Now that we have a decent idea of what a Snowball Earth was and how it all started from our part I of the article, it is time to answer a few more interesting questions like: How did it end? Did multi-cellular life really evolve during Snowball Earth? Will Snowball Earth happen again? How did early life forms survive during the global glaciation? These question definitely are intriguing and we believe that the time you spend exploring the answers will be time well-spent. So, if you have some 10-15 minutes of time to spare, join us on this incredible journey of find the answers to these remaining question. We promise that we will try to keep the answers simple and interesting.
Question: How did the Snowball Earth come to an end?
Answer: Before we start, we will like to remind you once again that Snowball Earth refers to Marinoan Glaciation that ended ~635 mya. That was actually a global glaciation and unfortunately that was not the only global glaciation. There were two more and sometimes, all these global glaciations are together known as snowball earths (note the plural form used).
According to experts, silicate weathering led to massive decline in CO2 content of the atmosphere, leading to global cooling, which eventually resulted into Snowball Earth (we are speaking about the Marinoan Glaciation). However, there was one problem. The path to recovery would be a long journey of the order of several billions of years because sun literally takes a billion year to grow only 6 % warmer. So, the solar evolution was definitely not the tool for ending Snowball Earth because only a few million years prior to Marinoan Glaciation there was Sturtian Snowball Earth.
So, what really happened? The answer to this lies in plate tectonics. In our first part we learned that silicate weathering leads to formation of calcium carbonate or CaCO3. Because of plate tectonics, organic matter along with calcium carbonate eventually flows down into the mantle of earth. This is known as subduction. When subduction happens, both the organic matter and the CaCO3 is subjected to enormous amount of heat and is converted into CO2. This CO2 is then emitted through volcanic activities.
The natural question that follows is, ‘what is plate tectonics anyway?’ Well, our earth’s crust is basically made up of rock plates. There are several rock plates that form everything from continents to ocean beds. These rock plates are at least 100 kilometers thick and are subject to gravitational pull. These plates interact with each other in a number of ways. One such interaction is where a cold rock plate sinks underneath another and results in volcanic activities that release CO2 back into the oceans and atmosphere.
Interestingly, though the planet remained covered in ice, plate tectonics continued without any restrictions. Millions of years of CaCO3 made it to hot mantle and voila! CO2 was back. Adding up to this, silicate weathering slowed down because of the ice which made earth’s global climate cold and dry. So, the amount of CO2 released by plate tectonics was not counterbalanced by quick reduction in CO2 content.
This lead to CO2-led greenhouse effect that eventually counterbalanced the loss of solar energy because of ice and snow. The first thing that happened then is that ice at the equator melted, exposing liquid water. The water then darkened the ice, which increased solar energy absorption and hence, further melting. Experts did some climate modeling to figure out that the millions of years of ice deposits melted in about 2,000 years only. As ice melted, the climate became warmer and wetter. On top of the loss of ice also exposed fresh rocks for silicate weathering.
Though the silicate weathering accelerated, it literally took tens of thousands of years for the CO2 levels to drop at normal levels. Unfortunately, immediately after the Snowball Earth came to an end, there was, what Joe Kirschvink (the person who coined the term Snowball Earth) called, ultra-greenhouse aftermath. This is precisely what happened, according to experts, in the two earlier global glaciation events, allowing earth to become blue again.
Question: How the hell did life forms manage to ride out the Snowball Earth and other global glaciations?
Answer: When we are referring to life during periods of global glaciations (that includes the Snowball Earth), we need to understand that complex life forms didn’t even exist. Animals that are above sponge grade appeared somewhere near 555 mya – long after the Snowball Earth came to an end. Fossil records of animals above sponge grade (known as bilaterian animals) have been dated to be about 555 million years old and they first evolved in Arctic Russia. Terrestrial fauna and other vascular land plants took more than 100 million years more to come into existence after the first bilaterian animals evolved. So, life as we know today never actually faced a single global glaciation.
However, that doesn’t mean that life never faced this terrible ordeal thrown on earth by nature. There were several microorganisms that had to live through global glaciation phases. These included:
- Prokaryotes (single-celled microscopic organisms with no nucleus bounded by membranes or separate organs bounded by membranes) – bacteria and archaea or archea.
- Eukaryotes (microscopic organisms which have membrane bound separate nuclei and other organelles that are bounded by membranes) – testate amoeba, algae etc.
Apart from the microorganisms, there were also a few other cm-scale organisms (or centimeter-scale organisms) present at the time of the global glaciations. These cm-scale organisms included:
- Coiled Grypania that lived ~ 1.9 billion years ago.
