Correspondence of δ18O record and modelled Greenland-Iceland-Norway seas sea ice fraction. (a) Severity of ice off North Iceland coast calculated from documentary evidence of the duration of ice (total weeks) multiplied by the extent along the coast27. (b) Area-averaged PMIP3 multi-model mean Greenland-Iceland-Norward seas sea ice fraction anomaly (blue) and bivalve Arctica islandica δ18O record (red), both timeseries are linearly detrended and sea ice fraction is normalised by its standard deviation. Between 1200 CE and 1600 CE the model sea ice timeseries is inverted (multiplied by −1). Note that following convention, the bivalve δ18O axis is inverted so that warming is represented by a positive change. Lower Panels: Correlation coefficients between the sea ice and δ18O timeseries for the intervals 950–1200 CE (c), 1200–1600 CE (d) and 1600–1849 CE (e) presented across a range of smoothing windows (see Methods). Stippling highlights significance (see Methods). Correlation coefficients are presented with smoothing windows of up to 20 years away from the 1:1 line to accommodate smoothing already introduced by calculating the multi-model mean. Maximum positive or negative correlations are −0.58, +0.58 and −0.34 for the intervals shown in panels c, d and e respectively, leading to a maximum variance explained in any phase (R2) of 34%.