Reliable estimates of future climate change in the Alps are relevant for large parts of the European society. At the same time, the complex Alpine region poses considerable challenges to climate models, which translate to uncertainties in the climate projections. Against this background, the present study reviews the state-of-knowledge about 21st century climate change in the Alps based on existing literature and additional analyses. In particular, it explicitly considers the reliability and uncertainty of climate projections. Results show that besides Alpine temperatures, also precipitation, global radiation, relative humidity, and closely related impacts like floods, droughts, snow cover, and natural hazards will be affected by global warming. Under the A1B emission scenario, about 0.25 °C warming per decade until the mid of the 21st century and accelerated 0.36 °C warming per decade in the second half of the century is expected. Warming will probably be associated with changes in the seasonality of precipitation, global radiation, and relative humidity, and more intense precipitation extremes and flooding potential in the colder part of the year. The conditions of currently record breaking warm or hot winter or summer seasons, respectively, may become normal at the end of the 21st century, and there is indication for droughts to become more severe in the future. Snow cover is expected to drastically decrease below 1500-2000 m and natural hazards related to glacier and permafrost retreat are expected to become more frequent. Such changes in climatic parameters and related quantities will have considerable impact on ecosystems and society and will challenge their adaptive capabilities.

[1] Regional climate models (RCMs) from the ENSEMBLES project are analyzed to assess projected changes in 21st century heavy and extreme precipitation events over Europe. A set of 10 RCMs with horizontal grid spacing of 25 km is considered, which are driven by six GCMs under an A1B greenhouse gas scenario. The diagnostics include basic precipitation indices (including mean, wet-day frequency, intensity, and percentile exceedance) and application of generalized extreme value theory for return periods up to 100 years. Changes in precipitation climate between present and future conditions are presented on a European scale and in more detail for 11 European regions (mostly in supplemental figures). On the European scale, projections show increases (decreases) in mean amounts and wet-day frequency in northern (southern) Europe. This pattern is oscillating with the seasonal cycle. Changes in extremes exhibit a similar pattern, but increases in heavy events reach much further south. For instance, during spring and fall, much of the Mediterranean is projected to experience decreases in mean precipitation but increases in heavy events. Thus, projected changes in mean and extremes may show different signals. The inter-model spread is partly attributable to a GCM-dependent clustering of the climate change signal, but also affected by RCM uncertainties, in particular in summer. Despite these uncertainties, many of the projected changes are statistically significant and consistent across models. For instance, for the Alps, all models project an intensification of heavy events during fall, and these changes are statistically significant for a majority of the models considered.

Projections of precipitation and its extremes over the European continent are analyzed in an extensive multimodel ensemble of 12 and 50 km resolution EURO‐CORDEX Regional Climate Models (RCMs) forced by RCP2.6, RCP4.5, and RCP8.5 (Representative Concentration Pathway) aerosol and greenhouse gas emission scenarios. A systematic intercomparison with ENSEMBLES RCMs is carried out, such that in total information is provided for an unprecedentedly large data set of 100 RCM simulations. An evaluation finds very reasonable skill for the EURO‐CORDEX models in simulating temporal and geographical variations of (mean and heavy) precipitation at both horizontal resolutions. Heavy and extreme precipitation events are projected to intensify across most of Europe throughout the whole year. All considered models agree on a distinct intensification of extremes by often more than +20% in winter and fall and over central and northern Europe. A reduction of rainy days and mean precipitation in summer is simulated by a large majority of models in the Mediterranean area, but intermodel spread between the simulations is large. In central Europe and France during summer, models project decreases in precipitation but more intense heavy and extreme rainfalls. Comparison to previous RCM projections from ENSEMBLES reveals consistency but slight differences in summer, where reductions in southern European precipitation are not as pronounced as previously projected. The projected changes of the European hydrological cycle may have substantial impact on environmental and anthropogenic systems. In particular, the simulations indicate a rising probability of summertime drought in southern Europe and more frequent and intense heavy rainfall across all of Europe.

Many climate studies assess trends and projections in heavy precipitation events using precipitation percentile (or quantile) indices. Here we investigate three different percentile indices that are commonly used. We demonstrate that these may produce very different results and thus require great care with interpretation. More specifically, consideration is given to two intensity-based indices and one frequency-based index, namely (a) all-day percentiles, (b) wet-day percentiles, and (c) frequency indices based on the exceedance of a percentile threshold.Climatic Change (2016) Wet-day percentiles are conditionally computed for the subset of wet events (with precipitation exceeding some threshold, e.g. 1 mm/d for daily precipitation). We present evidence that this commonly used methodology can lead to artifacts and misleading results if significant changes in the wet-day frequency are not accounted for. Percentile threshold indices measure the frequency of exceedance with respect to a percentile-based threshold. We show that these indices yield an assessment of changes in heavy precipitation events that is qualitatively consistent with all-day percentiles, but there are substantial differences in quantitative terms. We discuss the reasons for these effects, present a theoretical assessment, and provide a series of examples using global and regional climate models to quantify the effects in typical applications.Application to climate model output shows that these considerations are relevant to a wide range of typical climate-change applications. In particular, wet-day percentiles generally yield different results, and in most instances should not be used for the impact-oriented assessment of changes in heavy precipitation events.

[1] Summer temperature variability has been projected to increase in Central Europe in response to anthropogenic greenhouse gas forcing. Based on an unprecedented set of global and regional climate models from the ENSEMBLES project, we assess the robustness of these projections on interannual to daily time scales. In comparison to previous analyses using PRUDENCE simulations, we find a more diverse climate change signal for interannual summer temperature variability and a clear dependence upon presentday model performance. Models that realistically represent present-day variability, tend to consistently project increasing interannual variability at the end of the 21st century. We demonstrate that the partitioning of latent and sensible heat fluxes controlled by soil moisture is crucial to understand the projected changes across the multi-model experiment. The projected increase in daily summer temperature variability is more robust and consistently simulated by all models. Likewise, all models consistently project reduced daily temperature variability in winter. Thus, it is a robust signal across the entire ensemble that in summer and southcentral Europe hot extremes warm stronger than the mean, and in winter and northern Europe cold extremes warm stronger than mean temperatures. Citation: Fischer, E. M.,