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The increasing carbon dioxide (CO 2 ) concentration in the atmosphere in combination with climatic changes throughout the last century are likely to have had a profound effect on the physiology of trees: altering the carbon and water fluxes passing through the stomatal pores.However, the magnitude and spatial patterns of such changes in natural forests remain highly uncertain. Here, stable carbon isotope ratios from a network of 35 tree-ring sites located Central Europe, a region where summer soil-water availability decreased over the last century.We were able to demonstrate that the combined effects of increasing CO 2 and climate change leading to soil drying have resulted in an accelerated increase of iWUE. These findings will help to reduce uncertainties in the land surface schemes of global climate models, where vegetation-climate feedbacks are currently still poorly constrained by observational data. 4

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Though tree-ring chronologies are annually resolved, their dating has never been independently validated at the global scale. Moreover, it is unknown if atmospheric radiocarbon enrichment events of cosmogenic origin leave spatiotemporally consistent fingerprints. Here we measure the 14C content in 484 individual tree rings formed in the periods 770–780 and 990–1000 CE. Distinct 14C excursions starting in the boreal summer of 774 and the boreal spring of 993 ensure the precise dating of 44 tree-ring records from five continents. We also identify a meridional decline of 11-year mean atmospheric radiocarbon concentrations across both hemispheres. Corroborated by historical eye-witness accounts of red auroras, our results suggest a global exposure to strong solar proton radiation. To improve understanding of the return frequency and intensity of past cosmic events, which is particularly important for assessing the potential threat of space weather on our society, further annually resolved 14C measurements are needed.

A 1200-year multiproxy record of tree growth and summer temperature at the northern pine forest limit Published by: http://www.sagepublications.com can be found at: The Holocene Additional services and information for AbstractCombining nine tree growth proxies from four sites, from the west coast of Norway to the Kola Peninsula of NW Russia, provides a well replicated (> 100 annual measurements per year) mean index of tree growth over the last 1200 years that represents the growth of much of the northern pine timberline forests of northern Fennoscandia. The simple mean of the nine series, z-scored over their common period, correlates strongly with mean June to August temperature averaged over this region (r = 0.81), allowing reconstructions of summer temperature based on regression and variance scaling. The reconstructions correlate significantly with gridded summer temperatures across the whole of Fennoscandia, extending north across Svalbard and south into Denmark. Uncertainty in the reconstructions is estimated by combining the uncertainty in mean tree growth with the uncertainty in the regression models. Over the last seven centuries the uncertainty is < 4.5% higher than in the 20th century, and reaches a maximum of 12% above recent levels during the 10th century. The results suggest that the 20th century was the warmest of the last 1200 years, but that it was not significantly different from the 11th century. The coldest century was the 17th. The impact of volcanic eruptions is clear, and a delayed recovery from pairs or multiple eruptions suggests the presence of some positive feedback mechanism. There is no clear and consistent link between northern Fennoscandian summer temperatures and solar forcing.

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.

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Investigating the many internal feedbacks within the climate system is a vital component of the effort to quantify the full effects of future anthropogenic climate change. The stomatal apertures of plants tend to close and decrease in number under elevated CO2 concentrations, increasing water‐use efficiency (WUE) and reducing canopy evapotranspiration. Experimental and modelling studies reveal huge variations in these changes such that the warming associated with reduced evapotranspiration (known as physiological forcing) is neither well understood or constrained. Palaeo‐observations of changes in stomatal response and plant WUE under rising CO2 might be used to better understand the processes underlying the physiological forcing feedback and to link measured changes in plant WUE to a specific physiological change in stomata. Here we use time series of tree ring (Pinus sylvestris L.) δ13C and subfossil leaf (Betula nana L.) measurements of stomatal density and geometry to derive records of changes in intrinsic water‐use efficiency (iWUE) and maximum stomatal conductance in the Boreal zone of northern Finland and Sweden. We investigate the rate of change in both proxies, over the recent past. The independent lines of evidence from these two different Boreal species indicate increased iWUE and reduced maximum stomatal conductance of similar magnitude from preindustrial times (ca. ad 1850) to around ad 1970. After this maximum stomatal conductance continues to decrease to ad 2000 in B. nana but iWUE in P. sylvestris reaches a plateau. We suggest that northern boreal P. sylvestris might have reached a threshold in its ability to increase WUE as CO2 rises.