Gregg Easterbrook, A Moment on Earth: The Coming Age of Environmental Optimism, Viking Penguin, 1995, pp. 72-73:

One focus of recent study is a moment from the recent past with an odd name, the Younger Dryas. Toward the end of the latest ice age, around 13,000 years ago, glacial sheets were in retreat from most of North America and Eurasia. Most plant and animal life had rebounded to approximately what it is today, helped by global temperatures thought to have been rising gradualistically for a few thousand years. Then, suddenly, temperatures fell off the cliff. New evidence suggests that air temperature in Europe declined by 11 degrees Fahrenheit in less than a century, causing an essentially instantaneous return to ice-age climate. Oak trees and similar mild-weather vegetation vanished from Europe in fewer than 200 years, replaced by vast fields of dryas, a glacial flower from which the period draws its name.

Thus a nearly global-scale, natural environmental disaster injurious to innumerable species occurred in about the same amount of time, two centuries, as the period of the industrial era—the period current commentators persist in describing as the unprecedented peak moment of environmental stress in Earth history. And this fast, naturally driven disaster happened not in the primordial mists but at a time when our immediate ancestors walked the Earth, behaving much like us in many respects, except that they were far more vulnerable to fluctuations in the environment. This naturally driven environmental disaster also happened at a time when most current plants and animals existed in approximately their present forms.

Surely if oak forests and their attendant life-forms began disappearing from Europe at anything remotely like the speed with which they died during the Younger Dryas, contemporary commentators would proclaim an ecological catastrophe of hyper-mega-ultramagnitude. Yet even the worst-case interpretation of human-induced deforestation does not remotely approach the significance of the Younger Dryas weald wipeout.

What triggered the Younger Dryas? No one knows; the cause is among the keenly debated mysteries of natural science. Cyclical variations in the Earth’s axis and orbit around the sun, presumed to be the principal on/off switch of ice ages, could not have been the explanation, since at the time these variations were tending to warm the Northern Hemisphere. Some oceanographers have supposed that the oceans, which shift heat north toward Europe from the tropics, might hold the key to understanding the Younger Dryas. They may, but in a highly nonlinear way.

In 1992 Scott Lehman, a researcher at the Woods Hole Oceanographic Institute, shocked the climate-study business with a study suggesting that at the moment the mysterious cooling of the [73] Younger Dryas began, sea surface temperatures in the North Atlantic Ocean dropped by nine degrees Fahrenheit in less than 40 years. A climate swing of nine degrees in less than half a century represents roughly three times the fastest rate of temperature change projected by the most pessimistic computer models for an artificial greenhouse effect. It is roughly 20 times the highest claimed observed (actual, as opposed to computer-projected) global temperature change to have occurred during the last century, supposedly the century in which genus Homo has caused unparalleled climate mayhem. Aside from killer rock impacts, a nine-degree temperature change in 40 years may well be the fastest environmental alteration ever to have occurred on Earth. Lehman called this nonlinear effect a “climate collapse.”

Climate collapse. It’s just a descriptive term coined by a researcher, who may or may not be right about what he believes he has found. But what happened to the environment during the Younger Dryas was much worse than any effect so far caused or threatened by human malfeasance. The primary ecology of much of the globe was swept away—we’d say “destroyed forever”—in just two centuries, as temperate forests were replaced by glacial flowers and their accompanying cold-weather lifeforms. Then, in just a few centuries more, the Younger Dryas switched off. The boreal forests of North America and Eurasia were un-destroyed, returning to their previous expanse and magnitude. At this point any environmental commentator of the time would have proclaimed the glacial flower ecology “destroyed forever” and would no doubt have lamented that without the environmentally vital dryas plant, the European ecology could never again be the way nature intended it to be.

Just as the cause of the Younger Dryas is mysterious, the mechanism of ocean-triggered climate collapse is unknown. Lehman speculates that fresh meltwater returning to the seas as the continental ice sheets thawed may have altered global ocean currents in such a way as to slow the movement of warm water normally pumped northward from the tropics. When the ocean “pump” that sends warm waters north shut down, a boreal big chill followed. Eventually, this speculation continues, the ocean balance between salt and fresh water was restored. Warm water once again flowed north from the tropics. The planetary warming in progress before the Younger Dryas resumed its course.

Further reading: The Younger Dryas Event and the Prehistoric Period Extinction