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Rockfall in high-mountain regions is thought to be changing due to accelerating climate warming and permafrost degradation, possibly resulting in enhanced activity and larger volumes involved in individual falls. Yet the systematic lack of long-term observations of rockfall largely hampers an in-depth assessment of how activity may have been altered by a warming climate. Here we compile a continuous time series from 1920 to 2020 of periglacial rockfall activity using growth-ring records from 375 trees damaged by past rockfall at Täschgufer (Swiss Alps). We show that the ongoing warming favours the release of rockfall and that changes in activity correlate significantly with summer air temperatures at interannual and decadal timescales. An initial increase in rockfall occurred in the late 1940s to early 1950s following early twentieth century warming. From the mid-1980s, activity reached new and hitherto unprecedented levels. This long-term record of rockfall activity can help to inform the design of vital mitigation and risk reduction measures in inhabited mountain environments.

Explosive volcanism is a key contributor to climate variability on interannual to centennial timescales1. Understanding the far-field societal impacts of eruption-forced climatic changes requires firm event chronologies and reliable estimates of both the burden and altitude (that is, tropospheric versus stratospheric) of volcanic sulfate aerosol2,3. However, despite progress in ice-core dating, uncertainties remain in these key factors4. This particularly hinders investigation of the role of large, temporally clustered eruptions during the High Medieval Period (HMP, 1100–1300 ce), which have been implicated in the transition from the warm Medieval Climate Anomaly to the Little Ice Age5. Here we shed new light on explosive volcanism during the HMP, drawing on analysis of contemporary reports of total lunar eclipses, from which we derive a time series of stratospheric turbidity. By combining this new record with aerosol model simulations and tree-ring-based climate proxies, we refine the estimated dates of five notable eruptions and associate each with stratospheric aerosol veils. Five further eruptions, including one responsible for high sulfur deposition over Greenland circa 1182 ce, affected only the troposphere and had muted climatic consequences. Our findings offer support for further investigation of the decadal-scale to centennial-scale climate response to volcanic eruptions.

The trade winds cause strong upwelling in the eastern equatorial Pacific, and create the eastern Pacific Cold Tongue (EPCT) that has far‐reaching impacts on global climate. However, large discrepancies persist in quantifying 20th‐century EPCT sea surface temperature (SST) changes across different instrumental data sets. Here we synthesize four coral Sr/Ca‐SST records from the tropical central‐eastern Pacific to develop a Cold Tongue Index (CTI) reconstruction for 1887–1997. The coral CTI record shows a rapid 20th century warming of the EPCT, suggesting an underestimation of warming trends in instrumental CTI records. The decadal to multidecadal changes in reconstructed EPCT SST show an association with the Walker Circulation. Our reconstruction indicates that recent EPCT cooling during the global warming hiatus is not unusual in the context of the 20th century. Our results provide new evidence for 20th century EPCT SST changes and an observational constraint for predicting future tropical climate changes. The tropical Pacific exhibits an asymmetric pattern of sea surface temperatures (SST) along the equator, with a warm pool in the west and a cold tongue in the east. The Eastern Pacific Cold Tongue (EPCT) plays a crucial role in influencing global climate. To date, however, considerable disagreement persists about changes and causes of the EPCT SST over the 20th century due to the sparse instrumental observations before ∼1960. Here we synthesize published coral Sr/Ca‐SST records from the tropical central‐eastern Pacific to develop a coral Cold Tongue Index (CTIcoral) for the period 1887–1997. The CTIcoralrecord correlates significantly with instrumental data since 1960, and thus allows reliable assessment of 20th century EPCT SST changes. Our record exhibits a rapid 20th century warming trend of the EPCT, indicating an underestimation of warming trends in instrumental CTI records. The decadal to multidecadal changes in CTIcoralalso show an association with the Walker Circulation. Comparing the magnitude of the 1992–2011 trend in instrumental CTI with our reconstruction reveals that recent EPCT cooling during the global warming hiatus is not anomalous in the context of 20th century. Our findings have implications for predicting future EPCT SST changes by offering a constraint for model simulations. Synthesizing coral Sr/Ca‐SST records yields robust estimates of 20th century warming trend in the eastern Pacific Cold Tongue (EPCT)A coral‐based reconstruction indicates an underestimation of 20th century EPCT warming trends in instrumental SST recordsRecent EPCT cooling during the global warming hiatus is not anomalous in the context of the 20th century Synthesizing coral Sr/Ca‐SST records yields robust estimates of 20th century warming trend in the eastern Pacific Cold Tongue (EPCT) A coral‐based reconstruction indicates an underestimation of 20th century EPCT warming trends in instrumental SST records Recent EPCT cooling during the global warming hiatus is not anomalous in the context of the 20th century

