Revisiting the Younger Dryas

Unlike changes in global temperature (such as modern day global warming) which can be understood as a result of perturbations to the planetary energy balance, the millennial-scale climate changes during the last glaciation are viewed primarily from the lens of internal dynamics, including ice retreat and re-organizations of ocean circulation. They are not dominated by changes in global mean temperature but rather changes in temperature distribution, explained by changes in oceanic or atmospheric heat transport. In particular, proxies of deepwater formation show large reductions in the Atlantic meridional overturning circulation (AMOC) coincident with the start of the YD. This suggested weakening of overturning circulation provides immense explanatory power for the onset of the YD although no consensus has emerged concerning the trigger of the AMOC reduction. There are some radiative changes associated with millennial-scale climate change induced by the ice-albedo effect, extra dust loading out of Asia during cold snaps, as well as greenhouse gas feedbacks– although they are relatively small. However, pinning down the exact sequence of causes and effects is rather difficult since precise chronologies and global-scale reconstructions are difficult to come by prior to the Holocene.

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.

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