How do we even know that the Ice Age happened?

This is a brilliant question. How do we know what we know? How do we know that once upon a time there was ice over huge amounts of the earth? Especially since the idea of ice ages is so widely accepted today, I mean, we have 6 feature length Ice Age films. Well, I am here today to explain how we even know that ice ages or glacial maxima happened.

Well we partly know this because we lived through the ice ages. Homo sapiens evolved about 300000 years ago. We have, collectively, survived two glacial periods or ice ages since then. In fact, homo sapiens are the only hominin species to have survived through the last ice age, which is thought to be partly because humans are social and had stronger communities than other hominin species. 

We also know that ice ages happened because glacial periods leave scars. They leave deep and massive scars on the landscape in the form of glacial valleys. As glaciers melt and flow downwards they carve out huge U-shaped valleys in their wake. These are very distinctive landforms that can be identified centuries after the glacier has disappeared completely.

Here is a glacial valley in Antarcitica. 

Glaciers can also leave behind what geologists call a 'glacial erratic.' They are huge stones left behind by a melting glacier that do not belong to the geology of the places they are left. It kind of looks like they were dropped from the sky.  

Here is a 'glacial erratic.'

For a long time in places like Europe, people knew there must have been colder times before the present because they saw the shrinking glaciers in the Alps and the huge scars they left behind on the landscape. But it was only in the 1800s that a Swiss geologist popularised the idea of an ice age by looking at current glaciers in Switzerland. It was only later when Milutin Milankovitch calculated the earth's orbit, tilt and procession for his 1941 book that this theory was solidified. 

Okay, that's great and all, people now know that there were possibly ice ages in the past. But how do we actually know that they happened? How do back these theories with scientific evidence? 

Well, there are several ways to prove glaciation or ice age periods:

1. We can collect and test ice cores. 

2. We can use and test sediment cores in organic carbon burial sites. 

3. We can test oxygen isotopes in algae and phytoplankton.

But how can long dead algae and phytoplankton tell us anything about the climate? 

Well, being long dead and buried in sediments at the bottom of lakes or oceans means that these dead plants retain in their bodies atmospheric signatures that can tell us what the air and water were like when they were alive. One way in which we scientifically explore this is by looking at silica based diatoms from high altitude lakes. The methodology behind this is pretty interesting. First, scientists need to gather silica based algae from the bottoms of lakes. These silica based diatoms are single celled microalgae that are found across the globe. 

Diatoms under a microscope. 

Once they are collected, scientists must carefully remove the outside layer of the diatom cell as this may have absorbed oxygen data from more recent time periods. Then other debris like volcanic ash that can settle in the lakes must be removed. This is done with a tiny pipette and lots of careful work under a microscope. Scientists need millions of diatoms from a single sample to get the most accurate results. They then use gas source mass spectrometry to look at the isotopes within the diatoms long dead cells. This is a highly specialised process that looks at the stable isotopes of oxygen preserved in the microalgae. The main limitation to this process is that the equipment and people needed to perform this are so specialised that it can only be done in a few labs around the world. 

I won't bore you with the details of isotopes and why scientists use algae to look at them. What I will tell you is that oxygen isotopes can give us some of the clearest data on what the temperature of the past was. Using this information we can reconstruct the ice ages of the past. 

So, by using proxy data, we can in fact prove that there were ice ages before humans had a way of sharing information via written records. Diatoms are just one of many ways in which we can know that ice ages happened. But this reminds us that it is always important to question, how do we know what we know?