The European Alps have experienced remarkable climate changes since the beginning of the Industrial Age. In particular, mean air temperature in the region increased at a greater rate than global temperature, leading to the loss of nearly half of the glaciated area and to important changes in the ecosystems. Spanning 1,200 km in length, with peaks reaching over 4,000 meters above sea level (m asl), the Alps have a critical influence over the weather in most of Europe and separate the colder oceanic/continental climate in the north from the milder Mediterranean climate in the south. The climatic differences between the main slopes are reflected into different climate changes—whereas the northern slope got wetter, the southern slope got drier. The consequences of these climate changes are not confined to the Alpine region. Being located in the center of Europe, the Alps provide water and electricity for over 100 million people. Alpine run-off is a major contributor to the total discharge of several major European rivers such as the Rhine, the Rhône, the Po, and the Danube. Therefore, climate change in the Alps can have significant economic impacts on a continental scale. Their convenient geographical position allowed scientists to study the Alpine climate since the very beginning of the instrumental era. The first instrumental meteorological observations in an Alpine valley were taken as early as the mid-17th century, soon followed by measurements at higher elevations. Continuous records are available since the late 18th century, providing invaluable information on climate variability to modern-day researchers. Although there is overwhelming evidence of a dominant anthropogenic influence on the observed temperature increase, the causes of the changes that affected other variables have, in many cases, not been sufficiently investigated by the scientific community.
The European Alps feature a unique situation with the densest network of long-term instrumental climate observations and anthropogenic emission sources located in the immediate vicinity of glaciers suitable for ice core studies. To archive atmospheric changes in an undisturbed sequence of firn and ice layers, ice core drilling sites require temperatures low enough to minimize meltwater percolation. In the Alps, this implies a restriction to the highest summit glaciers of comparatively small horizontal and vertical extension (i.e., with typical ice thickness not much exceeding 100 m). As a result, Alpine ice cores offer either high-resolution or long-term records, depending on the net snow accumulation regime of the drilling site. High-accumulation Alpine ice cores have been used with great success to study the anthropogenic influence on aerosol-related atmospheric impurities over the last 100 years or so. However, respective long-term reconstructions (i.e., substantially exceeding the instrumental era) from low-accumulation sites remain comparatively sparse. Accordingly, deciphering Alpine ice cores as long-term climate records deserves special emphasis. Certain conditions must exist for Alpine ice cores to serve as climate archives, and this is important in particular regarding the challenges and achievements that have significance for ice cores from other mountain areas: (a) a reliable chronology is the fundamental prerequisite for interpreting any ice core proxy time series. Advances in radiometric ice dating and annual layer counting offer the tools to crucially increase dating precision in the preinstrumental era. (b) Glacier flow effects and spatio-seasonal snow deposition variability challenge linking the ice core proxy signals to the respective atmospheric variability (e.g., of temperature, mineral dust, and impurity concentrations). Here, assistance comes from combining multiple ice cores from one site and from complementary meteorological, glaciological, and geophysical surveys. (c) As Alpine ice cores continue to advance their contribution to Holocene climate science, exploring the link to instrumental, historical, and other natural climate archives gains increasing importance.
The free troposphere is the location of important weather and climate processes. Here, horizontal and vertical transport of energy, mass, and momentum take place, and it holds greenhouse gases, water vapor, and clouds. The free troposphere therefore plays an important role in global climate feedback processes. Mountains provide important ecosystem services for a large lowland population. Mountain ecosystems may react particularly strongly to climatic changes. This is because mountains intersect important environmental and geoecological boundaries such as the snow line and the tree line. In a changing climate, these boundaries may shift. Climate change thus affects mountain glaciers, water resources, and mountain ecosystems. Climates of mountains and of the free troposphere have attracted scientists of the enlightenment and have been studied scientifically at least since the 18th century. High-altitude observatories were installed in the late 19th century, and upper-air measurements were started soon afterwards. However, even in the early 21st century, the climate observing systems do not well cover mountain regions and specifically mountain peaks. The temperature of the free troposphere is dominated by horizontal and vertical transport of sensible and latent heat, condensation and release of latent heat, and radiation to space. Mountain peaks sometimes reach into the free troposphere, but at the same time also share characteristics of surface climate. They are strongly influenced by radiative processes of the surrounding surface, while during the day they are often within the atmospheric boundary layer. With respect to climate change, temperature trends are amplified in the tropical upper-troposphere relative to the surface due to latent heat release, while in the Arctic the surface warms faster than the free atmosphere due to strong inversions and due to feedback processes operating at the surface. Mountain peaks may see both types of amplification. Several processes have been suggested to cause an elevation dependent warming, the most important of which arguably is the snow-albedo feedback. Elevation dependent warming is also seen in model studies and in observations, although detecting this signal in observations turns out rather difficult outside the tropics due to high variability and sometimes low-data quality. The observed climatic changes are expected to continue into the future.
