I just changed my profile Twitter avatar for the first time in forever, so, enjoy me barely able to contain my excitement at holding a piece of the Greenland ice core that records the Younger Dryas, a famous abrupt cold event that started 12,900 years ago!

Layers in the ice core showed that climates cooled 4 to 10° C (7.2 to 18° F) in the northern hemisphere, in just a few decades. This showed definitively that Earth's climate could change really fast -- well within the lifespan of humans and other species.

Scientists had long recognized that as Earth was warming of the last glaciation, the climate took an abrupt turn back to cold conditions. In fact, the name "Dryas" comes from this lovely Arctic plant, because its pollen started showing up in sediment cores as the climate cooled.

(Yep, there's an Older Dryas, and even an Oldest Dryas!) What the ice core did, with its annual layers, was show just how FAST this cold event started. It also ended almost as quickly 11,700 years ago, with the onset of our current warm period.

The rate of warming we saw with the onset of the Holocene, our current interglacial, is the same as climate models predict in the next century. While the starting point was colder, it can be a good analog for studying species response to rapid climate change.

By now, you might be asking: why did the Younger Dryas happen? It's kind of a mystery. One of the prevailing hypotheses is that as the Laurentide ice sheet covering North America melted, fresh, cold water slowed or stopped the warm ocean currents that bring heat from the tropics.

This warm current is why London has a warmer climate than, for example, Maine, even though they're further north.

You may also recognize the ocean current hypothesis from the plot of The Day After Tomorrow, which was inspired by the Younger Dryas!

One issue with the "conveyor belt" hypothesis is that geologists have struggled to identify where the meltwater came into. Was it the Labrador Sea? The Mackenzie River? Where was the evidence? Arguments about this are seriously some of the most heated I've ever seen.

There have been some recent studies showing evidence for the Mackenzie River or the Beaufort Sea, which support the conveyor belt hypothesis. Others think the answer may lie in the atmosphere, where the position and strength of air currents also shape our climate.

Another hypothesis, suggested about a decade ago, posits that a bolide or comet impact caused the ice sheet to release meltwater, triggering climate change, extinctions, and other upheavals. But this has been extremely controversial, and is not widely accepted.

The discovery of the Hiawatha Crater under the Greenland Ice Sheet caused some to speculate that this might be the source, but the crater remains undated, and the ecological evidence on land doesn't support the idea that the Younger Dryas coincided with extinction or wildfires.

In any case, the Younger Dryas remains one of our best-studied examples of how the Earth's climate can change quickly. Its discovery launched widespread interest in understanding abrupt climate change. And it may hold clues for how to survive the coming decades.