Ice Ages
The last great ice age peaked about 20,000 years ago. Ice sheets a kilometre thick covered what is now London, New York, and Chicago. Sea levels were 120 metres lower. Britain was connected to Europe by dry land.
What causes ice ages
Ice ages are driven by Milankovitch cycles — slow, predictable changes in Earth's orbit and axial tilt that alter how much sunlight different parts of the planet receive at different times of year. These cycles operate over tens of thousands to hundreds of thousands of years. When Northern Hemisphere summers receive less sunlight, winter snow survives into the following year, glaciers grow, and the planet cools further in a feedback loop.
The world during an ice age
During the last glacial maximum, 20,000 years ago, woolly mammoths, cave lions, giant ground sloths, cave bears, and woolly rhinoceroses roamed alongside early humans. Enormous lakes existed in what are now deserts. The Amazon was more savanna than rainforest. The world looked completely different — and our ancestors navigated all of it on foot.
Ice ages shaped human history
The end of the last ice age, around 12,000 years ago, transformed the world. Melting ice raised sea levels, flooded coastal land bridges, and created the relatively stable, warm climate in which all of human civilisation developed. Agriculture, cities, writing, science — everything happened in this narrow window of unusually stable climate. That window may not last indefinitely.
Reading the ice
Ice cores drilled from Antarctica contain trapped air bubbles going back 800,000 years — a direct chemical record of past atmospheric composition and temperature across multiple ice age cycles. The data shows a tight, consistent correlation between CO₂ levels and global temperature. The current atmospheric CO₂ level is higher than anything recorded in those 800,000 years of ice core data.
The Younger Dryas was a dramatic cold snap that occurred about 12,900 years ago, interrupting the warming at the end of the last ice age. Within decades, temperatures in parts of the Northern Hemisphere plunged back to near-glacial levels and stayed there for about 1,200 years before abruptly warming again. Its cause is debated — leading theories include disruption of Atlantic Ocean circulation by meltwater floods. It demonstrates how rapidly Earth's climate system can shift.