A large amount of well-preserved timbers was found during several archaeological excavations of the Faravel mining site (Southern French Alps, between 1950- and 2150 m a.s.l.). 232 of these timbers were sampled for dendrochronological analysis and 67% of them were dated. These 156 larch (Larix deciduaMill.) series, crossdated against existing reference chronologies, were averaged for a site chronology spanning from 777 to 1243. From this dataset, 33 timbers with (almost) complete sapwood allowed us to obtain tree felling years with seasonal resolution. The chronological distribution of these felling years highlights nine distinct mining phases that occurred between 1059 and 1243, revealing a discontinuous exploitation of the study site during the medieval period. In addition, the presence of late wood in the vast majority of complete samples, demonstrates that logging mainly occurred during late fall and early winter. These results, combined with historical, palynological and archaeological investigations, plead for short, seasonal, and low-intensity, mining campaigns, mainly carried out after the bulk of agropastoral activities using rudimentary techniques with limited impact on the forest cover.

The eruption of Samalas in Indonesia in 1257 ranks among the largest sulfur-rich eruptions of the Common Era with sulfur deposition in ice cores reaching twice the volume of the Tambora eruption in 1815. Sedimentological analyses of deposits confirm the exceptional size of the event, which had both an eruption magnitude and a volcanic explosivity index of 7. During the Samalas eruption, more than 40 km3of dense magma was expelled and the eruption column is estimated to have reached altitudes of 43 km. However, the climatic response to the Samalas event is debated since climate model simulations generally predict a stronger and more prolonged surface air cooling of Northern Hemisphere summers than inferred from tree-ring-based temperature reconstructions. Here, we draw on historical archives, ice-core data and tree-ring records to reconstruct the spatial and temporal climate response to the Samalas eruption. We find that 1258 and 1259 experienced some of the coldest Northern Hemisphere summers of the past millennium. However, cooling across the Northern Hemisphere was spatially heterogeneous. Western Europe, Siberia and Japan experienced strong cooling, coinciding with warmer-than-average conditions over Alaska and northern Canada. We suggest that in North America, volcanic radiative forcing was modulated by a positive phase of the El Niño–Southern Oscillation. Contemporary records attest to severe famines in England and Japan, but these began prior to the eruption. We conclude that the Samalas eruption aggravated existing crises, but did not trigger the famines.

January 2018 was an unusually warm and wet month across the Western Alps, with widespread landslides at low elevations and massive snowfall higher up. This extreme month yields lessons for how mountain communities can prepare for a warmer future.

Explosive volcanism can alter global climate, and hence trigger economic, political and demographic change. The climatic impact of the largest volcanic events has been assessed in numerous modelling studies and tree-ring-based hemispheric temperature reconstructions. However, volcanic surface cooling derived from climate model simulations is systematically much stronger than the cooling seen in tree-ring-based proxies, suggesting that the proxies underestimate cooling; and/or the modelled forcing is unrealistically high. Here, we present summer temperature reconstructions for the Northern Hemisphere from tree-ring width and maximum latewood density over the past 1,500 years. We also simulate the climate effects of two large eruptions, in AD 1257 and 1815, using a climate model that accounts explicitly for self-limiting aerosol microphysical processes. Our tree-ring reconstructions show greater cooling than reconstructions with lower spatial coverage and based on tree-ring width alone, whereas our simulations show less cooling than previous simulations relying on poorly constrained eruption seasons and excluding nonlinear aerosol microphysics. Our tree-ring reconstructions and climate simulations are in agreement, with a mean Northern Hemisphere extra-tropical summer cooling over land of 0.8 to 1.3 °C for these eruptions. This reconciliation of proxy and model evidence paves the way to improved assessment of the role of both past and future volcanism in climate forcing.

