What Was the Ice Age?

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 about 34 million years ago, when ice first covered Antarctica, and its present phase of great northern ice sheets began about 2.6 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.

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 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

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.

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.

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, holding a climate record going back hundreds of thousands of years, with yearly layers countable for the most recent tens of thousands.

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.

Glaciers leave distinctive marks, and those marks were how people first worked out that ice had once covered places like Scotland, Switzerland and the northern United States. Moving ice carves river valleys into broad U shapes, scrapes parallel scratches into bedrock and drops boulders, called erratics, far from the rock they came from. In the nineteenth century the naturalist Louis Agassiz argued that such features across Europe could only be explained by vast ice sheets, an idea that met strong resistance before becoming accepted.

The ice also left a slower legacy. A sheet several kilometres thick pressed the crust down into the softer mantle beneath it. When the ice melted, the land began to rise again, and in some places it is still rising today. Parts of Scandinavia and the area around Hudson Bay in Canada continue to lift, which has left ancient beaches stranded well above modern sea level. In Britain, Scotland is generally rising while parts of southern England are gently sinking, a slow rebalancing that began when the ice left.

A common picture of the ice age is one long, unchanging deep freeze. The records described above tell a different story. Greenland ice cores show that during the last glacial period the climate swung repeatedly between very cold phases and milder ones, sometimes warming by many degrees within a few decades in the North Atlantic region. These abrupt shifts are named Dansgaard-Oeschger events after the scientists who identified them, and they are thought to be linked to changes in ocean circulation.

The most famous swing came right at the end. As the world warmed out of the last glacial period, parts of the Northern Hemisphere were plunged back into near glacial cold for over a thousand years, from roughly 12,900 to 11,700 years ago. This interval is called the Younger Dryas, named after a small Arctic flower, Dryas octopetala, whose pollen reappears in European lake sediments of that time. Its sudden start and end are a reminder that the climate system can change quickly once certain thresholds are crossed.

Most of the large animals listed above died out between roughly 50,000 and 10,000 years ago, and the cause is still genuinely debated. One explanation blames climate: as the ice retreated, the mammoth steppe gave way to forest and wet tundra, shrinking the open grasslands that big grazers depended on. Another blames people. Modern humans spread across the world during the same period, and even modest hunting can push slow-breeding animals such as mammoths into decline, because each female raised only a few young in her lifetime. Many researchers now think both pressures acted together, in different balances in different places.

The timing of the extinctions offers clues. In Australia many large animals vanished in the period after people arrived, tens of thousands of years before the end of the last glacial period, which many researchers see as pointing to a strong human role there. In the Americas the losses cluster near the end of the glacial period, when people and rapid climate change arrived together. Africa, where large animals had lived alongside human ancestors for a very long time, lost comparatively few. Some populations held on in isolated refuges. Woolly mammoths survived on Wrangel Island in the Arctic Ocean until roughly 4,000 years ago.

The orbital explanation took about a century to win acceptance. In the 1860s the Scottish scientist James Croll, largely self-taught and working as a caretaker at a Glasgow college, argued that changes in Earth's orbit could trigger ice ages. The Serbian engineer Milutin Milankovic later calculated by hand, over many years, how sunlight at different latitudes had varied over hundreds of thousands of years, continuing the work while interned in Budapest during the First World War. His results combined three rhythms: the shape of the orbit changes over about 100,000 years, the tilt over about 41,000, and the wobble of the axis over roughly 23,000.

The test came from the sea floor. Plankton shells settle layer by layer in deep ocean mud, and the oxygen isotopes in them record how much water was locked up in ice sheets. In 1976 James Hays, John Imbrie and Nicholas Shackleton showed that long sediment cores from the southern Indian Ocean contained the same three rhythms, and they called Earth's orbit the pacemaker of the ice ages. Yet over roughly the last million years, glacial cycles have followed the 100,000 year rhythm, though that orbital change has the weakest direct effect on sunlight, while earlier cycles kept a 41,000 year beat. Why the pattern switched is still debated.

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.