About 12,000 years ago, after several thousand of years of ice age cold, the glaciers that covered large parts of North America and Eurasia were melting and temperatures were gradually warming.

But then the cold returned.

A tremendous cold spell gripped the high latitude regions of the Northern Hemisphere. This wasn’t just a short-term weather event, or even a seasonal change. It was cold for about 1,300 years. In some places the glaciers regrew and it was as cold as the peak of the Ice Age.

It would seem that the cold returned much faster than an ice age cycle - within a century.

This time in Earth history is known as the Younger Dryas, after the vast quantities of pollen from tundra flower species, Dryas octopetala, found in sediment deposited at the time. It’s also called the Big Freeze for obvious reasons.

Why did this happen? Scientists are not sure whether the cooling was global or regional and they are not entirely certain about the cause or causes. However, many scientists hypothesize that a partial or total shutdown of the ocean’s thermohaline circulation may have been involved.

Here’s the hypothesis:

As the vast North American ice sheets began to melt, a huge lake formed from the meltwater in what is now central Canada. Today that lake no longer exists, but the sediment deposited on the bottom of the lake does. Scientists dubbed it Lake Agassiz and speculated that the water in the lake may have drained all at once flooding into the Atlantic. The influx of fresh water shut down the normal cycle of thermohaline circulation

Without thermohaline circulation warm, tropical waters wouldn’t flow north towards the Arctic and cold, polar waters wouldn’t flow south towards the equator which could change regional, and possibly even global, climate. The theory claims that this shutdown temporarily plunged at least some parts of the Northern Hemisphere back into peak "ice age" conditions. Eventually, the flow from Lake Agassiz subsided, and the thermohaline circulation re-established itself.

Could a repeat performance of this event happen in the future? Rapid melt of Arctic sea ice and the Greenland ice sheet are adding fresh water into the Atlantic. Would the melting ice provide enough fresh water to shut down or disrupt thermohaline circulation eventually? Nobody knows for sure. Most scientists agree that while there is low probability of this scenario, there would be very high consequences if it did happen. Some think that the story of the Big Freeze is a cautionary tale, reminding us that not all climate change is gradual, that a series of unlikely events can produce catastrophic change, and that climate warming can lead to freezing cold conditions because of the complexity of our planet.