The Younger Dryas

Podcast Transcript

Around 12,900 years ago, the last ice age was ending. Things were warming up, and the glaciers were starting to recede.

…and then something happened.

For about 1,200 years, the climate reversed and got colder again.

When this cooling trend ended and the ice age was finally over, it also happened to coincide with the rise of agriculture and human civilization.

Learn more about the Younger Dryas, some of its possible causes, and how it impacted humanity on this episode of Everything Everywhere Daily.

The Younger Dryas might not be something that you’ve heard of. It has mostly been a concern for ice age researchers.

However, it has become more relevant recently for reasons I’ll get to shortly.

Let’s start with, “What was the Younger Dryas?”

The Younger Dryas was a sudden and severe cold period that interrupted the general warming trend at the end of the last Ice Age. It lasted from approximately 12,900 to 11,700 years ago, representing one of the most dramatic and rapid climate shifts in Earth’s recent geological history.

The Younger Dryas is named after a small Arctic–alpine flower called Dryas octopetala, which became abundant in pollen records during that cold interval. Dryas grows today in tundra and subarctic regions, so its presence in ancient European sediments signals a return to cold, glacial-like conditions.

European scientists studying lake and peat deposits from the late Pleistocene found several distinct layers rich in Dryas pollen, each representing a cold phase.

They labeled them the Older Dryas, Middle Dryas, and Younger Dryas in chronological order. The Younger Dryas was the last and most recent of these cold episodes, occurring just before the onset of the stable Holocene climate, which is the geologic epoch we are currently living in.

How do we know that the Younger Dryas took place?

Evidence for the Younger Dryas comes from multiple paleoclimate proxies that all register a sharp, millennium-scale return to cold conditions beginning about 12,900 years ago and ending abruptly around 11,700 years ago.

The clearest evidence is found in Greenland ice cores. Stable isotopes of oxygen and hydrogen show a sharp drop in inferred temperatures over years to decades, alongside increases in wind-blown dust, sea salt, and calcium, indicating stronger, colder, stormier conditions over the North Atlantic.

These ice cores also trap ancient air. Methane concentrations fall at the onset of the Younger Dryas, consistent with a contraction of northern wetlands, and then rebound rapidly at its end, a pattern that matches the temperature shifts.

Lake and marine sediments independently mirror this story. Lakes in Scandinavia, Germany, and Japan preserve year-by-year changes in sediment composition that mark colder, drier, more erosive climates.

In Venezuela and in North Atlantic cores, finely layered sediments and microfossils show sea surface cooling and changes in plankton communities.

Glaciers add a physical record. In Scotland, the Loch Lomond moraines capture a pronounced glacier readvance that aligns with the Younger Dryas. Glaciers in the European Alps, the Andes, and New Zealand also advanced or stabilized, marking cooler conditions.

Finally, cave deposits, which can be dated precisely by uranium-thorium methods, provide some of the best geologic clocks. Stalactites and stalagmites from caves in Israel, China, and elsewhere display abrupt shifts in oxygen isotopes that track regional hydroclimate responses to the North Atlantic cooling.

These records indicate that the Younger Dryas began relatively quickly within decades and ended just as rapidly about 1,200 years later.

If you remember way back, I previously did an episode on Milankovitch cycles. These are the long-term cycles based on the tilt of the Earth’s axis and the precession of its orbit around the sun. These cycles can affect the amount of sunlight hitting the Earth and can change the planet’s climate.

However, this couldn’t be responsible for the Younger Dryas because these cycles work on the scale of tens to hundreds of thousands of years.

The big question, then, is why this is important. If the Earth happened to cool down for 1,200 years over 11,000 years ago, why is that relevant?

If you’ve been paying attention, that date, about 11,000 years ago, has cropped up again and again in various episodes.

This was the earliest time we can point to the domestication of crops and animals, as well as the earliest human-made structures, such as Göbekli Tepe in Turkey.

When the Younger Dryas ended about 11,700 years ago, the climate in the Northern Hemisphere shifted from a volatile, near-glacial climate to the relatively warm and stable conditions of the early Holocene.

That change happened fast on human timescales, with decades to centuries of abrupt warming. The new baseline reduced the frequency of extreme cold spells and reorganized winds and rainfall, which in turn expanded grasslands, forests, and wetlands.

For people who had endured a millennium of cooler, drier conditions, this meant a broader and more reliable number of plants and animals, longer growing seasons, and predictable water on the landscape.

Once the climate settled, experiments in saving seeds and corralling animals could pay off across generations rather than being undone by a change in climate.

With the sudden climate rebound, fields of wild wheat and barley expanded again, and the incentives flipped toward permanent settlement and active cultivation.

Over the following centuries, those experiments hardened into the domestication of cereals and pulses, followed by sheep, goats, and later cattle.

Similar events unfolded independently in other regions as climates stabilized regional water cycles. In the Yellow River basin, the Yangtze wetlands, along the tropical regions that supported maize and squash in the Americas, and in the high valleys that nurtured tubers in the Andes, the new climate created ecological windows where repeated planting, harvesting, and selection could produce results.

