38: Ice Ages

For this week’s plog we will be following the cold theme and looking at ice ages. The words “Ice Age” may make you think of mammoths and sabre tooth tigers wandering around a little over 10,000 years ago (or of a Disney movie with a rather wonderful sabre tooth squirrel). However, this recent age of mammoths hasn’t been the only ice age earth has experienced. In fact the earth has fluctuated over the last few million years between warm and cold phases. The colder periods which we call ice ages are formally known as glacial periods, which are defined by the presence of large ice sheets in the northern and southern hemispheres.

Sound familiar? That’s because that’s what we have right now. Yes, technically we are in an ice age right now, though coming out of it. Does that mean that the warming of the planet is natural and we shouldn’t blame ourselves? Yes and no, but more on that later. First lets looks a little more at what glacial periods have happened, what causes them, and what they do to the planet.

Mammoths were around during the last ice age, but there have actually been many ice ages before them too. (Picture: Ice Age Centre/Wikimedia Commons/ CC BY-SA 4.0)


There have been five major glacial periods in earth’s history so far. The first was the Huronian glaciation, which lasted from 2.4-2.1 billion years ago. It’s the longest glaciation period we’ve had. At that time life on earth was single celled organisms which relied on CO2 to survive. Some of them photosynthesised, so took in CO2 and released oxygen as a waste product like plants today. However, a side effect (or perhaps a cause?) of the Huronian glacial period was that there was actually quite little CO2 in the atmosphere. As life at the time not only relied on the CO2 to survive but actually found oxygen toxic, this all came together to cause a mass extinction of these little single celled blobs.  It is thought that during the Huronian glaciation, the earth was almost totally covered in ice.

What Earth may have looked like during the Huronian glaciation period. Pretty chilly… (Image: Neethis/Royalty Free)


After this the world warmed up a little, and the ice melted. Then fast forward a few million years and we reach the Cryogenian period which lasted from roughly 720-635 million years ago (so recent, I know!). Whilst much shorter than the last glacial period, this one is thought to have been colder, with the entire world covered in ice, sometimes called ‘snowball earth’, though there is some debate about if there was a little band of ocean at the equator at the time. Life had come on a little at that point becoming multicellular, with amoebas, sponges, algae and marine plankton. At the end of this period the first plankton which ate other plankton started to develop, seeing more complex life forms becoming more common.

Creatures like amoebas were floating about during the Cryogenian period. Life was pretty tiny back then. (Picture: Wikimedia Commons/CC BY-SA 4.0)


Fast forward again through another spot of warming and we hit the Andean-Saharan glaciation. Thus called because the glaciers covered the Andes and the Sahara (now desert), this glaciation period lasted from 450-420 million years ago, though it should be noted that with tectonic plates moving, the Andes and the Sahara were in totally different places than today.

This period included the second largest mass extinction of species in earth’s history, with 85% of all species dying. This happened in three phases, with the causes for the first slightly unknown but large drops of creatures in the fossil record seen. The second drop in species coinciding with sea levels falling due to all the water getting locked up as ice, and the final phase of species loss coinciding with sea level rises. The cold temperatures are thought to have been a large contributor too, but the exact details of the extinctions are still somewhat of a mystery.  It was a long time ago after all.

A map of the south pole during the Middle Ordovician 450 millions years ago, showing how the continents and landmasses we know today were in very different locations (Image: Björn Kröger , Thomas Servais, Yunbai Zhang/CC BY-SA 4.0)

Karoo Ice Age

A little warming and cooling later we hit the Karoo ice age from 360-260 million years ago. Land plants had evolved around this time, so were busy sucking CO2 out of the atmosphere and pumping out oxygen. This reduced the greenhouse effect (opposite of today) causing everything to cool. As there was all this extra oxygen around, species that relied on oxygen to survive were able to grow larger than they are today. This includes things like the Meganeura, a dragonfly-like creature with a wingspan of up to 75 cm, the millipede-like Arthropleura reaching 1.8 m in length and scorpions reaching 70 cm in length. I said big, I didn’t say cute…

Arthropleura which lived during the Karoo Ice Age. It was a pretty sizeable bug! Reconstruction: Tim Bertelink


Finally we reach the last glaciation period, the Quaternary glaciation which started 2.58 million years ago and is thought to be going on today as we still have ice sheets. Whilst we are in the Quaternary glaciation period overall, within these periods you can get glacial and interglacial phases, where glaciers grow and shrink. As we have less glaciers than when the mammoths were roaming around (also still part of the Quaternary glaciation), we are in an interglacial phase of this period.

In answer to the earlier point of “so is warming of the planet natural or humans fault then?”, it’s a bit of both. The earth is naturally in a cool period which will be followed by a warmer period (we don’t know exactly when). However, this ‘natural’ warming and cooling happens over time spans of thousands or millions of years. With human activities pumping lots of greenhouse gases like CO2 into the atmosphere, insulating the planet, we are causing it to warm up much faster. In terms of earth, the geology doesn’t really care about this. This lump of rock we call our planet will survive, as will life in some form or another no matter what we do, but as for our species and many others, we don’t want to trigger another mass extinction within which we may be included, which is why we have to be careful about how we affect the climate.

There have also been a bunch of smaller glacial periods between all these major ones too, but it would be quite a long list to go through them all, so I’ll stop there with only covering the major ones.

Graph of glacial and interglacial periods in only the last 450,000 years. Whilst there have been five major glacial periods, there has still been fluctuations in between. Picture: Utah Geological Survey

Causes and Effects

So far we have seen that the causes of the various major glaciation periods include gas fluctuations in the atmosphere. The earth’s orbit, tilt, and how close it is to the sun, therefore how much sunlight it receives at different times of year, also have an effect. I have attached a video at the bottom with more info on this.

These glacial periods have also had impacts on our planet. As also mentioned, the big changes in climate have cause extinctions of many different species and have caused sea levels to rise and fall. The development of massive ice sheets have weighed down some areas of land, and now that this ice has melted the land is very slowly springing back up again, freed from the weight. This in our eyes is slow, though in geological time could be considered pretty fast, in some areas rising by around 1 cm/year.

As these glaciers and ice sheets were melting, many moved and flowed through the landscape. The movement of these glaciers paired with their massive weight carved out the landscape beneath them, creating the valleys, fjords and mountains that we see today. In this way, the ice ages have really played a huge rule in shaping our planet physically as well as affecting the life that was and still remains on it. It seems like the only constant on this planet really is change!

For more info:

Here’s another article with more details: https://www.history.com/topics/pre-history/ice-age

What causes ice ages:

Music: kongano.com

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