Five different times during the Earth’s history, the planet has entered a prolonged period of reduced temperatures. When this happens, massive ice sheets form, and sea levels drop.
While some of these events occurred billions of years ago, not all of them were in the distant past.
In fact, the last such event had a profound impact on the development of humans as a species.
Learn more about ice ages, how they affect the planet and how they affect humanity, on this episode of Everything Everywhere Daily.
Let’s start out this discussion of ice ages with what an ice age is.
An ice age is nothing more than a prolonged period of time of reduced temperatures such that large continental-size ice sheets and glaciers will form.
These periods do not last decades, centuries, or even millennia. Ice ages are periods that are millions of years long. Much of this confusion comes about from the colloquial use of the term “the ice age” which is in reference to the most recent glaciation, which I’ll get to in a bit.
According to geologists, there have been at least five ice ages in the history of the Earth.
The earliest would be the Pongolan Glaciation which occurred about 2.9 to 2.7 billion years ago. This is often not included on the list of ice ages because it occurred so long ago that the evidence is the shakiest for it. This is why I said there were “at least five” ice ages.
What is usually considered the first ice age was the Huronian Glaciation which was around 2.4 to 2.1 billion years ago.
The next was the Cryogenian. This occurred about 850 to 635 million years ago. This was still before the Cambrian Explosion which saw the rapid rise of multicellular life.
One or both of these early ice ages might have been a period known as Snowball Earth.
The Snowball Earth hypothesis holds at that some point in the distant history of the Earth, the entire planet, or at least most of it down to tropical latitudes, was covered in ice.
The planet wouldn’t have been green or blue, but rather white from the frozen seas.
The evidence suggesting there was a snowball Earth comes from the existence of glacial sediment at what was then tropical latitudes.
The Snowball Earth hypothesis is far from agreed upon amongst geologists, and evidence from that far back is always difficult to interpret.
Most of the difficulty comes from determining what the paleolatitudes were for during different periods. Due to plate tectonics, the Earth’s landmasses are slowly roaming around the planet over the course of tens and hundreds of millions of years.
We can use paleomagnetic evidence to determine what latitudes were when rock formed, but when you go that far back, there is much we don’t know about the Earth’s magnetosphere from that time.
If true, the Snowball Earth hypothesis might have profound implications for the development of early life on Earth. It could have been a shock to the planet which drove the evolution of bacteria, which had to adapt to survive.
The next ice age was the Andean-Saharan which occurred 450 to 420 million years ago. It is named after the glacial evidence which has been found in South America and Northern Africa. This was also one of the first major extinction events on Earth, which was when animals such as trilobites went extinct.
The ice age after that was the Karoo which went from 360–289 million years ago. Plantlife was rampant on Earth during this period which took much of the carbon dioxide out of the atmosphere and reduced the ability of the atmosphere to retain heat.
After the Karoo ice age, there was a long period of almost 300 million years where there were no ice ages at all.
The last and most recent ice age is the Quarternary which began 2.58 million years ago. Technically speaking, we are still in the middle of an ice age right now.
Before I mention that there was some confusion about the term “ice age”, and this is where it comes from.
Ice ages are not monolithic periods where there is ice and cold and nothing else. There are periods within each ice age known as glaciations and interglacial periods. The ice caps will ebb and flow back and forth over periods of tens of thousands of years or longer.
So within the ice age that we are currently living in, we are currently in an interglacial period, which is the period between massive ice sheets. The last glacial period was known as the Wisconsin Glacial Episode and it ended about 11,000 years ago.
When you hear people talking about “the ice age”, this is almost always what they are talking about. The most geologically recent period where massive ice sheets covered much of North America and Europe.
During the last 2.5 million years that the quaternary ice age has been taking place, the period between glaciations has been between 40,000 and 100,000 years.
The Wisconsin Glaciation started about 75,000 years ago and reached its maximum about 22,000 years ago.
I’m going to spend the rest of the episode talking about the Wisconsin Glaciation because that is what we know the most about given how recently it ended.
Before I do that, I should address the big question of why ice ages happen.
As far as we can tell, there is no one single cause of ice ages. They can be caused by a combination of plate tectonics, changes to the atmosphere, as well as Milankovitch cycles, which are the various cycles of the earth’s orbit and tilt which can augment or nullify each other.
This is an area of research where there is still a lot of debate. The Milankovitch cycles don’t fit perfectly to the glaciation cycles of the last 2.5 million years. This is known as the 100,000-year problem and there are numerous theories that try to explain the discrepancy.
For a glacier to form, all you need is for the rate of snow accumulation to be greater than the rate of melting. If more snow can be deposited in the winter than can be removed in the summer, you will see the gradual formation of glaciers.
