This was what the equator looked like 700 million years ago. And the rest of the world too (Image: John Eastcott and Yva Momatiuk/National Geographic Creative)
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These days, if you want to see a glacier near the equator, you must scale the rarefied heights of Mount Kenya or the Ecuadorian Andes. Around 700 million years ago it was a bit less of an effort. In fact, you’d struggle to find somewhere that wasn’t frozen over.
You’ve arrived in the middle of the Cryogenian period, so called because the planet was repeatedly sheathed in ice in a series of “snowball Earth” episodes. The greatest of these Cryogenian snowballs, the Sturtian glaciation, began 716.5 million years ago. In the space of a few years, land and sea across the globe were swallowed up by sheets of ice that eventually became kilometres thick. They did not melt for another 55 million years. Earth was literally a snowball, like today’s Antarctica from pole to pole.
“In the space of a few years, the land and sea were swallowed by ice sheets”
That, at least, is the story many geologists have come to accept since Joseph Kirschvink of the California Institute of Technology in Pasadena first advanced the idea of snowball Earths in the early 1990s. Ancient glacial deposits laid down at tropical latitudes – for example in north-western Canada, which 700-odd million years ago straddled the equator – tell a story of sea ice between 1.5 and 3 kilometres thick, says Kirschvink.
The same region also provides clues to the Sturtian glaciation’s cause. The Franklin Large Igneous Province, a vast area of volcanic rock covering more than 1 million square kilometres, can be dated to shortly before the glacial layers. It seems the eruption of a supervolcano brought vast volumes of basalt to the surface that quickly weathered under tropical rainstorms – a chemical process that sucked huge amounts of the greenhouse gas carbon dioxide out of the atmosphere. Temperatures plunged and the polar ice caps began to advance.
From then on things proceeded with a speed unusual for Earth processes. As the seas froze, water vapour, itself a potent greenhouse gas, could no longer evaporate into the atmosphere in the usual quantities. “It was like throwing a master off-switch on the hydrological cycle,” says Kirschvink – and the big freeze gathered pace towards the equator. You won’t want to open the door of your time machine, because even at the equator it will be -50 °C, the sort of cold you can only reliably find today deep in the Antarctic.
Perhaps because of its sheer drama, the snowball Earth idea remains controversial. Some geologists opt for a less harsh “slushball Earth” variant. But Kirschvink thinks the sheer geographical spread of glacial deposits now dated to the same time tell their own story. “It’s a hard snowball, dammit,” he says.
Eventually, CO2 seeping out from undersea volcanoes began to warm things again, and cracks in the ice stayed open. Then, snowball Earth was over almost as quickly as it began. “You throw water vapour into the atmosphere, and fresh meltwater absorbs sunlight and warms,” says Kirschvink. “As you start to open the snowball, it drives it till it’s gone.”
Yours will be the only eyes around to see these startling transitions: the most advanced witnesses to the beginning of the Sturtian glaciation were single-celled zooplankton. Kirschvink wonders whether these little critters, engaged in an increasingly desperate doggy paddle for survival, might have inadvertently contributed to their own predicament. “My pet hypothesis is that zooplankton evolved that excreted fecal pellets which sank rapidly to the bottom, burying carbon there and getting the CO2 out of the system,” he says.
Certainly life seems to have had a hand in an earlier snowball episode beginning around 2.4 billion years ago. “There we think it had a biological trigger from day one,” says Kirschvink – the evolution of photosynthesising cyanobacteria that sucked CO2 from the atmosphere.
Whatever their triggers, the rapidity with which Earth succumbed to the deep freeze is a reminder that small perturbations can have huge consequences in our planet’s complex and sensitive climate system – a lesson we would do well to take to heart, says Kirschvink. “We’re twiddling knobs where we don’t know what they connect to.”
Read more: “A time traveller’s guide to Earth”
This article appeared in print under the headline “Snowball Earth”
