The current lack of information on past summer sunshine duration variability from annually resolved palaeoclimatological archives is hindering progress in the understanding and modelling of the earth climate system. We show that a combination of tree-ring carbon and oxygen isotopes from Siberia provides robust information on summer sunshine duration, which we use for an annual 1505-year reconstruction of July sunshine duration variability (1,5K-SIB-JSDR). We found that the Medieval maximum is 56 % higher than the average over 1505 years. Rapid and drastic decreases in sunshine duration up to 60 % correspond to major stratospheric volcanic eruptions. Grand Solar Minima and total sunspot numbers are also well preserved in the 1,5K-SIB-JSDR. Coherency with a global air temperature composite and spring Arctic Oscillation indicate that a large-scale climate signal is retained in our sunshine reconstruction.