THE END OF THE LAST ICE AGE

RETREATING ICE AND RISING SEA LEVELS

By 20,000 years ago, thanks to slow global warming, the great ice sheets covering northern Siberia, half of Europe, and a third of North America had begun to melt back. Vegetation crept northward into the soggy landscapes that were now free of ice. Wildlife and people followed. Almost everywhere, climate started slowly to improve from the human point of view. In Africa and South Asia, where aridity more than cold had constrained human endeavor during the Ice Age, conditions grew wetter as well as warmer. Deserts, like the northern ice sheets, retreated, opening new land for plants, animals, and humans. The changes took place too slowly for anyone to have noticed but were large enough to help human population to grow a little faster during the final stages of the Paleolithic.

Some ground was lost while climate warmed. Sea level rose because of melting glaciers, which drowned coastal plains and pushed people inland, if only by a few inches every year. Indonesia, Britain, and Japan eventually became the archipelagoes we know today, cut off from the Eurasian landmass. Rising seas separated Taiwan from mainland China, Tasmania and New Guinea from Australia, and Newfoundland from North America.

Usually, the rising seas claimed low-lying lands slowly; but whenever rising waters overtopped a ledge or broke through an ice-dam, epic floods resulted. According to one theory, the Black Sea, formerly an immense inland lake, filled up with Mediterranean seawater splashing over the sills of the Dardanelles and Bosporus with the force of 200 Niagara Falls. In the span of a single year (perhaps around 7400 BCE), the former shores of the inland lake were flooded and a larger, saltwater Black Sea was born. It’s possible that the great flood sagas, such as those in the Epic of Gilgamesh (the first known story, written in Mesopotamia about 2100 BCE) or in the Bible’s Book of Genesis, derive from folk memories of such events. But scholars don’t know: the archeological evidence isn’t conclusive, and the flood stories may have nothing at all to do with the forming of the Black Sea.

As of about 6000 BCE, the melting was almost over and sea level was close to where it is now. Over 15,000 years, it had risen by 430 ft. (130 m)—about the height of a 25-story building. With these environmental changes, the human population expanded, spreading out into formerly ice-covered northern (and mountain) zones and into former deserts, even as people had to abandon former coastlines and retreat inland. Far more land had been gained than lost thanks to the defrosting.

THE YOUNGER DRYAS

The Ice Age had one last shiver in it. Between 12,900 and 11,700 years ago (10,900 to 9700 BCE), another brutally cold and dry spell hit the Northern Hemisphere. It is called the Younger Dryas, named for a tundra flower that turns up often in sediments from that time. It probably resulted from a massive outflow of fresh water from an ancient North American lake (Lake Agassiz, centered on what is now the Canadian province of Manitoba and larger than all the Great Lakes put together) that had formed from glacial meltwater during the warming. When those waters broke through into the North Atlantic, they suddenly made it less salty, changing the Atlantic Ocean’s circulation pattern and plunging the Northern Hemisphere back into the deep freeze for more than a thousand years.

The Younger Dryas seems to have been coldest around the North Atlantic, in Europe especially. In Europe, the summers were 9 to 13 degrees Fahrenheit (5 to 7 degrees Celsius) cooler, and in winter more like 18 to 21 degrees F (10 to 12 degrees C) colder—the difference between sweatshirt weather and parka weather. The change apparently came on suddenly, within a decade or two, which is unusual in the annals of climate change, and made adjustment especially challenging. As during ice ages, in the Younger Dryas conditions were dry throughout the Northern Hemisphere as well as colder. In the tropics, so far as anyone can tell, the Younger Dryas was mainly dryer. The Younger Dryas came as a catastrophe for people, animals, and plants that had been getting used to warmer and wetter conditions.

EMERGENCE OF A MORE STABLE CLIMATE

But when the Younger Dryas ended, the forecast was good: the warmth and wet returned, and more important still, the chaotic flickering so characteristic of the Ice Age climate came to an end—or at least a long interlude. From the end of the Younger Dryas until now, a roughly 11,700-year period known as the Holocene (from ancient Greek meaning “wholly recent”), climate has been remarkably stable. We of course regard our weather as unstable and unpredictable, and over short time spans it is just that. But Holocene climate fluctuates much less than climate did during the last ice age. In particular, the probability of extremely cold winters far outside the statistical norm was significantly reduced after the Younger Dryas. This relative stability of climate made it much easier for people to settle down in one place and rely on the plants that grew there. When and where climate fluctuated chaotically, people had to stay on the move. With the stability of Holocene climate, they could become sedentary with less risk.