AbstractOn forested paths, dendrogeomorphology has been demonstrated to represent a powerful tool to reconstruct past activity of avalanches, an indispensable step in avalanche hazard assessment. Several quantitative and qualitative approaches have been shown to yield reasonable event chronologies but the question of the completeness of tree-ring records remains debatable. Here, we present an alternative semi-quantitative approach for the determination of past snow avalanche events. The approach relies on the assessment of the number and position of disturbed trees within avalanche paths as well as on the intensity of reactions in trees. In order to demonstrate that no bias was induced by the dendrogeomorphic expert, we carry out a statistical evaluation (Classification and Regression Trees, or CART) of the approach. Results point to the consistency and replicability of the procedure and to the fact that the approach is not restricted to the identification of high-magnitude avalanches. Evaluation of the semi-quantitative approach is illustrated on a well-documented path in Chamonix, French Alps. For the period 1905–2010, comparison between the avalanche years recorded in a substantial database (Enquête Permanente sur les Avalanches, or EPA) and those defined with dendrogeomorphic techniques shows that the avalanche record reconstructed from tree-ring series contains 38% of the observed events.

Depuis deux décennies, de nombreuses publications scientifiques ont mis en évidence les impacts du changement climatique sur la fréquence des chutes de pierres dans les parois englacées de haute altitude. À plus basse altitude, les inventaires rares et incomplets existants n’ont pas permis d’identifier un impact significatif de l’augmentation des températures. Dans ce travail, nous avons utilisé une approche dendrogéomorphologique, basée sur l’étude des perturbations dans les cernes de croissance des arbres, pour reconstituer l’activité passée des chutes de pierres sur deux versants forestiers des Préalpes françaises calcaires (massifs du Vercors et du Diois). Les incertitudes liées à la diminution du nombre d’arbres dans le temps ont été précisément quantifiées grâce à un inventaire forestier exhaustif et à une cartographie à haute résolution du peuplement forestier. En accord avec la littérature, la comparaison des reconstitutions dendrogéomorphologiques avec les réanalyses météorologiques SAFRAN montre que les précipitations estivales et les évènements pluvieux intenses sont les principaux facteurs qui conditionnent l’activité de l’aléa sur les deux sites. En revanche, aucun impact significatif des températures ou des cycles de gel–dégel n’a pu être détecté. À l’échelle décennale (60 dernières années), nous expliquons l’augmentation apparente de l’activité des chutes de pierres depuis 1959 dans le Vercors par la recolonisation forestière rapide de la parcelle étudiée au cours des dernières décennies et par une sur-représentation des jeunes arbres, plus sensibles aux impacts. Dans le massif du Diois, l’absence de tendance dans la reconstitution dendrogéomorphologique au cours des dernières décennies suggère que le réchauffement climatique n’a pas eu d’impact sur la fréquence de l’aléa à basse altitude. Cependant, le nombre réduit de sites et la robustesse limitée des analyses statistiques développées suggèrent que ces résultats doivent être interprétés avec prudence et devront nécessairement être répliqués dans le futur afin de conclure définitivement quant à l’absence de réponse au réchauffement de l’aléa rocheux dans les zones de basse altitude des préalpes calcaires.

Les chutes de blocs représentent un aléa majeur dans les zones montagneuses, menaçant infrastructures collectives, zones urbanisées et vies humaines. Les conséquences de ces événements peuvent être importantes pour les collectivités locales ainsi que les pouvoirs publics, qui restent démunis en matière de méthode de diagnostic et d’analyse du risque. Dans ce contexte, l’évaluation du risque rocheux par une approche de type QRA (quantitative risk assessment) est devenue incontournable pour l’aménagement des territoires de montagne et le choix des stratégies destinées à réduire le risque. Cependant, en pratique, la QRA reste peu utilisée et développée. À cet égard, cet article propose de renforcer les bases formelles du calcul du risque dans le domaine des chutes de blocs et démontre sa faisabilité sur des zones urbanisées/urbanisables. Les effets de la non-stationnarité du phénomène, et l’apport de nouvelles mesures de risque permettant les arbitrages court terme/long terme, sont également abordés. Le potentiel de l’approche est illustré par le cas d’étude réel de la commune de Crolles, dans les Alpes françaises.

