Not all ice ages are equally brutal. In the most extreme glaciation events known to science, ice stretches out from Earth’s polar regions and extends all the way into lower latitudes, literally reshaping the face of the planet.
Evidence for such epic transitions can be found in the geologic record, most recently in formidable glaciations during the Cryogenian period. Scientists think these extreme cooling events had potentially global reach and ramifications: a phenomenon called ‘Snowball Earth’.
But what on Earth could unleash such devastating, unrelenting cold that most – or all – of our planet would end up sheathed in a frozen sphere of ice and snow? While the exact triggers remain unknown, researchers have now unearthed a new theoretical explanation for how such a thing could happen.
“There are lots of ideas for what caused these global glaciations, but they all really boil down to some implicit modification of solar radiation coming in,” says planetary science researcher Constantin Arnscheidt from MIT.
“But generally it’s been studied in the context of crossing a threshold.”
In other words, the conventional explanation for how a Snowball Earth could transpire is that, in some kind of cataclysmic throwing of shade, a reduced amount of sunlight would reach the surface of the planet, resulting in a cooler Earth that proceeds to freeze over.
Another hypothetical explanation, to do with the carbon cycle, would be the opposite of the global warming crisis facing Earth now: what if our planet had so little heat-trapping carbon dioxide in the atmosphere that we lost Earth’s temperate climate altogether, with its warmth drifting off into space?
“Although there remains debate about the specific triggers of the low-latitude glaciations in Earth’s geologic past, there is a general understanding that glaciation is initiated when changes in radiative fluxes or in CO2 fluxes exceed a critical threshold,” Arnscheidt and his co-author, MIT geophysicist Daniel Rothman, explain in a new paper.
While the theory has focused on such critical thresholds before, the researchers have struck upon a new avenue for thinking about Snowball Earth precursors: what if it wasn’t a critical threshold that was met (in terms of solar radiation plunging, for example), but a critical rate of change that was reached?
In new modelling, the researchers simulated Earth’s dynamic systems in glaciation scenarios – chiefly the interplay of ice-albedo feedback and the carbonate-silicate cycle.
The former is an example of positive feedback. As Earth gets icier and approaches Snowball Earth, it’s frosty coat of ice and snow end up reflecting more sunlight away from the planet, which in turn accelerates the cooling effects already in process.