Eightpetal mountain-avens

In the geological record, the change looks abrupt, a sharp turn downward, a return to colder conditions that lasted for roughly one thousand two hundred years, ending around eleven thousand seven hundred years ago.

Scientists call this interval the Younger Dryas, named after Dryas octopetala, a cold-loving Arctic wildflower whose pollen becomes more common in sediments from this period.

It was a tiny botanical signal pointing to an enormous transformation.

The first really dramatic evidence came from Greenland.