Les bases de données événementielles recensant les chutes de blocs et leurs conséquences restent peu développées. Dans les Alpes françaises, un territoire pourtant propice et où les enjeux sont nombreux et diversifiés, l’information existante est dispersée et hétérogène. Dans le contexte du Projet National C2ROP, un premier effort de capitalisation, d’homogénéisation et d’exploitation de l’existant a été réalisé. La base de données C2ROP-INRAE construite couvre les six départements des Alpes françaises et rassemble d’ores et déjà plus de 8000 événements au sein d’un environnement commun géo-référencé incluant date et lieu d’occurrence et volume des événements. En outre, les informations textuelles disponibles au format numérique ont été dépouillées pour évaluer les dommages occasionnés de façon aussi complète que possible. Ce travail confirme la réalité de l’aléa et du risque pour les Alpes françaises. Les données rassemblées sont à présent disponibles pour des analyses plus poussées et/ou ciblées. Des pistes pour approfondir et étendre la base C2ROP-INRAE sont également proposées.

Abstract: •Introduction: In the context of climate change, assessing climate–growth relationships is of high importance in order to understand how forest ecosystems evolve and to test climate models at regional scale. •Objectives: This study aims to identify the climate variables that explain most of the variation in Pinus cembra radial growth at different spatial–temporal scales, response functions and moving response functions were processed on chronologies from a dense tree ring network. This original network is centred on the French Alps influenced by the Atlantic, Continental, and Mediterranean synoptic systems. •Results: A spatial clustering pattern matching the latitudinal climatic gradient was observed in tree ring indexed chronologies and climate responses. The stationary response functions computed for each cluster-averaged population evidenced a thermo-dependent northern cluster (Cl1), a rainfall-dependent southern cluster (Cl3), and an alternative response for the intermediate cluster in between (Cl2). Since the late nineteenth century, the moving response functions indicate an increasing influence of winter precipitation for the northern and the intermediate clusters. •Conclusion: Considering the increase in temperature and reduction both of summer precipitation predicted by general circulation models and of the snow cover and duration in the French Alps, stone pine is likely to be exposed to stressful conditions during the twenty-first century.

This paper reports on climate-induced growth changes in relict, low-altitude mountain pines (Pinus uncinataMill. ex. Mirb.) from two refugia with cold microclimates located in the Northern French Alps. The P. uncinatastands analyzed grow at the lower bound of their ecological limit and are thus thought to be sensitive indicators of ongoing climate change. Using dendroecological approaches, we compare tree-ring growth at two closely spaced low-altitude stands in the Chartreuse massif (French Alps): La Plagne and Cirque de Bresson. La Plagne is a N-NW-exposed, ventilated slope with cold air circulating in the scree during summer, and the presence of sporadic permafrost as well as ground overcooling, whereas Cirque de Bresson is located on a small, S-exposed fan with sporadic avalanche activity. At both sites, growth responses of P. uncinatato changes in twentieth and twenty-first centuries temperature and precipitation conditions were investigated by means of moving correlation analyses. At Cirque de Bresson, a significant and rapid decline in tree-ring widths has been observed since the early 1990s. We attribute this decline to (i) increasing air temperatures at the beginning of the growing season (May–June) as well as to (ii) a decrease in soil water potential. At La Plagne, we do not detect any significant trend between the higher summer temperatures and tree growth, presumably as a result of the circulation of cold air in the scree slope, which is thought to maintain fresh and humid soil conditions and therefore favor tree growth. These forest stands provide prime examples on how dendroecology can contribute to the study of the dynamics and local variability of tree growth and climate change in relict forest populations with high ecological and conservation values.