Greenland Warming at Last Deglaciation: the Younger-Dryas Not So Cold

by Emil Morhardt

I’ve been blogging recently about papers that claim the thousand-year cessation of global warming in the midst of the last deglaciation—known as the Younger-Dryas (Y-D)—was triggered by a comet. Buizert et al.’s (2014) paper on Y-D temperature changes doesn’t address the comet question, but another equally interesting one: why did the sudden reversal of temperature 12,800 years ago (whatever it was triggered by) cause the temperature to plunge clear back to what it was before any warming had started? That’s what the relative deuterium and oxygen-18 concentrations from the Greenland Ice Sheet ice cores imply—more about that in a moment. Nevertheless, it seemed unlikely because at the time of the Y-D, a considerable amount of CO2 had accumulated in the atmosphere and Antarctica was warming apace. The answer, according to this paper is that temperatures did not cool down so much after all; things cooled off for sure, and warming was delayed for another thousand years, but at the depth of the Y-D cooling most of Greenland was on the order of 4˚C warmer than it had been 4,000–5,000 years before—but still quite cold. Continue reading

Comets, Climate Change, and Extinctions—2

 

by Emil Morhardt

When a large meteorite struck the earth 65 million years ago, it killed off the dinosaurs by abrupt climate change; the energy of the strike sharply raised global temperatures, ignited massive wildfires, and filled the atmosphere both with smoke from the fires and dust ejected from the crater which presumably prevented plants from thriving for a long enough time to starve all but the smallest animals (allowing, as it happened, the evolution of humans.) That’s a different cause of climate change than now, and most of us, if we worry about climate change at all, don’t much worry about it being caused by another meteorite strike. But, on August 16, I wrote about a paper published in 2007 that proposed a similar, though not so severe, extraterrestrial impact and abrupt climate change about 12,800 years ago—the initiation of the Younger Dryas (YD) cooling episode that stopped the Continue reading

Comets, Climate Change, and Extinctions—1

by Emil Morhardt

At the end of the last ice age as the Earth was warming to its present condition there was an unexplained 1000-year pause and partial reversal in the warming (called the Younger-Dryas stadial). The result was a millennium of very cold weather in the Northern Hemisphere. The cause was widely attributed to the abrupt stoppage of the Gulf Stream; warm water was no longer transported from the equator north past the US east coast and Europe toward Greenland. The physical cause of the stoppage was presumably the melting of the Laurentide Ice Sheet covering Canada; enough freshwater flowed out over the North Atlantic near Greenland, that it formed a thick layer on top of the ocean that was not dense enough to sink through the underlying salt water. It is sinking saltwater off Greenland that drives the major global ocean currents—the Meridional Overturning Circulation (MOC)—of which the Gulf Stream is the last leg. Scientists are somewhat worried that under the current warming conditions, enough meltwater could flow off the Greenland Ice sheet to wreak the same sort of havoc…a much colder North America and Europe in the midst of a generally warming globe. In 2007, Firestone et al. presented an unexpected theory that the trigger for the freshwater outflow 12,900 years ago was an extraterrestrial (ET) impact event—a comet or meteorite—that also directly led to the Continue reading