Is There a Future Tipping Point in the Atlantic Meridional Thermohaline Circulation (AMOC)?

by Emil Morhardt

The AMOC is the set of ocean currents that begins with cold seawater off Greenland sinking to the bottom and flowing south, being replaced by warmer water flowing at the surface past Florida from the south, transferring warmth from the tropics to the east cost of North America and the West Coast of Europe. The full round trip cycle from, and back to, Greenland takes a thousand years. Without it, both western Europe and eastern North America would cool significantly, with large numbers of potential adverse effects. We know from ice core record temperature data from both the Greenland Ice Sheet (GrIS) and from Antarctic ice sheets, that the AMOC has come to an abrupt halt many times, and has characteristically taken a millennium to recover. The comparative abruptness of these cessations has led to the fear that there may be some threshold that once crossed—a tipping point—that cessation is inevitable. This would be nice to avoid. Continue reading

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