- Horodyskia that lived ~ 1.5 billion years ago – They had a protective layer that was hard, which means they had soft tissue inside. They were colonial by nature and resembled a necklace.
- Parmia that lived ~ 1 billion years ago. They looked like worms.
Not only the microbes lived through the global glaciation period but the cm-scale organisms also survived. The question is, how did they manage to survive? Looking for answers to this question Jim Kasting and Dave Pollard from Pennsylvania State University looked into ice dynamics for evidences and came up with a model in which they argued that there was an equatorial zone during Snowball Earth where instead of ice sheets of more than 100 meters of thickness, ice stayed only 2 meters thick. This zone had a width of 2,000 kilometers. According to the model, 2m thick clear ice allowed sunlight to penetrate in ample quantities that allowed a healthy rate of photosynthesis just beneath the ice. Additionally, the model also suggests that since the ice was only 2 meters thick, there were ample cracks in the ice sheet that allowed marine photosynthesis. However, this model requires the presence of nutrition dust (trace elements, phosphorus and nutrient iron) along with fixed nitrogen. This problem was probably solved by volcanic activities as well as extraterrestrial material that reached earth’s surface.
Well, that’s the closest explanation given so far but some scientists really find it difficult to accept that eukaryotic ancestors of modern life form managed to survive in the narrow escape of global glaciation. However, until a more relevant and acceptable theory comes along, this is what we need to accept no matter how outlandish it may sound.
Question: Did multi-cellular life really evolve during Snowball Earth?
Answer: Okay, this question needs to be slightly modified. It should be ‘Did multi-cellular life really evolve during global glaciations of snowball earths?’ Fossil records show that multi-cellular eukaryotic microbes existed at least 1.9 billion years ago. Remember the date of first global glaciation or the Makganyene Glaciation? That happened anywhere between 2.4 and 2.2 billion years ago. This had led to the believe that eukaryotic organisms evolved after Makganyene Glaciation and the multi-cellular macroscopic animals evolved after Sturtian and Marinoan snowball earths.
One of the most popular theories of such evolutionary leap is that ice killed many of then surviving species of organisms. The catastrophic environment forced the handful of individual organisms to become very dependent on each other, forcing the single cells (single-celled organisms and even individual cells in multi-cellular eukaryotes) to give up their reproduction ability and get into more specialized functions. That is, cells started behaving altruistically so that others can benefit from such behavior. The conditions during Snowball Earth gave rise to high relatedness among the cells. The pressure of relatedness was so high that the cost of forming complex multi-cellular animals (here cost refers to giving up of ability of cells to reproduce on their own) was actually overcome because of survival needs. Basically it is the theory of Kin Selection.
Question: Will Snowball Earth happen again?
Answer: The chances of that happening are pretty slim. Since the last Snowball Earth, the luminosity of sun has increased by at least 4%. This increase in luminosity will continue to happen. On top of that, the present configuration of the continents has actually led to large boreal lands (i.e. northern lands) and subtropical lands. This has warmed up the earth. On top of that human activities have increased global warming to great extent. These factors together conspire against the possibility of another global glaciation.
Some say that CO2 emission has actually decreased because of decreased volcanic activities. Going deep in the oceans carbonate sediments in Pacific Ocean bed is also very low. This means that less CO2 is released from Pacific Ring of Fire. Experts however hold the view that despite low CO2 emission by volcanoes (both underwater and land), earth will warm up even further in future because of two reasons:
- Subduction will switch from Pacific to Atlantic Ocean. Atlantic Ocean is rich in carbonate sediment and this will lead to release of massive amounts of CO2.
- The boreal lands are pretty large. This is actually a safety switch because if at all global CO2 decreases dramatically, those lands will get ice sheet blankets and suppress silicate weathering significantly, thereby preventing global glaciation.
Given these scenarios, it is very unlikely that a Snowball Earth will occur again unless there is a major continental rearrangement approaching.
The final question: Is Snowball Earth theory widely accepted?
Answer: So far (including Part I) we have studied Snowball Earth theory is great details. However, some do believe that actually earth never completely froze. Several simulations under different conditions have been performed which only returned the results that:
- Only if solar radiation was very low, heat transport of oceans were very low and CO2 in atmosphere was very low, it was possible to have a Snowball Earth.
- If all three conditions were not met at a time (especially considering the ocean physics, i.e. how oceans behave and transport heat), it was really not possible to have the entire earth covered by an ice sheet. What really was possible was that ice sheets separately formed in continents of low altitude but left the oceans open and free of ice, making them oases for life.
Based on these models, many people believe that earth did not become a snowball but rather became a slushball.