What Was the Ice Age?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
By the ice age, people usually mean the last glacial period, which peaked around twenty thousand years ago when enormous ice sheets covered much of northern Europe and North America. The full picture is more interesting, because ice ages come in cycles and we are arguably living in one right now.
When it was
Geologists use the term ice age for a long period when permanent ice sheets exist on Earth. Within an ice age, colder glacial periods alternate with warmer interglacial periods. The current ice age began around two and a half million years ago.
The last glacial maximum was about twenty thousand years ago. Ice sheets several kilometres thick covered Canada, Scandinavia and much of northern Britain. So much water was locked in ice that sea levels were roughly 120 metres lower than today.
What the world looked like
Lower sea levels exposed land that is now underwater. Britain was connected to mainland Europe, and a land bridge joined Siberia to Alaska, which is how people and animals moved into the Americas.
South of the ice lay vast cold grasslands known as mammoth steppe, dry and windy but surprisingly productive. This landscape supported large herds of grazing animals across a band stretching from western Europe deep into Asia.
The animals
The ice age fauna is famous for its size and its adaptations to cold:
- •Woolly mammoths, with thick fur and small ears to limit heat loss
- •Woolly rhinoceroses, grazing the steppe alongside them
- •Cave lions and cave bears across Europe
- •Sabre toothed cats in the Americas
- •Giant ground sloths, some as tall as a house
Why ice ages happen
The main driver is a set of regular variations in Earth's orbit and tilt, often called Milankovitch cycles. They change how much sunlight reaches the high northern latitudes in summer, and if summers are cool enough that winter snow survives, ice accumulates.
These orbital changes are small on their own. They work because they trigger feedbacks: ice reflects sunlight, which cools things further, and cold oceans absorb more carbon dioxide, which reduces the greenhouse effect and cools things further again.
Are we still in one?
Technically yes. Permanent ice sheets remain on Greenland and Antarctica, which means the current ice age has not ended. We are in an interglacial period that began about eleven and a half thousand years ago.
That warm window is when agriculture, cities and written history all appeared. Human civilisation has existed entirely within one relatively stable interglacial, which is worth remembering when thinking about how much climate stability we have been relying on.
How we read past climates
Almost everything known about ice age temperatures comes from proxy records, since nobody was taking measurements. Ice cores drilled from Greenland and Antarctica are the most valuable, containing annual layers going back hundreds of thousands of years.
Trapped air bubbles preserve samples of the ancient atmosphere directly, and the ratio of oxygen isotopes in the ice indicates the temperature when the snow fell. Ocean sediment cores, tree rings, cave formations and pollen records provide independent checks. The strength of the reconstruction comes from these separate methods agreeing with each other.
The takeaway
The ice age usually refers to the last glacial period around twenty thousand years ago, driven by orbital cycles and their feedbacks, and since permanent ice sheets still exist we are living in a warm interval within an ice age rather than after one.