Here are some ways the world might end in ice, according to science fiction. Earth freezes. Humankind’s remnants huddle in ice caves, fending off giant alien monsters (Snowball Earth). Or people shelter in the New York Public Library as ice encases the Empire State Building (The Day After Tomorrow). Or a train hurtles in an endless loop, both protection and prison for Earth’s scant survivors (Snowpiercer).
These stories are all set in Earth’s near future, within a decade from now. And what makes the visions so chilling is that there’s a snowflake of truth in them. Well, space monsters haven’t invaded Earth — yet. And a lot of ink has been spilled describing the scientific fallacies of the movie The Day After Tomorrow.
But there have been at least five ice ages in Earth’s history, the most severe one during the aptly named Cryogenian Period, between 720 million and 635 million years ago. At least twice during this period, glaciers extended from pole to pole, creating a “Snowball Earth.”
Or “maybe it was more of a ‘Slushball Earth,’” says geoscientist Thomas Algeo of the University of Cincinnati. Fossils of seafloor-dwelling algae, which would have needed sunlight to survive, suggest that even in the coldest times, there were patches of open water near the equator that may have harbored life.
Earth’s past deep freezes came from a combination of geologic processes. The Cryogenian followed after shifting tectonic plates caused a supercontinent to split apart. That increased rainfall and weathering, which dramatically reduced atmospheric carbon dioxide, causing runaway cooling. Widespread ice exacerbated the cooling, reflecting sunlight back to space rather than the ground absorbing the heat.
These processes are slow, which is not ideal for dramatic storytelling. However, sci-fi has the advantage of playing with time, taking known natural — or human-caused — climate interactions and feedback effects and accelerating them or amping them up for drama.
The Day After Tomorrow, for example, is based on a real-world scenario: the potential shutdown of the Atlantic Meridional Overturning Circulation, a part of a global system of ocean currents that transports heat around the world, keeping regions that might otherwise be frigid relatively warm.
Geochemist Wallace Broecker suggested in the mid-1980s that large fluxes of freshwater to the North Atlantic Ocean could halt this circulation. Ocean current slowdowns or shutdowns have contributed to past ice ages, he said. Scientists fear it could happen again, and soon. But soon, geologically, means decades to a century. In The Day After Tomorrow, a new ice age arrives in weeks.
Snowpiercer, a graphic novel, movie and TV series, imagines a different deep-freeze scenario: Geoengineering gone haywire. To mitigate global warming, scientists released aerosols into the atmosphere to reflect sunlight to space. The idea is based on an actual type of geoengineering known as solar radiation management.
But the aerosols plunged the planet into a deep freeze within weeks (or hours, in the novel). Geoengineering strategies could cool Earth, but not that quickly (more like within a few years). And not by that much, or by accident. To bring on an ice age, aerosols would have to be deliberately and repeatedly injected over generations, says climate engineer Douglas MacMartin of Cornell University.
How Earth freezes in Snowball Earth is still something of a mystery, as the manga is still in progress (and an animated version of it is slated to debut in April). What is known is that the transition to an ice world takes about a decade. And — spoiler alert — the giant alien monsters might have something to do with it. For that, scientists don’t really have an answer.
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