Revisiting the Younger Dryas

Guest Commentary by Chris Colose

One of the most intriguing and well-studied climatic events in the past is the Younger Dryas (YD), a rather abrupt climate change between ~12.9 and 11.6 thousand years ago. As the world was slowly warming and ice was retreating from the last glaciation, the YD effectively halted the transition to today’s relatively warm, interglacial conditions in many parts of the world. This event is associated with cold and dry conditions increasing with latitude in the North, temperature and precipitation influences on tropical and boreal wetlands, Siberian-like winters in much of the North Atlantic, weakening of monsoon intensity, and southward displacement of tropical rainfall patterns. RealClimate has previously discussed the YD (here and here) however there have been a number of developments in recent years which deserve further attention, particularly with respect to the spatial characteristics and causes of the YD.

The YD is often discussed in the same context as the ‘Dansgaard-Oeschger’ events seen in the ice cores during full glacial conditions, and the ‘Heinrich events’ of layers of ice-rafted debris in North Atlantic ocean sediments. Indeed, some people occasionally refer to the YD as Heinrich event 0, but this implies that the YD cooling was caused by an ice-rafting event (probably untrue) and should be avoided.  The YD occurred last of several prominent and abrupt deglacial events including Heinrich Event 1 (~17.5 to 16 ka) which is an event contained within the Older Dryas (18 to 14.7 ka), followed by the Bølling-Allerød warm period (~14.7 to 12.9 ka) whose end then marks the start of the YD. The end of the YD can be said to be the start of the Holocene. It has been proposed that the warmings before and after the YD can be viewed as Dansgaard-Oeschger events with the YD just a regular cold (i.e. stadial) phase in between (Rahmstorf 2002, 2003). In Antarctica (~15 to 13 ka), the most featured event is that as the Younger Dryas begins, warming is occurring in Antarctica.  The cold period in Antarctica that precedes the Younger Dryas is referred to as the Antarctic Cold Reversal (ACR) (see figure, from Shakun and Carlson, 2010) and was once thought to be in phase with the YD.  They are neither directly in phase nor anti-phased with one another (see e.g. Steig and Alley, 2002).

Fig. 1. Deglacial ice core time series and insolation. (a) GISP2 δ18O (black step plot) (Blunier and Brook, 2001). (b) Byrd δ18O (grey step plot) (Blunier and Brook, 2001). (c) Insolation (Incoming Solar Radiation) for 60ºN on June 21 (black line) and for 60ºS on December 21 (dashed black line) (Berger and Loutre, 1991). The timing of the Younger Dryas (YD), Bølling/Allerød (B/A), Heinrich Event 1 (H1), Oldest Dryas (OD) and Antarctic Cold Reversal

(ACR) are denoted.

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.

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