Mike Mann on the Younger Dryas – Could it Happen Again?
March 24, 2015
Paleoclimatologist Michael Mann has just released a paper discussing a slowdown in the North Atlantic Current, the Atlantic Meridional Overturning Circulation. If you haven’t seen that video and post yet, go here.
Historically, scientists believe that a complete shutdown of that current happened about 12000 years ago, as the earth was coming out of the last glacial period, plunging a large part of the northern hemisphere back into ice-age like conditions, for most of a thousand years.
The movie “The Day After Tomorrow” was based on an exageratted, dramatized idea of what might happen if global warming caused a similar shutdown in the modern day.
As part of a wide ranging interview on the new paper, I asked Mike Mann to put the Younger Dryas in context of current conditions.
Below, ice core temperature proxies show the Younger Dryas in the paleo record. (NOAA)
The Younger Dryas is one of the most well-known examples of abrupt change. About 14,500 years ago, the Earth’s climate began to shift from a cold glacial world to a warmer interglacial state. Partway through this transition, temperatures in the Northern Hemisphere suddenly returned to near-glacial conditions (Figure 6). This near-glacial period is called the Younger Dryas, named after a flower (Dryas octopetala) that grows in cold conditions and became common in Europe during this time. The end of the Younger Dryas, about 11,500 years ago, was particularly abrupt. In Greenland, temperatures rose 10° C (18° F) in a decade (Figure 6; Cuffey and Clow, 1997).
What caused the Younger Dryas?
The Younger Dryas occurred during the transition from the last glacial period into the present interglacial (the Holocene). During this time, the continental ice sheets were rapidly melting and adding freshwater to the North Atlantic. Figure 6 shows the reconstructed freshwater flux from the melting Laurentide ice sheet through the St. Lawrence River. Just prior to the Younger Dryas, meltwater fluxes into the North Atlantic increased dramatically. In addition, there was probably a short-lived period of particularly high freshwater flux about 13,000 years ago that is not shown in this figure, resulting from a large discharge of freshwater from a glacial lake in North America. Scientists have hypothesized that meltwater floods reduced the salinity and density of the surface ocean in the North Atlantic, causing a reduction in the ocean’s thermohaline circulation and climate changes around the world. Eventually, as the meltwater flux abated, the thermohaline circulation strengthened again and climate recovered.
The record from Dome C in Antarctica supports this explanation. If the thermohaline circulation were to slow, less heat would be transported from the South Atlantic to the North Atlantic (Crowley, 1992; Broecker, 1998). This would cause the South Atlantic to warm and the North Atlantic to cool. This pattern, sometimes called the “bipolar see-saw”, is observable when comparing the GISP2 and Dome C records for the Younger Dryas.
Notice the second period of large freshwater discharge following the Younger Dryas in Figure 6. Interestingly, this discharge did not cause a second major climate change similar to the Younger Dryas. One possible explanation for this is that, after the Younger Dryas, the thermohaline circulation had become more vigorous as the climate finally entered the interglacial. A vigorous thermohaline circulation might be less susceptible to freshwater discharges.