Climate Number: 18 Degrees Fahrenheit
Earth 13,000 years ago was in the process of thawing from the coldest part of the last ice age. Then, something sudden and catastrophic happened: within a few decades, northern Europe’s average temperature dropped by 18 degrees Fahrenheit. The sudden cold period that followed is called the Younger Dryas, named after the Arctic tundra wildflower that expanded south across Scandinavia at that time. While this cold period was a global event, its most pronounced effects were experienced in northern Europe. Today, the thermohaline circulation in the North Atlantic is driven by sinking cold and salty waters off the coast of Greenland. As these waters sink, they draw up warm waters from the south. The warm waters bring warm air masses to Europe, keeping the continent warmer than it would otherwise be at its relatively high latitude. As the planet was thawing 13,000 years ago, North America’s Laurentide Ice Sheet was melting. This melt allowed previously ice dammed glacial lakes, such as the now dry Lake Agassiz located in modern day Manitoba, to suddenly drain. Shortly after 13,000 years ago, the ice dam that was keeping Lake Agassiz from draining overland into the Arctic Ocean melted, releasing about 2,300 cubic miles of water (almost 24 years worth of discharge from the Mississppi River!) in less than a year. This drainage upset the balance between salt and fresh water that keeps the sinking in the North Atlantic going. As the sinking suddenly stopped, so did the flow of warm waters and air masses to Europe. The interruption of this flow caused the sudden 18 degree Fahrenheit drop in the northern Europe’s temperature.
For comparison: A sudden 18 degree drop in average temperature is equivalent to the climate of Memphis, Tennessee suddenly becoming like the climate of Chicago, Illinois.
Seasons: Winter, Spring, Summer, Fall
Sources: Murton, JB et al. “Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean.” Nature 464 (2010): 740-743 and “River reveals chilling tracks of ancient flood.” Nature 464 (2010): 657.
