Climate Change and Glacier Reaction in the European Alps

Glaciers are probably the most obvious features of Earth’s changing climate. They enable one to see the effects of a warming or a cooling of the atmosphere by landscape changes on time scales short enough to be perceived or recognized by humans. However, the relationship between a retreating and advancing glacier and the climate is not linear, as glacier flow can filter the direct signal of the climate. Thus, glaciers can advance during periods of warming or, vice versa, retreat during periods of cooling. In fact, it is the mass change of the glacier (i.e., the mass balance) that directly links the glacier reaction to an atmospheric signal. The mechanism-based understanding of the relationship between the changing climate and glacier reaction received important and significant momentum from the science of the Alpine region. This strong momentum from the Alps has to do with the well-established science tradition in Europe in the 19th and beginning of the 20th century, which resulted in a series of important inventions to measure climate and glacier properties. Even at that time, knowledge was gained that is still valid in the early 21st century (e.g., the climate is changing and fluctuating; glacier changes are caused by changing climate; and the ice age was the result of shifting climate). Above all others, Albrecht Penck and Eduard Brückner were the key scientists in this blossoming era of glacier climatology. Interest in a better understanding of the relationship of climate to glaciers was not only driven by curiosity, but also by several impacts of glaciers on human life in the Alps. Investigations of climate–glacier relationships in the Alps began with the expiration of the Little Ice Age (LIA) period when glaciers were particularly large but began to retreat significantly. Observations of post-LIA glacier front positions showed a sharp decline after their maximum extent in about 1850 until the turn of the 19th to 20th centuries, when they began to grow and advance again. They were also forming a prominent moraine around 1920, which was, however, far behind the 1850 extent. Interestingly, climate time series of the post LIA period show a general long-term cooling of summer temperatures and several decades of precipitation deficit in the second half of the 19th century. Thus, the retreat forced by climate changes cannot be simply explained by increasing air temperatures, though calibrated glacier mass balance models are able to simulate this period quite well. Additional effects related to the albedo could be a source for a better understanding. From 1920 onward, the climate moved into a period of warm and high-sunshine summers, which peaked in the 1940s until 1950. Glaciers started again to melt strongly and related discharges of pro-glacial rivers were exceptionally high during this period as glaciers were still quite large and the available energy for melt from radiation was enhanced. With the shift of the Atlantic meridional overturning (AMO), which is an important driver of European climate, into a negative mode in the 1960s, the mass balances of Alpine glaciers experienced more and more positive mass balance years. This finally resulted in a period of advancing glaciers and the development of frontal moraines around 1980 for a large number of glaciers. Thereafter, from 1980 onward, Alpine glaciers moved into an era of continuous negative mass balances and particularly strong retreat. The anthropogenic forcing from greenhouse gases together with global brightening and the increase of anticyclonic weather types in summer moved the climate and thus the mass balances of glaciers into a state far away from equilibrium. Given available scenarios of future climate, this retreat will continue and, even under the optimistic RCP2.6 scenario, glaciers (as derived from model simulations for the future) will not return to an equilibrium mass balance before the end of the 21st century. According to a glacier inventory for the European Alps from Landsat Thematic Mapper scenes of 2003, published by Paul and coworkers in 2011, the total surface of all glaciers and ice patches in the European Alps in 2003 was 2,056 km² (50% in Switzerland, 19% in Italy, 18% in Austria, 13% in France, and <1% in Germany). Generally, the reaction of Alpine glaciers to climate perturbations is rather well understood. For the glaciers of the Alps, important processes of glacier changes are related to the surface energy balance during the ablation season when radiation is the primary source of energy for snow and ice melt. Other ablation processes, such as sublimation and internal and basal ablation, are small compared to surface melt. This specificity enables the use of simple temperature-based models to simulate the mass balance of glaciers sufficiently well. Besides atmospheric forcing of glacier mass balance, glacier flow (which is related to englacial temperature distribution) plays a role, in particular, for observed front position changes of glaciers. Glaciers are continuously adapting their size to the climate, which could work much faster for smaller glaciers compared to large valley glaciers of the Alps having a response time of about 100 years.

Terrestrial Processes and Their Roles in Climate Change

The land surface is where humans live and where they source their water and food. The land surface plays an important role in climate and anthropogenic climate change both as a driver of change and as a system that responds to change. Soils and vegetation influence the exchanges of water, energy and carbon between the land and the overlying atmosphere and thus contribute to the variability and the evolution of climate. But the role of the land in climate is scale dependent which means different processes matter on different timescales and over different spatial scales. Climate change alters the functioning of the land with changes in the seasonal cycle of ecosystem growth, in the extent of forests, the melt of permafrost, the magnitude and frequency of disturbances such as fire, drought, … Those changes feedback into climate at both the global and the regional scales. In addition, humans perturb the land conditions via deforestation, irrigation, urbanization, … and this directly affects climatic conditions at the local to regional scales with also sometimes global consequences via the release of greenhouse gases. Not accounting for land surface processes in climate modelling, whatever the spatial scale, will result in biases in the climate simulations.

Plant Phenology of the European Alps

Phenology is the study of the seasonal timing of life cycle events. The Belgian botanist Charles Morren introduced the term in 1853, which is a combination of two Greek words, φαίνω, which means to show, to bring to light, make to appear, and λόγος, which means study, discourse, or reasoning. The global change discussion has stimulated phenological research, which as a consequence greatly advanced as science and evolved to one of the main climate impact indicators. Many of the earliest systematic efforts to collect phenological observations took place in countries sharing the Alps, most of which are still operating phenological networks. These phenological data sets are generally freely available to researchers, and numerous essential contributions to the topic of phenology and climate have been built on those data sets. Plant physiological processes underlying the ability of the plants to adapt to the year-to-year variability of the climate still constitutes largely a black box. Since the experiments of René Antoine Ferchault de Reaumur in the 18th century, it is known that temperature constitutes the main environmental driver of the seasonal development of the mid- to high-latitude plants. Second to temperature, day length governs the seasonal cycle of some species as an additional factor. Therefore, temperature-driven phenological models are able to simulate the year-to-year variability of phenological entry dates accurately enough for various applications, such as climate change impact research or numerical pollen forecast models, where the beginning of flowering of some plants is linked with the release of allergic pollen into the atmosphere. Large-scale circulation patterns, like the North Atlantic Oscillation, determine the frequency and intensity of warm and cold spells and decadal temperature trends over Europe. Combined anthropogenic and natural forcings explain the advance of spring phenology over the last 50 years, which is also clearly discernible in the area of the Alps. The early phenological spring starts in Western Europe, whereas later in the season it makes progress with a stronger southerly component across the Alps. The combined temporal and spatial trends have been studied along elevational gradients. Trends toward earlier entry dates are stronger at higher elevations, which indicates that the elevational phenological gradient has weakened since the mid-20th century. Similarly, the vegetation response to temperature is observed to decrease when moving from high to low latitudes. In contrast, the temporal response of plant phenology to increasing temperatures is less clear. Some works indeed demonstrate a decreasing temperature sensitivity with increasing temperature, which is explained as a result of a reduced winter chilling that delays spring phenology or of a limiting effect due to a shorter photoperiod. Other works report no change of temporal temperature sensitivity with increasing temperatures. