Understanding past climates requires data extending beyond modern instrumental records. This study shows that blue intensity (BI) measurements from European larch trees in the Southern Rhaetian Alps provide a stronger proxy for reconstructing past summer temperatures than traditional tree-ring-width data. BI processing enables regional-scale reconstructions and helps extend these reconstructions to the Mediterranean Basin and northern Europe, with excellent correlations to existing data.

During the Last Deglaciation, global surface temperature rose by about 4–7 °C over several millennia. We show that changes in year-to-year up to century-to-century fluctuations of temperature and precipitation during the Deglaciation were mostly larger than during either the preceding or succeeding more stable periods in 15 climate model simulations. The analysis demonstrates how ice sheets, meltwater, and volcanism influence simulated variability to inform future simulation protocols.