Revisiting the Younger Dryas

A new study though (Shakun and Carlson, 2010) has compiled over 100 high-resolution proxy records to characterize the timing and extent of the Last Glacial Maximum (LGM) and the deglacial evolution into the Holocene, including the shorter-lived Younger Dryas. Several of the key features of the study include:

  1. The global mean cooling of the LGM relative to the peak of our current interglacial is approximately 5ºC as a minimum value. It is likely larger than this since many of the records are from the ocean which are typically less sensitive to temperature change than landmasses, and further, adiabatic cooling of marine air advected over land masses would result from the ~120 m reduction in sea level. The cooling is global in scale and largest at high latitudes, as expected from polar amplification.
  2. In contrast, during the YD, there is much more spatial heterogeneity as the North became colder and drier (increasing with latitude) while the South became warmer and wetter in the opposite sense. The global mean cooling during the YD is only ~0.6ºC .  The tropics cooled by 2.5ºC (with an error of about a degree in either direction) at the LGM, yet exhibited very little temperature change during the YD. Thus, while the YD was a global scale climate change event with widespread signatures, it was not a widespread global cooling event. Fig. 2. Magnitude of the glacial-interglacial temperature change relative to absolute latitude. (Shakun and Carlson 2010)Fig. 3. Magnitude of the Younger Dryas temperature change. Map of the Younger Dryas temperature anomaly (a). Circle denotes the size of the temperature change. Blue is cooling, red warming (Shakun and Carlson 2010).
  3. The timing of the LGM and peak interglacial is synchronized between hemispheres on orbital timescales, which the authors attribute primarily to the global radiative forcing provided by CO2. As has been noted in the past, the CO2 lags the onset of deglaciation in most records, as this is paced by summer insolation changes. However the CO2 still acts as the dominant temperature-change influence throughout the deglacial period and provides an effective means to communicate temperature anomalies to the tropics. On the other hand, the YD exhibits the well-known bipolar see-saw effect which involves a reduction in northward heat transport, which warms the South. The see-saw is best expressed in the mid to high latitudes, although the see-saw model is a poor descriptor for the tropical variability.

The see-saw effect during millennial-scale climate changes has been confirmed before (also discussed at RealClimate in the context of the somewhat similar Dansgaard-Oeshger events) and is consistent with modeling efforts of the climate evolution during the last deglaciation, including Liu et al., 2009 (discussed here) who show that current state-of-the-art models can simulate the magnitude of abrupt climate changes well.

So what caused the reduction in the AMOC?

The most prevalent concept for slowing AMOC involves a reduction in the surface water density at the ocean surface via adding freshwater into the ocean. The preferred location is primarily the North Atlantic, which is a key point for deep ocean convection. The original idea for this to cause a YD-event was proposed in 1976 by Johnson and McClure, and involved the opening of eastern Lake Agassiz outlet via northward retreat of the Laurentide Ice Sheet out of Lake Superior. This re-routed drainage from the Mississippi to the St. Lawrence River.

There is a difference between the diversion of continental runoff from the Mississippi River (routing) and the relatively fast pro-glacial lake drainage to a new level (flooding). In contrast to the Johnson and McClure paper, many recent studies have focused on short-lived floods, although the re-routing mechanism might be a necessary, and in fact primary ingredient (Carlson et al., 2007; Carlson and Clark, 2008) in accord with modeling studies which require a persistent forcing to substantially alter AMOC (Meissner and Clark, 2006).

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