None of this means that the Younger Dryas by itself caused civilization. Social innovations, local ecosystems, and cultural choices were essential, and civilization timelines differed in different regions.

The end of the Younger Dryas created a stable environment and a burst of ecological productivity, making cultural evolution much more likely to persist over generations.

If the improved climate served as a carrot, then the extinction of large megafauna might have served as the stick.

As temperatures rose and ice sheets retreated, tundra and steppe habitats that had supported mammoths, woolly rhinos, and giant bison were replaced by forests and wetlands. The specialized grazers that thrived in open, cold environments lost their food sources and range, while smaller, more adaptable species survived.

The decrease in large animals, which were the focus of most nomadic hunting societies, may have necessitated many of them to become more settled, a change that was now possible in a warmer environment.

So, the Younger Dryas was very important in the development of human civilization, and there is plenty of evidence we can point to for its existence.

However, there is one thing I haven’t touched on yet, which is a topic of great debate and controversy. Why did the Younger Dryas happen?

What was the reason why the Earth’s climate so quickly reversed for over a millennium and then reversed again almost as quickly?

The leading theory is called the Meltwater Pulse Hypothesis. As the ice sheets over Northern Europe and North America melted at the end of the last Ice Age, large proglacial lakes formed on their margins.

A proglacial lake is a body of water that forms in front of a glacier or ice sheet, created when meltwater becomes trapped by the ice itself or by moraines and other debris left behind as the glacier retreats.

This theory suggests that a freshwater influx from the glacial lakes disrupted the Atlantic Meridional Overturning Circulation or AMOC, which is the ocean conveyor belt that brings warm water northward.

Fresh water is less dense than salt water, so the influx may have prevented the normal sinking of dense, salty water in the North Atlantic that drives this circulation. Without this heat distribution system, the Northern Hemisphere cooled dramatically.

There are different theories on the routing of this meltwater. Some evidence suggests it flowed through the St. Lawrence River into the North Atlantic, while other data points to routing through the Mackenzie River into the Arctic Ocean, or the Mississippi River into the Gulf of Mexico.

All of these are physically plausible and consistent with the pattern of strongest cooling occurring around the North Atlantic, with smaller changes in the tropics and Southern Hemisphere.

A second, less popular theory, at least amongst professional geologists, is known as the Cosmic Impact Hypothesis.

Proposed in 2007, this controversial theory suggests that a comet or asteroid impact or airburst over North America triggered the Younger Dryas.

Proponents argue that a disintegrating comet or meteor airburst ignited widespread biomass burning, darkened ice, and perturbed atmospheric chemistry, which then caused rapid cooling and perhaps helped destabilize ice margins to release meltwater.

They cite layers with elevated nanodiamonds, magnetic spherules, shocked minerals, and sometimes a carbon-rich “black mat” at sites in North America and elsewhere.

Critics counter that these signals are patchy, that some claimed impact markers are produced by ordinary terrestrial processes, and that there is no confirmed crater of the right age and scale.

Proponents reply by noting that if there was an airburst, like in the Tunguska Event, which I covered in a previous episode, there wouldn’t be a crater, and there also wouldn’t be a crater if it hit the ice sheet itself, which would also have caused a catastrophic release of freshwater into the ocean.

While Meltwater Pulse and Cosmic Impact are the two most widely debated theories, they are not the only ones that have been proposed.

Volcanic forcing has also been proposed, particularly the large Laacher See eruption in the Eifel region of Western Germany, which occurred very close in time to the Younger Dryas.

A major volcanic eruption injects sulfate aerosols into the stratosphere, reflecting sunlight and cooling the surface for a few years. This happened after the Mount Pinatubo eruption in the Philippines in 1991.

Advocates suggest that such a short-lived cooling, superimposed on a climate system already primed by meltwater, could have nudged the Atlantic circulation into a weaker mode that persisted for a millennium.

The main objections are that volcanic aerosol effects are too brief to explain a 1,200-year event on their own. Additionally, precise dating places the eruption slightly before the full onset of the Younger Dryas, which weakens a direct causal link.

A fourth line of thinking treats the Younger Dryas as an internally generated oscillation of the ocean–ice–atmosphere system that was tempered by slow orbital changes but not directly caused by them.

In this view, the North Atlantic has multiple stable circulation states. As ice sheets retreated and freshwater flows varied, the system crossed thresholds that flipped it into a cold, sea-ice-rich mode without requiring a single external shock.

Once in that mode, strong sea ice and environmental feedbacks reinforced the cold, and only later did the background warming and changing freshwater balance allow a rapid return to the warmer conditions.

There is no single smoking gun that commands universal agreement on the cause of the Younger Dryas. The agreement of many records on an abrupt surge in freshwater in the North Atlantic and a weakened ocean circulation makes meltwater forcing the most widely accepted driver, with debate focused on the exact routing and whether the freshwater release was catastrophic or more gradual.

Regardless of how it happened, the Younger Dryas, or more specifically, the end of the Younger Dryas, is one of the most important events in the history of humanity.

This abrupt warming and the end of the ice age ushered in a host of changes, which included agriculture, animal domestication, and monument construction.

All of which were the first steps in the very long journey in the creation of human civilization.