Earth looked like this 700 million years ago:
That’s right, the entire planet was covered in ice sheets that reached to the equator, the globe was on an ice age on massive steroids. This event is known as Snowball Earth, the most recent of which happened in the Cryogenian period (720–635 million years ago) during the Neoproterozoic era. Earlier global freezing did happen in the Paleoproterozoic (Huronian glaciation), but I’ll be focusing on the Sturtian and Marinoan glaciations in specific.
There are 2 main things that determine Earth’s temperature-
- The sun’s luminosity
- Atmospheric gases
Back in the Cryogenian, the sun was 6–7% dimmer than it is today (the sun gets brighter over time), meaning baseline temperatures were a lot lower on average, which made Earth much more vulnerable to an albedo runaway, which I’ll get to in a bit.
In the preceding Tonian period, continents were lined in the equator, this is huge as chemical weathering is strongest at the equator due to intense rainfalls. Chemical weathering locks CO2 gases into carbonate rocks, this happens when CO2 rains down upon silicate rocks, which are common in volcanic regions and continental merging. Carbon dioxide is a greenhouse gas, which means it traps the sun’s heat in the atmosphere, unlike how oxygen is. So when CO2 gets locked away in carbonate rocks, the amount of greenhouse gases in the atmosphere decreases.
This process happened over in the millions of years leading up to the Cryogenian, CO2 would keep plummeting as more and more carbonate rocks were formed due to chemical weathering, which continuously cooled the planet. Eventually, ice around the poles started to form, ice is one of the best deflectors of sunlight on Earth, otherwise known as albedo. As the amount of greenhouse gases lowered, more and more ice started to form, which reflected more sunlight, which made the Earth even colder, which meant more ice.
Once the ice reached around 40° degrees latitude, the albedo runaway effect became unstoppable, causing ice to eventually reach the equator, thus beginning Snowball Earth.
Cryogenian Earth 🧊
It’s estimated that at least 80–90% of the planet during this time would’ve been frozen rock solid, with possible small patches of equatorial ocean remaining intact, if any. Otherwise, the continents were covered in ice sheets kilometers thick, with oceans being covered in ice at least a couple hundred meters thick.
Temperatures around this time are estimated to have averaged around -50°C globally, with the equator being around -30°C on average, and the poles being a whopping -80°C on average! To put that into perspective, Antarctica's record low of -89°C is barely colder than the average South Pole day of Snowball Earth.
The absolute coldest days of Cryogenian period would’ve likely happened in the South Pole at winter, it’s been estimated that the record low could've reached a monumental -110°C to -130°C, that’s as freezing as the average day on Mars’s poles at night. These were very likely the coldest days in Earth’s history.
However, below the ice sheets covering the sea, life was still enduring, the earliest and most primitive of animals still hanged on in these hellish conditions, such as Otavia antiqua, possibly by being near hydrothermal vents and small patches of water where sunlight hit.
And as for how Snowball Earth ended? Volcanos. 🌋
Even during this global freezing, volcanos remained active and contributors of CO2 gases, and now that Earth was frozen, chemical weathering became pretty much nonexistent, so nothing was able to prevent CO2 gases from accumulating over millions of years.
Eventually, enough CO2 accumulated that ice began to melt at the equator before eventually, Earth was ice free.
Since then, Snowball Earth has never happened again, the last time this phenomena could’ve been possible under the perfect conditions was around 540–520 million years ago in the early Cambrian, afterwards, the sun became too bright to allow ice to reach the equator before melting. Even today, we are nearing the threshold for ice ages in the geological time scale. Snowball Earth will forever be exclusive to the Proterozoic eon.
-Cesar Alcaraz