A 500-year record of summer near-ground solar radiation from tree-ring stable carbon isotopes

Tree-ring stable carbon isotope ratios (d¹³C) in environments of low moisture stress are likely to be controlled primarily by photosynthetic rate. Therefore, sunshine, rather than temperature, represents the more direct controlling factor. Temperature reconstructions based on tree-ring d¹³C results thus rest on the assumption that temperature and sunshine are strongly coupled. This assumption is tested using a d¹³C series from pine trees in NW Norway, where there are long (>100 yr) records of both summer temperature and cloud cover. It is demonstrated that when summer temperature and d¹³C diverge, summer temperature and cloud cover also diverge, and that cloud cover/sunshine may provide a stronger and more consistent parameter with which to calibrate tree-ring d¹³C series in this area. When a 500-year reconstruction of summer cloudiness is compared with a published reconstruction of summer temperatures in northern Sweden based on tree-ring maximum densities, the two time-series are largely parallel, with high levels of annual—decadal coherence. We identify, however, three distinct periods of lower frequency divergence: two (AD 1600—1650 and AD 1900—1927) when we propose summers were cool but sunny and one during the first half of the sixteenth century when summers were warm but cloudy. These episodes where temperature and sunshine decouple may represent large-scale changes in circulation as recorded in the Arctic Oscillation (AO) index. Strongly negative values of the summer AO index, as occurred during the early twentieth century, are associated with persistent high pressure over northern Norway and Fennoscandia, bringing cool summers with clear skies. Long reconstructions of cloudiness (near-ground radiation), based on tree-ring d¹³C series from suitable sites, would be extremely valuable for testing General Circulation Models (GCMs), because the generation of cloud is a strong control on temperature evolution, but remains a major source of uncertainty.