744 – 705 million years ago
The United States has been hit by a major winter storm. I’ve been out shoveling snow from the driveway several times today. In-person classes at my university were cancelled, and I’ll probably take the light rail into school tomorrow rather than braving the streets with my car.
But things have been worse. Before 720 million years ago, we find thick limestone deposits left by decaying algae. These were sequestering carbon, taking carbon dioxide out of the atmosphere, and cooling the Earth. At some point a positive feedback cycle kicked in, as polar seas froze and reflected more sunlight, cooling the planet further. The result was a succession of extreme Ice Ages. The Ice Age of the last two million years, which merely covered high latitudes with glaciers, off and on, were nothing compared to the Snowball Earth of the Cryogenian: at a minimum, polar seas were frozen, and tropical seas were slushy with icebergs. It’s possible that things were even more extreme: the entire sea may have been covered by a thick layer of ice, with a few photosynthetic algae surviving in the ice, and other organisms hanging on around deep sea hot water vents. A limited amount of oxygenated meltwater seeping into the ocean from under the glaciers may have kept early oxygen-breathers alive. For a hundred million years, climate oscillated abruptly between two steady states, frozen and warm.
It’s only in the last two decades we’ve begun to figure out this amazing story. If there’s a lesson here, it’s that Earth over the long run is far from a stable system. We will see again and again that the history of life, like human history, has been punctuated by catastrophes.
Above, a rock dropped from an iceberg or glacier into the middle of a tropical ocean