Snowball Earth

We know that the earth was snow covered, even at/very close to the equator, repeatedly. There’s even a very evocative term for it: “snowball earth”. The most recent series of snowball earths, comprising of four great ice ages, started about 750 million years back, writes Nick Lane in Oxygen. What may have set off that chain makes for fascinating reading.

 

The most plausible explanation is based on those tectonic plates moving around. If you didn’t know already, the movement of those tectonic plates is what causes earthquakes, created the Himalayas, and why the coasts of Africa and South America seem like jig saw puzzle pieces that fit so well. But what’s that got to do with ice ages?

 

Lane explains. When rock is exposed to carbon dioxide in the air, it is eroded (since CO2 is weakly acidic). In the process, carbonates are formed and the CO2 level in the air reduces correspondingly. Now imagine an ice age where ice covers the land. In this scenario, the rocks and CO2 can’t react (since the ice/snow blocks their interaction), and the CO2 stays in the air. And CO2, of course, is a greenhouse gas. To add to that, volcanoes erupt and spew more CO2 into the atmosphere. All this increase in CO2 eventually adds up and increases global temperature, which in turn causes the ice to melt, thereby breaking the ice age.

 

Now consider the time when the tectonic plates moved to a position whereby all the continents were joined together and clumped near the equator. The ice age now starts off in the oceans, not on land. The rocks, now all near the equator, continue to react with the CO2 thereby decreasing the CO2 in the atmosphere. As the greenhouse gas reduces, the temperature of the earth falls further. In addition, all that snow and ice reflects back the sunlight, which means even less of the heat stays on earth. See how this has become a vicious cycle where it keeps getting colder and colder?

 

So what broke the vicious cycle of snowball earth intermittently? Volcanoes that spewed CO2 into the air, raising temperatures enough to break the ice age. But not for long:

“The juxtaposition of the continents around the Equator continued to set the same snare.”

And so continued the multiple rounds of snowball earth. Until the tectonic plates moved enough and “the continents were finally dispersed to the four corners of the earth”.

 

Yes, this is still a theory, but it aligns with many other facts and certainly explains the repeated cycles of snowball earth. No wonder then it is the most plausible theory as of today.