have devastated ocean ecosystems by cutting them off from sunlight. Yet, only small extinctions appear in the Cryogenian fossil record.
Adding to the mystery, there’s evidence of an active water cycle during this time — something unlikely if the oceans were sealed under thick ice.
"The idea that Earth was entirely covered by ice — continents buried under glaciers and oceans capped by thick sea ice — remains controversial," says physicist Richard Peltier of the University of Toronto.
As a result, an alternative theory has gained traction: the slushball Earth. Under this scenario, the planet became largely ice-covered, but open water persisted near the equator. Many geologists favor the slushball hypothesis, as it aligns more closely with the available evidence.
A hard snowball may have happened earlier in Earth’s history, though. Around 2.2 billion years ago, during the Paleoproterozoic Era, extensive glaciation suggests a global ice cover was plausible. Back then, the Sun was even dimmer — 16% fainter than today — and the evolution of photosynthetic life may have reduced greenhouse gases significantly, cooling the planet.
Reconstructing the Ancient Climate
To better understand the Cryogenian, Sohl’s team is developing climate models to recreate Earth’s conditions nearly a billion years ago. They use NASA's GISS Earth System Model (ModelE2-R) — a sophisticated tool also used by the Intergovernmental Panel on Climate Change (IPCC) — but adjust it for ancient conditions. The Sun’s brightness is reduced by 6%, and the continents are arranged into a single supercontinent near the equator.
"We need this flexibility to accurately study past climates," says Sohl. "ModelE2-R is probably one of the most advanced tools available for paleoclimate research."
While some researchers have aimed to force their models into a hard snowball scenario, Sohl's team lets the simulations reveal what would naturally happen. They’ve found that, similar to modern-day ocean currents like the Gulf Stream, ancient ocean circulation played a significant role in distributing heat.
"Ocean circulation seems to help prevent a complete freeze-over," explains Sohl. Their early results show that even as glaciers covered much of the land, tropical oceans retained areas of open water — supporting the slushball model.
The team is also examining other factors that shaped ancient climates. For example, Earth's day was shorter during the Cryogenian — about 21.9 hours long — which likely influenced atmospheric dynamics.
Peltier, though not part of Sohl's project, emphasizes the role of topography — variations in elevation. High-altitude regions may have encouraged glaciation, even when other factors opposed it.
Beyond Earth: Other Ice Worlds
Sohl points out that her team's results add to a growing realization: freezing a planet solid isn't easy. But that message hasn’t fully reached the astrobiology community.
Astrobiologists often treat the hard snowball state as the outer boundary of habitability, assuming planets beyond a certain distance from their star — outside the so-called habitable zone — are frozen and lifeless. But those who study climate know freezing depends on much more than distance.
An artist's concept of Kepler-62f, a potentially water-rich exoplanet in its star’s outer habitable zone, illustrates this complexity. Without a dense CO₂ atmosphere, Kepler-62f's water would likely be ice — yet, whether it's entirely frozen remains uncertain.
Sohl hopes her research will help expand how we define habitable planets. "In the end, I think we’ll realize the habitable zone is broader than we originally thought," she says.
This article was originally prepared as a feature for Astrobiology Magazine.
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