Younger Dryas

MARKING THE BOUNDARY between the Holocene and Pleistocene epochs, the Younger Dryas, a period of glacial conditions between 12,900 and 11,500 years ago, is named for Dryas octopetala, a flower that is adapted to the cold. Dryas pollen is found in abundance in strata of this age. Dryas pollen is also found in older strata, necessitating the term Younger Dryas to distinguish this time from older periods in which Dryas pollen is abundant. Locked in an ice age, earth had finally warmed and the glaciers had begun to retreat 15,000 years ago. Counteracting this warming trend, the Younger Dryas reduced temperatures 50 degrees F in only a decade. Glaciers once more advanced in North America and Europe. Rainfall diminished, and frigid winds carried dust from central Asia throughout Europe.

Climatologists have identified three causes of the Younger Dryas, though it is uncertain whether all three operated at the same time. The fact that the Southern Hemisphere cooled before the Northern Hemisphere suggests that some mechanism cooled the south, whereas no mechanism was then operating in the north. The rapid change in climate that was the Younger Dryas may have caused the extinction of large mammals in North America and the collapse of the first Native American culture. In western Asia, the Younger Dryas may have prompted humans to invent agriculture. The end of the Younger Dryas ushered in the modern climate.

The Younger Dryas was part of the Cenozoic Ice Age, which locked the world in glaciers 100,000 years ago. The climate was particularly cold as recently as 18,000 years ago. From these frigid conditions, the climate gradually warmed until 15,000 years ago, the glaciers began to retreat. The Younger Dryas interrupted this warming trend, restoring glacial conditions to earth.

Climatologists have advanced three causes of the Younger Dryas. The leading explanation focuses on ocean currents. The Gulf Stream brings warm water from the tropics to the North Atlantic Ocean, warming the Atlantic coasts of North America and Europe. The Gulf Stream remained undisturbed as the North American glacier began to retreat north 15,000 years ago. In the initial centuries of retreat, the ice sheet emptied its water down the Mississippi River and into the Gulf of Mexico. By 12,900 years ago, however, the North American glacier had retreated to the Great Lakes. Melted water no longer flowed south down the Mississippi River but now went east along the St. Lawrence River to the Atlantic Ocean. This cold water shut down the Gulf Stream, robbing North America and Europe of its warmth and returning the climate to glacial conditions.

Climatologists have identified a second cause in the impact of an asteroid near the Great Lakes 12,900 years ago. Upon impact, the asteroid ejected enormous amounts of debris, dust, and ash into the atmosphere, blocking out sunlight and cooling the Earth. A third cause might have been the sudden, and unexplained, cessation of El Nino. Every two to seven years, warm water from the western Pacific Ocean and Indian Ocean flows east, warming the west coasts of South and North America. Without El Nino, these continents cooled, returning them to glacial conditions. Possibly more than one cause initiated the Younger Dryas.

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