2012: Advent Atmosphere: Younger Dryas

by on December 9, 2012

The Younger Dryas event is named after the kind of pollen used as a proxy for the temperature. Dryas octopetala grows at the edge of icy regions. So if you find its pollen at a particular time in a particular place, you know that’s where the edge of the ice was, and you can track how it moved.

The Younger Dryas event is, basically, a brief cold snap at the end of the most recent ice age. Take a look at this graph:

This is the last forty thousand years of temperature data. The present is on the left, and the higher the lines are, the warmer things were. Right now, things are lovely and warm and steady. As you move right, that first sudden dip you come to is the Younger Dryas.

By this point in history, humans are living everywhere in the world. It’s the end of the mesolithic period, 10,800 – 9500 BC. In 500 years, you find the first evidence of human habitation at Jericho. In 1500 years, the Mesopotamians will invent agriculture. 4000 years after the Younger Dryas and the end of the ice age, writing will be invented in Sumeria and history will begin. The people of 10,000 BC were hunter-gatherers, moving north as the ice retreated and opened up new living space.

The dramatic thing about the Younger Dryas is just how suddenly it happened. Look at the red line in that graph. See how sharp the rise is at the end of the cold snap? That’s no longer than a human lifetime. 50 years at most, from ice age to modern climate. Maybe as little as 10 years. By the end, there were people in northern Europe who remembered when it had all been ice fields. And the beginning, the drop from nearly modern temperatures back to ice age, was nearly as fast.

So why? Why did the temperature suddenly change, and why only in that part of the world? It’s a distinctly European event, you don’t see that cold snap elsewhere. Mesolithic people had no way of knowing what was happening, but with hindsight and a global view, we’ve got a pretty good idea nowadays, and we’ll look at it tomorrow.

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