It does seem a bit of a stretch to suggest that two eruptions in 536 and 540 (actually three, but I dont think the 547 eruption was that big in my opinion), could cause 125 years of global cooling. Certainly the effects of the 540 eruption are much greater than 536, but yet it has seemingly escaped the historical record, while the 536 effects are well recorded (the 18 month dust veil for example).

The authors though suggest that it was a combination of effects, namely the eruptions which triggered a decade of extreme cooling, accompanied by weak solar activity and with ocean and sea ice-feedback mechanisms sustaining the cooling. I havent looked at the data in great detail, but it does appear to me that cooling had commenced before AD 536, but it was these two large eruptions in close proximity that caused a sort of "climatic phase change".

Strangely enough, I have seen a similar effect for the 4.2 kya event, where the 4.2 kya event seems to have an abrupt trigger in 2206 BC, though due to dating of the ice cores in this epoch, there is no clear signal of any volcanic eruption. In case you are wondering, this was published in conference proceedings in October but there is a six month embargo on it appearing digitally (mark the end of April in your diary ).

The reference is Baillie and McAneney 2015 "Why we shouldn’t ignore the mid-24th century BC when discussing the 2200-2000 BC climate anomaly" in 2200 BC : a climatic breakdown as a cause for the collapse of the old world? : 7th Archaeological Conference of Central Germany, October 23-26, 2014 in Halle (Saale), if you are interested. Alas, we were not at the conference, so it doesnt appear in the abstract list.

Jonny

The path to good scholarship is paved with imagined patterns. - David M Raup