PCA ordination of temperature data from our field sites. This figure shows axes 1 and 2, which together represent 74.5% of the total variation. Ovals indicate variation in each region (turquoise, Iceland (circles, Ice_1; filled squares, Ice_2; uptriangles, Ice_3; diamonds, Ice_4; asterisks, Ice_5; pluses, Ice_6; open squares, Ice_7; crosses, Ice_8); light green, Ireland (circles, Ire_1; diamonds, Ire_2); red, Norway (circles, Nor_1; filled squares, Nor_2; triangles, Nor_3; diamonds, Nor_4; asterisks, Nor_5; pluses, Nor_6; open squares, Nor_7; crosses, Nor_8); dark blue, Scotland (circles, Sco_1; uptriangles, Sco_2; downtriangles, Sco_3; diamonds, Sco_4; asterisks, Sco_5; pluses, Sco_6; squares, Sco_7; crosses, Sco_8); orange, Sweden (circles, Swe_1; filled squares, Swe_2; uptriangles, Swe_3; diamonds, Swe_4; asterisks, Swe_5; pluses, Swe_6; open squares, Swe_7; crosses, Swe_8); dark green, Wales (circles, Wal_1; squares, Wal_2)). Details are given in the electronic supplementary materials.