Younger Dry-as dust?

The Younger Dryas is so called because it corresponds, in the pollen record from Europe, to the latest (i.e. youngest) appearance of the Dryas octopetala pollen, an alpine flower in regions that are now far from alpine. It marks a clear period towards the end of the last ice age when the warming trend of the deglaciation in Europe particularly was interrupted for a period of about 1300 years before it got going again. There were clear glacier advances during this time and the moraines can be seen very clearly all around Europe and Scandinavia.

The clues to what caused this remarkable, if temporary, turnaround have always lain in assessing its spatial extent, the exact timing and correspondence with other events. Two recent papers have shed some welcome and potentially controversial light on the subject.

To appreciate those papers though, you need a little background. It is clear from the Greenland ice cores that the Younger Dryas was a huge event in that region – 10 to 15ºC cooling at Summit – and this is confirmed by studies of ocean sediments in the North Atlantic which also show large temperature drops (a few degrees) over this period. Particularly clear records of climate impacts are seen off Portugal and as far south as the Cariaco Basin off Venezuela. New evidence from proxy circulation tracers suggest that the North Atlantic overturning decreased significantly during the YD, possibly shutting down completely. This has all lent support to the theory, first suggested over a decade ago, that glacial meltwater interfered with the N. Atl. circulation causing an interruption of the ocean heat flow to the North. This is of course the prototype of the “ocean circulation changes imply a new ice age” meme which has been so hard to get rid of in recent years.

But how far afield did this climate change reach? The event is been clearly seen in sediments off Santa Barbara (California) and in cave records from China, but both of these areas are still in the Northern Hemisphere, and exactly what is recorded (wind speed change and precipitation amounts?) is still a little ambiguous. But what about the south?

The initial results from Antarctic ice cores at first seemed to show something very similar – the long warming through the deglaciation was interrupted by a cold reversal half way along. The relative dating was not very good at that time and it was quite plausible that the two events in Greenland and Antarctica were one and the same. When glacial advances in New Zealand were found to be around the same time, it seemed clear that the YD cooling had extended the entire length of the Atlantic! The only problem was that the favored mechanism, an ocean circulation change, no longer matched the data. Models of these shutdowns found it very hard (actually impossible) to get a cooling in the North and South at the same time. Lots of other ideas were suggested, but none that were really convincing. So scientists went on thinking that it probably was the ocean, but always with a bit of unease about the southern hemisphere results or the models.

Clarity started to emerge when new techniques for lining up the ice cores in Antarctica and Greenland were developed. One technique used the very rapid changes in methane (which could be measured in both poles) to synchronise the chronologies. The thought being that methane changes are well mixed and so large changes in one hemisphere get transmitted very quickly to the other. With this came a big surprise – the Antarctic Cold Reversal started hundreds of years before the Younger Dryas! In fact, Antarctica stopped cooling just as the YD was getting started. This was evidence of a bi-polar see-saw in the ocean – something the models did seem happy to show.

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