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

Abstract

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.

Climate-related changes in peatland carbon accumulation during the last millennium

“…We explored the potential reduction of PAR by late-lying snow cover by calculating the PAR for days without snow and with temperature > 0 • C. The correlation between snow-free PAR0 and total carbon accumulated over the last 1000 yr was not significant (p = 0.23), so cloudiness is the more likely cause of change in PAR and carbon accumulation. The idea that the Northern Hemisphere LIA was characterised by greater summer cloudiness is consistent with historical documentary data (Grove, 2004), although recent findings from δ 13 C in Fennoscandian tree ring studies suggest that there may have been regional differentiation of this tendency (Young et al, 2010;Gagen et al, 2011).…”

Bayesian multiproxy temperature reconstruction with black spruce ring widths and stable isotopes from the northern Quebec taiga

“…Gagen et al (2011) have also found high correlations of their δ 13 C series with cloud cover and sunshine and with summer temperature. Young et al (2010) showed that temperature and cloud cover may be in phase (positively correlated) or in opposition (negatively correlated), which explained the divergence between the δ 13 C series and temperature over the reconstructed 500-years period of their study. In northeastern Canada, we do not exclude the possibility that δ 13 C series could, additionally to temperature, be correlated to sunshine or cloud cover over the last millennium.…”

Arctic hydroclimate variability during the last 2000 years – current understanding and research challenges

“…However, because moisture is rarely the dominant tree-growth limiting factor across much of the Arctic region, there 10 is a limitation of the hydroclimate information that can be reconstructed using the isotopic approach. Using a multiparameter approach several studies (Loader et al, 2013;Young et al, 2010;2012;Gagen et al, 2011) provided sunshine/cloud estimates and were able to demonstrate large-scale shifts in the dominance of Arctic and Maritime air masses over the Northern Fennoscandian region during the Little Ice Age and Mediaeval period. Such multi-parameter studies are potentially very powerful as they help to develop testable hypotheses relating to the future response of the Arctic atmosphere 15 and provide a foundation for developing a circum-polar isotope network to track changes in atmospheric circulation and its relationship to climate throughout the Common Era.…”

“…Waterhouse et al, 2000;Holzkämper et al, 2008;2012;Sidorova et al, 2008;Porter et al, 2009). The combination of long-lived trees, robust dendrochronologies and excellent sample preservation both on land and in lakes have facilitated the development of several multi-centennial to millennial length isotopic records (Boettger et al, 2003;Kremenetski et al, 2004;Sidorova et al, 2008;Young et al, 2010;Gagen et al, 2011;Loader et al, 2013;Porter et al, 2014). However, because moisture is rarely the dominant tree-growth limiting factor across much of the Arctic region, there 10 is a limitation of the hydroclimate information that can be reconstructed using the isotopic approach.…”