ABSTRACT
To assess the impact for any given region of the climate downturn from c. 1275 that trend must first be viewed hemispherically. Acidity measurements from core ice from Crête in central Greenland reveal a volcanic eruption at an unknown location in 1257/8 which generated four times as much acidity as did Krakatoa in 1883. That event excepted, a sharp contrast can be drawn between very low levels of vulcanism, 1110 to 1265, and the highish incidence from then to 1500. Moreover, this second phase is characterised by clusterings on decadal timescales – notably, for our immediate purposes, 1285 to 1300, 1310 to 1317, and 1340 through 1350.194
Obviously, vulcanism can affect inter alia the spread of pack ice. But so, too, can other factors, among them internal oceanic convolutions. Christian Pfister et al. believe that how the colder winters were distributed in the fourteenth century does suggest that North Atlantic deep water could have been a forcing agent.195 That argument lends added interest to the spread of sea ice as an indicator, but also as a determinant, of wider change. Unfortunately the evidence for the critical Icelandic sector is piecemeal for the period 1250 to 1350, even when supplemented by data about Iceland itself. The general inference drawn by Astrid Ogilvie is as follows. The last two decades of the thirteenth century were severe in the winter half of most years but the first couple of the fourteenth were tolerably mild, 1313 excepted. Then 1320, 1321 and 1323 had severe weather with sea ice. After that, only 1331 and 1333 were as severe. In short, the accent was still on erraticism more than on a downward secular trend. Not until 1350 does the weather turn more consistently severe.196