Michael Kuhn and Marc Olefs
Elevation-dependent climate change has been observed in the European Alps in the context of global warming and as a consequence of Alpine orography. It is most obvious in elevation-dependent warming, conveniently defined as the linear regression of the time series of temperatures against elevation, and it reaches values of several tenths of a degree per 1,000 m elevation per decade. Observed changes in temperature have forced changes in atmospheric pressure, water vapor, cloud condensation, fluxes of infrared and solar radiation, snow cover, and evaporation, which have affected the Alpine surface energy and water balance in different ways at different elevations. At the same time, changes in atmospheric aerosol optical depth, in atmospheric circulation, and in the frequency of weather types have contributed to the observed elevation-dependent climate change in the European Alps. To a large extent, these observations have been reproduced by model simulations.
Andreas Gobiet and Sven Kotlarski
The analysis of state-of-the-art regional climate projections indicates a number of robust changes of the climate of the European Alps by the end of this century. Among these are a temperature increase in all seasons and at all elevations and a significant decrease in natural snow cover. Precipitation changes, however, are more subtle and subject to larger uncertainties. While annual precipitation sums are projected to remain rather constant until the end of the century, winter precipitation is projected to increase. Summer precipitation changes will most likely be negative, but increases are possible as well and are covered by the model uncertainty range. Precipitation extremes are expected to intensify in all seasons. The projected changes by the end of the century considerably depend on the greenhouse-gas scenario assumed, with the high-end RCP8.5 scenario being associated with the most prominent changes. Until the middle of the 21st century, however, it is projected that climate change in the Alpine area will only slightly depend on the specific emission scenario. These results indicate that harmful weather events in the Alpine area are likely to intensify in future. This particularly refers to extreme precipitation events, which can trigger flash floods and gravitational mass movements (debris flows, landslides) and lead to substantial damage. Convective precipitation extremes (thunderstorms) are additionally a threat to agriculture, forestry, and infrastructure, since they are often associated with strong wind gusts that cause windbreak in forests and with hail that causes damage in agriculture and infrastructure. Less spectacular but still very relevant is the effect of soil erosion on inclined arable land, caused by heavy precipitation. At the same time rising temperatures lead to longer vegetation periods, increased evapotranspiration, and subsequently to higher risk of droughts in the drier valleys of the Alps. Earlier snowmelt is expected to lead to a seasonal runoff shift in many catchments and the projected strong decrease of the natural snow cover will have an impact on tourism and, last but not least, on the cultural identity of many inhabitants of the Alpine area.
Wilfried Haeberli, Johannes Oerlemans, and Michael Zemp
Like many comparable mountain ranges at lower latitudes, the European Alps are increasingly losing their glaciers. Following roughly 10,000 years of limited climate and glacier variability, with a slight trend of increasing glacier sizes to Holocene maximum extents of the Little Ice Age, glaciers in the Alps started to generally retreat after 1850. Long-term observations with a monitoring network of unique density document this development. Strong acceleration of mass losses started to take place after 1980 as related to accelerating atmospheric temperature rise. Model calculations, using simple to high-complexity approaches and relating to individual glaciers as well as to large samples of glaciers, provide robust results concerning scenarios for the future: under the influence of greenhouse-gas forced global warming, glaciers in the Alps will largely disappear within the 21st century. Anticipating and modeling new landscapes and land-forming processes in de-glaciating areas is an emerging research field based on modeled glacier-bed topographies that are likely to become future surface topographies. Such analyses provide a knowledge basis to early planning of sustainable adaptation strategies, for example, concerning opportunities and risks related to the formation of glacial lakes in over-deepened parts of presently still ice-covered glacier beds.