We are all familiar with the idea of an ice age. But as a rule, even in the worst of winters the tropics stay warm and ice-free. But then why are geologists finding the signature evidence of glaciers throughout previously tropical areas ? And what kind of Earth could exist that would allow even the tropics to freeze? One of the most intriguing explanations is that, in our distant past, the Earth itself completely froze over, a hypothesis known as Snowball Earth.
It would start as a normal period of global cooling, much like what happened 12,000 years ago in the Younger Dryas period. In that case the dip in temperatures was likely caused by disruption of ocean currents, but anything could start it off. Glaciers would swell and progress southward. For a little while it would look like any other ice age. But a quirk in our planet’s climate would change all of that soon enough and plunge us into a seemingly never-ending freeze.
Ice, you see, is brighter and more reflective than either land or ocean water. Anyone who has experienced snow blindness knows this. As the glaciers spread, the reflectivity (or albedo) of the surface of the earth increases. The problem is that the more reflective ice shunts more sunlight and more importantly heat back into space. Normally other climate factors can correct this, but only up to a point. After that the process only accelerates itself, the ice creating lower temperatures, the lower temperatures creating more ice. It would only stop once the entire globe was frozen. This would last for as long as thirty million years.
And the thing is? According to the hypothesis, this didn’t just happen once. At least three periods of deep glaciation have been identified, the latest during the Ediacaran period, just as life was beginning to diversify into recognizable forms. Funnily enough, this may have actually accelerated evolutionary change due to the extreme competition for what little resources survived (and the extinction of less fit lineages of life).
Eventually, however, everything must come to an end. Once again, quirks of the climate cycle would effect the entire globe. Carbon dioxide, the most famous greenhouse gas, is normally removed from the atmosphere by two means: photosynthesis and open water. But with life barely hanging on in shallow icemelts or geothermal hotspots, photosynthesis would be virtually nonexistent. Furthermore, Ice sheets would also prevent any contact with the ocean. The gas couldn’t go anywhere, but volcanoes and other geothermal activities would ensure a steady income of it. The CO2 would slowly build and build and build.
Eventually, the temperatures would rise up again and the cycle would reverse itself. Open water would appear and absorb sunlight again, continuing to drive up temperatures. Life, meanwhile, would find that conditions would sudden become much more favorable. Temperatures would climb, oceans would open, and the melting glaciers would drop tons of locked nutrients into the sea. The ocean would become chock full of resources for the newly freed life and they would go nuts in one of the single biggest evolutionary events ever, a period known as the Cambrian Explosion.
Luckily for life, there have been no other Snowball Earths since then. But the quirks and mechanisms that allowed it to happen once still exist today. It is not inconceivable that sometime in the far future the Earth may once again be ruled by ice.