Slow growing juniper shrubs from northern Iceland record summer temperature changes over the last 800 years

Iceland's unique environment is particularly sensitive to the effects of anthropogenic warming. Contextualising recent trends and extremes against past ranges is, however, limited by a lack of high-resolution temperature reconstructions. Here, we present ring widths measurements and wood anatomical observations from 68 living and relict juniper shrubs from northern Iceland. The preservation of these junipers is linked to a complex of favourable abiotic and anthropogenic factors. The combined dendrochronological and wood anatomical assessment of Iceland's oldest living organisms resulted in a continuous chronology back to 1159 CE, which is the longest of its kind for the Arctic. Our new summer temperature reconstruction provides evidence for the late Medieval Warm Period (circa 1260s–1370s) and various Little Ice Age Type Events (between circa 1380s and 1810s). The recent warming over northern Iceland is comparable to pre-industrial warm phases during the first half of the 18th century and the 1550s–1590s. Our wood anatomical investigation reveals reductions in cell wall lignification (i.e. Blue Rings) following volcanic eruptions, such as 1597, 1755, 1783, 1815, 1823, 1829, 1882–83, 1906.