So what exactly happened during the last glaciation? What was the Earth like back then?
For starters, the massive ice sheet on the northern hemisphere wasn’t uniform. Many people think of it as a semicircular cap that just would sit on the top of the Earth.
The ice sheet was actually very uneven in what it covered. Most of Canada, the Midwestern United States, New England, Scandinavia, and Northern Europe all the way through to Siberia was covered in ice.
However, the northern parts of Asia were not covered in ice. This would include the far eastern parts of modern Russia. Likewise, most of Alaska was not covered in ice as well.
The furthest extent south the ice extended in North America was about the middle of Illinois, Indiana, and Ohio. The ice didn’t get down as far in Europe, however, the northern parts of the island of Great Britain would have been covered in ice.
The only real glaciation in the southern hemisphere would have been in Antarctica, which is still covered in ice, and much more glaciers in the Andes Mountains in South America.
The ice sheets during the last glaciation would have been incredibly thick. The average thickness would have been about 4,000 meters or 2.5 miles. This is on a par with the thickness of ice on Antarctica today.
Thousands of meters of ice weighs a lot, and all that ice impacted the land underneath it. The mass of the ice literally deformed the land below it.
When the ice retreated all of the land which was pushed down by the ice began springing back up. This is called isostatic rebound. This process is still ongoing today in many parts of the world. There are some parts of the world where the land is rising as fast as 1 centimeter a year.
The country of Finland is growing by seven square kilometers per year due to isostatic rebound. The Gulf of Bothnia between Sweden and Finland will eventually totally disappear due to the uplift of the land underneath it.
Likewise, many of the lakes in Canada, Minnesota, and Wisconsin will also disappear at some point as the land rises, and all the water slowly drains away.
It is estimated that the post-glacial rebound of the land will probably last another 10,000 years.
If you could have looked at a map of the Earth during the glacial maximum 22,000 years ago, it would have looked radically different. All of the water which made up the ice in the ice sheets had to come from somewhere.
The result was dramatically lower sea levels than what we have today. During the glacial maximum, sea levels would have been 125 meters or 410 feet, below what the sea level is today.
That means much of Indonesia and the Philippines would have been connected by land to the rest of Asia. Australia would have been connected to Papua New Guinea. All the islands in Japan would have been connected, and southern Japan would have also been connected to Asia. The British Isles would have all been connected to Europe, with a huge grassy plain between them and Norway.
Most importantly, Alaska would have been connected to Siberia by what is known as the Bering Land Bridge. This was the most probable route that the first humans probably took to the Western Hemisphere.
Many people envision the first people crossing the Bering Land Bridge as walking over a glacier while wearing animals skins. Most likely, it was a grassy plain and they just chased game into the area and kept on going. They weren’t necessarily looking for a new home.
The movement across the land bridge wasn’t one way. It is believed that horses originally migrated to Asia from North America.
The last glaciation began to retreat about 19,000 years ago, and there was a reverse in the warming of the planet from 12,900 to 11,700 years ago. This period was known as the Younger Dryas, and it seems to play a very important role in the development of modern humans. Many of the first signs of human civilization we see, such as Gobekli Tepe, appear about the same time.
One theory for the sudden reversal in temperatures might have been a comet or meteor impact in North America on the ice sheet….but I will leave that story for another day.
Ice ages have played an important part in the history of the Earth and the development of life. The most recent ice age, the one that we are probably still in the middle of, played a major role in the development of modern humanity.
Even though the massive ice sheets have been gone for over 10,000 years, they are still shaping the world we live in today.
Everything Everywhere Daily is an Airwave Media Podcast.
The associate producers are Thor Thomsen and Peter Bennett.
Today’s review comes from listener cStyle over at Podchaser. They write:
Great podcast that covers a new topic in medium detail each day. Some days the topic will be related to the prior episode while other days it will be something totally random (but still interesting.) Love the podcast! My only feedback is I don’t think you need to mention “this will be covered in a future episode” as often. I think most of your listeners realize this. Perhaps just say this once a week or two. Either way, keep up the good work!
Thanks, cStyle! One of the reasons why I say something will be a future episode is that it can show the process of how one show can lead to another show. One of the most frequent questions I get is how I come up with ideas for shows.
Many if not most of my ideas come from researching past shows. One idea can spawn multiple more. For example, I can think of about five different episode ideas which relate to this episode’s ice age topic, and those five might then lead to even more.
All of those get put onto the great master list of show ideas, which currently has 734 ideas listed.
Remember, if you leave a review over at Podchaser until the end of April, they will make a donation to help feed Ukrainian refugees, which will be matched by several other podcast companies.