An atlas of megadroughts in Europe and in the Mediterranean Basin during the Common Era provides insights into climate variability.

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Midsummer (July) temperatures are reconstructed for the last 7500 years using the long ring-width chronology of Scots pine (Pinus sylvestris L.) from northern Finland. The chronology was built using regional curve standardization (RCS), which allows for long-term (low-frequency) variability to be extracted from this annually resolved record of 1087 samples from living trees and subfossil timber. Short-and long-term changes in reconstructed July temperatures are presented. The regression model accounts for 37% of the dependent instrumental temperature variance between ad 1879 and 1992. The warmest 30-year periods were ad 560–531, ad 560–531, 1190–1161 bc and ad 1541–1570, and the coldest 5240–5211, 5150–5121 and 3710–3681 bc. The warmest 100-year periods were ad 1501–1600, 600–501 bc and 300–201 bc, and the coldest 5200–5101, 2500–2401 and 1500–1401 bc. Broad comparisons are made with dendrochronological, lacustrine and glacial proxy evidence.

This paper reviews the development of the current 'supra-long' pine chronology for northern Finnish Lapland. In the forest-tundra ecotone region of northern Finnish Lapland over 250 samples from living Scots pines (Pinus sylvestris L.) and over 1700 samples of subfossil pines have been collected for dendrochronological studies. In addition, over 1400 subfossils have been sampled from the forested area of Finnish Lapland. The goal of the research was to build a more than 7000-year long continuous pine ring-width chronology. The construction of the chronology is now completed. The intensive phase of the data collection and chronology building lasted about 10 years, 1989 to 1999. The major part of the Finnish Lapland master curve was constructed several years ago, but it was extremely dif cult to bridge the c. 300-year gap, prior to 165 bc between the 'absolute' younger part of the chronology and the ' oating' older part. The crucial samples were identi ed and assembled in the chronology in early 1999, and there is now an unbroken pine chronology about 7500 years long constructed from the subfossil forest-limit pines of northern Finnish Lapland. The severe growth depression centred on 330 bc is likely to have been caused by increased wetness. A brief summary is presented of inferred tree-line changes from the location of the samples.

Various studies report substantial increases in intrinsic water-use efficiency (W i), estimated using carbon isotopes in tree rings, suggesting trees are gaining increasingly more carbon per unit water lost due to increases in atmospheric CO2. Usually, reconstructions do not, however, correct for the effect of intrinsic developmental changes in W i as trees grow larger. Here we show, by comparing W i across varying tree sizes at one CO2 level, that ignoring such developmental effects can severely affect inferences of trees’ W i. W i doubled or even tripled over a trees’ lifespan in three broadleaf species due to changes in tree height and light availability alone, and there are also weak trends for Pine trees. Developmental trends in broadleaf species are as large as the trends previously assigned to CO2 and climate. Credible future tree ring isotope studies require explicit accounting for species-specific developmental effects before CO2 and climate effects are inferred.

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Tree-ring chronologies of maximum latewood density are most suitable to reconstruct annually resolved summer temperature variations of the late Holocene. The two longest such chronologies have been developed in northern Europe stretching back to the 2nd century BC, and the 5th century AD. We show where similarities between the two chronologies exist, and combine portions of both into a new summer temperature reconstruction for the Common Era. To minimize the transfer of potential biases, we assess the contribution of the candidate reconstructions' measurements, and exclude data (i) from exceptionally young and old trees, and (ii) produced by different laboratory technologies. Our new composite reconstruction reveals warmer conditions during Roman, Medieval and recent times, separated by prolonged cooling during the Migration period and Little Ice Age. Twentieth century warmth, as indicated in one of the existing density records, is reduced in the new reconstruction, also affecting the overall, millennial-scale, cooling trend over the late Holocene (À0.30˚C per 1000 years). Due to the reduced biological memory, typical for tree-ring density measurements, the new reconstruction is most suitable for evaluating the rate and speed of abrupt summer cooling following large volcanic eruptions.

X-ray microdensitometry on annually resolved tree-ring samples has gained an exceptional position in last-millennium paleoclimatology through the maximum latewood density (MXD) parameter, but also increasingly through other density parameters. For 50 years, X-ray based measurement techniques have been the de facto standard. However, studies report offsets in the mean levels for MXD measurements derived from different laboratories, indicating challenges of accuracy and precision. Moreover, reflected visible light-based techniques are becoming increasingly popular, and wood anatomical techniques are emerging as a potentially powerful pathway to extract density information at the highest resolution. Here we review the current understanding and merits of wood density for tree-ring research, associated microdensitometric techniques, and analytical measurement challenges. The review is further complemented with a careful comparison of new measurements derived at 17 laboratories, using several different techniques. The new experiment allowed us to corroborate and refresh "long-standing wisdom" but also provide new insights. Key outcomes include (i) a demonstration of the need for mass/volume-based recalibration to accurately estimate average ring density; (ii) a substantiation of systematic differences in MXD measurements that cautions for great care when combining density data sets for climate reconstructions; and (iii) insights into the relevance of analytical measurement resolution in signals derived from tree-ring density data. Finally, we provide recommendations expected to facilitate future inter-comparability and interpretations for global change research.Plain Language Summary Paleoclimatology, the study of how the climate has changed throughout earth history, is an important component of climate change research. The wood density of tree