If you ever catch yourself complaining about winter – the wind, the ice, the snow – just remember that it could always be worse. Much, much worse. For one thing, as our favorite Game of Thrones characters echo time and time again: “Winter is coming.” (To see a scientific take on why winter is coming, check out this link.) More importantly, over Earth’s geological history, there have been several episodes when the planet froze over completely – land, oceans, everything. With this in mind and in the spirit of staying warm, I have decided to intersperse photos of glacier throughout this article.
The first episode, the Huronian glaciation, occurred between 2.4 and 2.1 billion years ago. It followed the Great Oxygenation Event, during which oxygen levels in the atmosphere rose quickly due to photosynthesis by cyanobacteria. As oxygen became available, some of it reacted with methane, common in the atmosphere at that time, to form carbon dioxide. Since methane is a much more effective greenhouse gas than CO2, this caused the planet to lose part of its ‘heat blanket’, sending it into a 300 million year ice age.
More recently, the planet was frozen during parts of the Cryogenian period, between 850 and 635 million years ago (in fact, ‘cryos’ is the Greek word for ‘icy cold’). These episodes are commonly referred to as ‘Snowball Earth‘. While the cause is still under debate, there are several possibilities. It could be that Earth received less energy from the Sun due to a change in orbit or solar output. Or a supervolcano released enough fine particles into the upper atmosphere to reflect much of the Sun’s heat back into space. What we do know is that once ice sheets begin to expand from the poles towards lower latitudes, a positive feedback cycle is set off. This is called ice-albedo feedback, albedo referring to reflection of the Sun’s radiation. Lighter colors reflect more radiation than dark colors, thus ice and snow reflects the Sun’s heat, cooling the planet further and allowing ice sheets to continue to grow. Once ice sheets have reached latitudes of 30 degrees, this feedback is strong enough to freeze the rest of the planet. With so much ice albedo and few clouds to trap heat (water freezes before clouds can form), the temperature around the equator would have been close to that of Antarctica today.
While this all sounds lovely, you might be asking how we can possibly know what happened 700 million years ago. You might be surprised just how much evidence is really out there. But since you probably don’t want to spend all day reading about rocks, I will present only my favorite. Enter: the BIF. BIFs or Banded Iron Formations are layered rocks that alternate between bands of iron oxides (magnetite and hematite) and chert. They form in a very specific set of conditions and can tell us a lot about the environment at the time of formation. In today’s oxygen-rich oceans, iron has a very low solubility. As soon as iron reaches the ocean, it reacts to form iron oxides that fall to the sea floor. In anoxic conditions (no oxygen), iron can accumulate. During Snowball Earth, the frozen oceans were cut off from the atmosphere, and there was not enough oxygen left to react with iron. When the ice melted, all the iron that had accumulated quickly precipitated, forming thick BIF deposits.
So we know that Snowball Earth did exist, and we know how it might have been caused. What we haven’t explored yet is how and why the ice melted. What you have to remember is that Earth systems operate largely independently of one another. While the planet was frozen, volcanoes continued to release greenhouse gases such as CO2 and methane. When we talk about climate change today, we talk about carbon dioxide levels rising by tens or hundreds of parts per million (levels are currently around 400 ppm or 0.04%). To melt Snowball Earth, carbon dioxide would have had to rise to 13% of the entire atmosphere! As ice around the equators began to melt, another positive feedback developed, this time in the opposite direction. Darker colored water has a lower albedo than ice and therefore absorbs more heat, melting the ice further. This is the same trend that we see today with melting Arctic sea ice. In the sauna that followed the icebox planet, cyanobacteria quickly bounced back and the atmosphere was reoxygenated, paving the way for the development of multicellular life.
National Geographic’s take on: “What if Snowball Earth happened today?” “First of all, Canadians will have to dig much deeper holes when they go ice fishing.”
This has been your daily dose of mostly harmless science.
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All images courtesy of Wikimedia Commons. Except the cover photo. The cover photo is mine. All mine! (Muhahaha)