Looking back at "Snowball Earth"
Long ago, our planet was in a deep freeze. It's a chilling reminder of the power of climate feedback loops.
If you’re searching for a mental escape from the summer heat, let me take you on a journey deep into Earth’s history, back to when our planet froze.
“Snowball Earth” is the nickname for the times long ago when temperatures plunged and ice extended from the poles toward the equator, which resembled modern-day Antarctica.
Scientists believe there were at least two Snowball Earths—the Sturtian and Marinoan—during the Cryogenian Period, which lasted from around 720 to 635 million years ago. More than 2 billion years ago, there was also extensive glaciation, but less is known about that phase in Earth’s evolution.
These episodes of runaway climate change might seem irrelevant in the age of global warming. Yet the process by which the planet froze and then melted is tied closely to the greenhouse effect and something known as ice-albedo feedback, in which an increasingly frosty planet reflects more and more sunlight, cooling the planet further, freezing more ice, and so on.
Spoiler alert: global warming threatens to spin the vicious circle in reverse by melting the planet’s cryosphere, causing the darker water and ground underneath to absorb more energy from the sun, which would only warm the planet further, leading to more melting, less reflectivity, more warming . . . you get the idea.
The Snowball Earth hypothesis
Across its 4.5 billion-year history, Earth has been a greenhouse, an icehouse, and everything in between, so scientists have long been interested in why the planet’s thermostat has been set at such different levels, especially now that human-caused climate change has attached our collective hands to the dial.
The coldest chapters in our planet’s history would make the recent Ice Ages feel balmy.
In a 2000 story in Scientific American, two leading Snowball Earth researchers, Paul Hoffman and Daniel Schrag, describe the conditions that prevailed during these episodes:
Imagine the earth hurtling through space like a cosmic snowball for 10 million years or more. Heat escaping from the molten core prevents the oceans from freezing to the bottom, but ice grows a kilometer thick in the –50 degree Celsius [-58°F] cold. All but a tiny fraction of the planet’s primitive organisms die. Aside from grinding glaciers and groaning sea ice, the only stir comes from a smattering of volcanoes forcing their hot heads above the frigid surface. Although it seems the planet might never wake from its cryogenic slumber, the volcanoes slowly manufacture an escape from the chill: carbon dioxide.
Hoffman and Schrag were among the authors of a seminal 1998 paper in Science that used geological evidence from Namibia to argue for the Snowball Earth hypothesis and explain how “biological productivity in the surface ocean collapsed for millions of years” due to global glaciation.
Decades earlier, in 1964, W. Brian Harland of the University of Cambridge was the first geologist to suggest Earth had iced over during the Neoproterozoic, which lasted from about 1 billion to 540 million years ago.
Credit for the Snowball Earth moniker goes to Joe Kirschvink, a Caltech scientist whose idea “was published in 1992 as an unrefereed, seven-paragraph-long, article buried in a 1348-page book,” according to snowballearth.org, a site created by Hoffman.
What did Snowball Earth look like?
Here’s how Hoffman described Snowball Earth in a 2019 Q&A in Astronomy (at the time, he was 77 and still doing fieldwork in Namibia):
The name describes its appearance from outer space — a glistening white ball. The ice surface is mostly coated with frost and tiny ice crystals that settled out of the cold dry air, which is far below freezing everywhere. Gale-force winds howl in low latitudes. Beneath the floating ice shelf, a dark and briny ocean is continually stirred by tides and turbulent eddies generated by geothermal heat slowly entering from the ocean floor.
It’s worth remembering the Earth’s surface looked vastly different from today even before it froze. The cool video below from EarthByte, a geoscience collaboration, shows the motions of the continents (gray) and plate boundaries (orange) from 850 to 540 million years ago. As plate tectonics re-arranges the continents, the Earth occasionally plunges into deep freezes, which are noted by the snowflakes.
This video stems from a February 2024 paper in Geology that argued the Sturtian glaciation lasted longer than any other icehouse climate in Earth’s history (717 to 661 million years ago) due to extremely low outgassing of volcanoes along mid-ocean ridges, which dramatically reduced carbon dioxide levels in the atmosphere.
Some researchers have argued for a “Slushball Earth” in which the oceans didn’t totally freeze at lower latitudes. The graphic below, from an April 2023 paper in Nature Communications, includes illustrations of how Snowball/Slushball Earth might have appeared from space. No need to study the stuff on the right side—it won’t be on the midterm or final exam—but it’s all about how ocean life might have responded to the varying conditions.
Why did Snowball Earth start—and end?
The planet’s icy past raises several burning questions: Why on Earth would it freeze? And then why did it rapidly melt, only to freeze once more? Could Snowball Earth happen again?
Scientists have come up with a number of possible causes for the life and death of Snowball Earth.
For starters, the sun back in the Neoproterozoic was about 6% dimmer, so the planet was more susceptible to freezing. “But convincing geologic evidence suggests that no such glaciations occurred in the billion or so years before the Neoproterozoic, when the sun was even cooler,” Hoffman and Schrag write.
