Point to Ponder...Perhaps?

Afternoon All, 

The last few posts have generally accepted that the Younger Dryas (YD) was caused by freshwater outbursts disrupting the thermohaline circulation. A few theories have been proposed, some have been rejected but to date I have not posted about the end of the stadial.  

This post will produce evidence by three studies which suggest that the YD stadial event abruptly ended within 3-50 years. In 1989, a paper reviewing the evidence to support the abrupt termination of the YD was written by Dansgaard et al (1). In this paper, the late glacial and the early Holocene is recorded from the ice core taken by the Greenland Ice Sheet Project in South Greenland. The Younger Dryas is recorded in Delta Oxygen-18 isotope analyses as shown in Figure 1, b (Dansgaard et al). 

Figure 1: Greenland ice core evidence showing δO-18, deuterium excess and dust variation supporting a rapid end to the Younger Dryas

In Figure 1, d-f parts of the image zoom in on the end of the Younger Dryas and start of the Pre-Boreal warmer stage. d shows a sharp increase in delta Oxygen-18 (δO-18) as the over a 50 year period. When an increase in δO-18 is seen in ice cores, compared with the concentrations of standard mean ocean water (SMOW), it is assumed that this precipitation will have been enriched in δO-18 which requires greater heat energy, and thus higher surface and air temperatures, to evaporate it before falling as precipitation. There is also less likelihood that the δO-18 will fall on the ice core as it is heavier. Therefore, a higher proportion of δO-18 will increase the chances of it falling at higher latitudes. 

Part e of Figure 1 shows evidence of a decrease in deuterium excess, defined as δDeuterium - 8* δO-18, over just 20 years. Dansgaard and his colleagues attribute this decrease to the northward movement of the Polar front and sea-ice margin causing greater moisture supply to the ice cores leading to a lower deuterium signal.

Part f provides evidence for a decrease in dust in the ice core over just 20 years. The prevalence of dust in an ice core is correlated with storminess (to transport the dust to the ice core) and dry climatic conditions (to allow for increased dust entrainment). The dust signal is negatively correlated with δO-18 and so cooler conditions give rise to more dust. Such a decrease in dust over just 20 years shows that temperatures increased rapidly over this period.

Looking at the evidence, the three proxies of δO-18 isotope analysis, deuterium excess and dust concentrations all point to higher temperatures over a very rapid time-series. Dansgaard estimates the 5‰ increase in δO-18 over 50 years to be equivalent to a 7°C warming. Another paper by Alley et al (1993) (2) has found evidence for increased ice accumulation over an abrupt period of just 3 years at the onset of the YD's termination. Figure 2 (Alley et al) shows ice accumulation in the Late Glacial into the early Holocene.

Figure 2: Ice accumulation rates in Greenland showing evidence of rapid shifts during abrupt climate changes

The areas of abrupt change are focussed on each showing an abrupt change in ice accumulation as temperatures varied. Alley et al suggest that a near doubling of ice accumulation rates would require a c. 7°C warming in line with Dansgaard's predictions. Ice accumulation rate increases with temperature as more snow is able to fall when temperatures allow for greater air humidity increasing snowfall.

If the previous two articles weren't enough evidence for rapid abrupt climatic change at the end of the YD, the paper by Steffensen et al (2008) which I posted in my second post "Late Glacial and Holocene climate variability" details a deuterium excess shift occurring over just 1-3 years. 

Today's post has provided some ice core evidence to suggest that the Younger Dryas ended very abruptly. It has also shown that abrupt climate shifts can occur in human life expectancy time-scales. Towards the end of this blog, I will be assessing past changes and how they could be realised in the future. Although the YD was a significant event in Earth's recent history, a large increase in freshwater from ice-cap melting could feasibly occur again over an alarmingly rapid time-period, possible within our life-time. Next time, I will wrap up my discussion of the Younger Dryas before moving on to another large abrupt climate shift which occurred 8.2 ka BP.

(1) Dansgaard et al (1989) doi: 10.1038/339532a0
(2) Alley et al (1993) doi: 10.1038/362527a0
(3) Steffensen et al. (2008) doi: 10.0.4.102/science.1157707