The Earth has already frozen, as climate history shows (looking at you, Cryogenian period)
Here are some ways the world could end up in ice, according to science fiction. The Earth is freezing. The remnants of humanity gather in ice caves, fending off giant alien monsters (Snowball Earth). Or people shelter in the New York Public Library while ice covers the Empire State Building (The day after tomorrow). Or a train hurtling in an endless loop, both protection and prison for the rare survivors of Earth (Snowdrops).
These stories all take place in Earth’s near future, a decade from now. And what makes visions so frightening is that they contain a flake of truth. Well, space monsters haven’t invaded Earth yet. And much ink has been spilled to describe scientific errors from the movie The day after tomorrow.
But there have been at least five ice ages in Earth’s history, with the most severe occurring during the Cryogenian period, between 720 million and 635 million years ago. At least twice during this period, glaciers expanded 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 algae, which would have required sunlight to survive, suggest that even during the coldest periods there were areas of open water near the equator that could have hosted life.
Earth’s past frosts were the result of a combination of geological processes. The Cryogenian followed the movement of tectonic plates caused the separation of a supercontinent. This increased precipitation and severe weather, which significantly reduced atmospheric carbon dioxide, causing uncontrollable cooling. Widespread ice exacerbated the cooling, reflecting sunlight back into space rather than the ground absorbing the heat.
These processes are slow, which is not ideal for dramatic storytelling. However, science fiction has the advantage of playing with time, taking into account known climate interactions and feedback effects, natural or man-made, and speeding up or amplifying them for drama.
The day after tomorrowfor example, is based on a real-world scenario: the potential shutdown of the Atlantic meridional overturning circulation, part of a global system of ocean currents that transports heat around the world, keeping regions that otherwise might be relatively warm.
Geochemist Wallace Broecker suggested in the mid-1980s that significant flows of fresh water to the North Atlantic Ocean could stop this traffic. Slowdowns or stops in ocean currents contributed to past ice ages, he said. Scientists fear it could happen again, and soon. But soon, geologically, that means decades, even a century. In The day after tomorrowa new ice age is coming in a few weeks.
Snowdropsa graphic novel, film, and television series, imagines another freezing scenario: geoengineering gone haywire. To mitigate global warming, scientists have released aerosols into the atmosphere to reflect sunlight back into space. The idea is based on a real type of geoengineering known as management of solar radiation.
But the aerosols plunged the planet into a deep freeze within weeks (or hours, in the novel). Geoengineering strategies could cool the Earth, but not that quickly (rather in a few years). And not so much, nor by accident. To cause an ice age, you would have to deliberately and repeatedly inject aerosols over generations, says climate engineer Douglas MacMartin of Cornell University.
How the Earth freezes Snowball Earth is still a mystery, as the manga is still ongoing (and an animated version is expected to debut in April). What is known is that the transition to an ice world takes about a decade. And – spoiler alert – giant alien monsters might have something to do with it. Scientists don’t really have an answer to this question.