Another important factor was the configuration of the continents and the effects on Earth’s carbon cycle. The animation I shared above shows the continents were clustered around the equator during the Cryogenian, whereas today some are closer to the poles. The breakup of the supercontinent Rodinia exposed more water between the continental pieces, which would have increased rainfall over land and boosted the chemical weathering of rocks. That process pulls carbon dioxide out of the atmosphere, thereby reducing the planet-warming greenhouse effect.1
Volcanic activity also helps explain the life cycle of Snowball Earth. Prior to the Big Chill, massive eruptions may have ejected enough aerosols to dim incoming sunlight and cool the planet. A 2022 study in Science Advances argued that epic eruptions in the Franklin Large Igneous Province in Arctic Canada preceded the Sturtian Snowball Earth by 900,000 to 1.6 million years. That created a massive amount of volcanic rock that was subsequently weathered, causing enough carbon dioxide to be sequestered that it “nudged Earth over the threshold for runaway ice-albedo feedback,” according to the researchers.
Once the planet’s surface was frozen, volcanoes would still do their thing, and eventually those hotspots would release enough carbon dioxide through the ice to kick-start global warming. Moreover, if nearly all of the Earth’s land mass were under ice and there was no liquid precipitation, there would be hardly any weathering of rocks, so the carbon dioxide would accumulate much more rapidly in the atmosphere, causing global temperatures to skyrocket.
Yet another hypothetical trigger for Snowball Earth was an asteroid similar to the one that wiped out the non-avian dinosaurs, known as the Chicxulub impact, although there isn’t any evidence that such an event actually occurred in the Cryogenian. A February 2024 study in Science Advances concluded that “the impact winter following an asteroid impact comparable in size to the Chicxulub impact could have led to a runaway ice-albedo feedback and global glaciation.”
The nature of Earth’s climate when the asteroid hit would be key. If the impact took place during warm times, such as those in the Cretaceous Period (145 to 66 million years ago), or the very recent preindustrial climate (150 years ago), a Snowball Earth wouldn’t result, according to the study.
But if the asteroid struck when the Earth’s climate was similar to the Last Glacial Maximum (the height of the most recent Ice Age, 21,000 years ago) or the Neoproterozoic (1 billion to 540 million years ago, when the last two Snowball Earths occurred), ice could rapidly spread across the planet’s oceans.
“What surprised me most in our results is that, given sufficiently cold initial climate conditions, a ‘Snowball’ state after an asteroid impact can develop over the global ocean in a matter of just one decade,” co-author Alexey Fedorov, a Yale professor of ocean and atmospheric sciences, said in a press release. “By then the thickness of sea ice at the Equator would reach about 10 meters. This should be compared to a typical sea ice thickness of one to three meters in the modern Arctic.”
How could life survive on Snowball Earth?
A frozen planet was hardly fecund, but life somehow survived these episodes and would eventually take off in complexity and diversity.
Some scientists believe Snowball Earth may have played a leading role in sparking the Cambrian Explosion, starting around 540 million years ago, when multi-cellular life rapidly evolved, diversified, and spread across the planet. “All 11 animal phyla ever to inhabit the earth emerged within a narrow window of time in the aftermath of the last snowball earth,” Hoffman and Schrag write.
When the Snowball Earths of the Cryogenian occurred, life was found in the oceans and not too sexy: bacteria, algae, single-celled organisms.
While the planetary freezes may have administered a sterilizing shock, scientists believe some organisms were able to persist in open patches of water or where the ice was thin enough to let some sunlight through.
A 2019 paper in the Proceedings of the National Academy of Sciences concluded that ocean life was able to hang on because enough oxygen made its way into the water via glacial meltwater streams. One researcher called it the “first direct evidence for oxygen-rich marine environments during Snowball Earth.”
We now know that undersea hydrothermal vents support microbes that don’t need the sun to survive, with some researchers pointing to these locations as a possible origin of life on Earth. Amazingly, our planet is now home to “extremophiles” that also live in frozen seawater, acid mine drainage, and other inhospitable environments.
Some researchers argue that if Snowball Earth confined the planet’s early organisms to isolated refuges, then subjected them to the dramatic rebound in global temperatures—which may have reached around 50°C/122°F—the extreme environments may have hastened evolution. “A series of global freeze-fry events would have imposed an environmental filter on the evolution of life,” Hoffman and Schrag write. Hoffman explained a possible mechanism in his interview with Astronomy:
One idea is that on Snowball Earth, ecosystems may have been more isolated from one another and this might be a situation that would be helpful for evolving new forms of life, and particularly forms of life that are altruistic — ones with cells that find that there is an advantage in working together rather than working individually. So more isolation of different ecosystems might have allowed certain ecosystems that had a higher proportion of these multicellular altruists to establish a foothold.
Parting shots: La Plata’s last snow
There ain’t much snow left here in Colorado, but I was psyched to see and touch some on a recent hike along the Colorado Trail in the La Plata Mountains near Durango. The snow patch in the photos below, located on the north-facing slopes of Cumberland Mountain, near Kennebec Pass, was surrounded by wildflowers and lush ground cover. Summer in the San Juans is short but sweet.
The process of burying carbon works like this: rainwater absorbs carbon dioxide from the air, creating a weak acid that reacts with certain types of rocks and erodes them. This material, along with the dissolved carbon dioxide, eventually gets washed into rivers and the ocean, where it forms materials such as limestone that settle on the seafloor and store the carbon there.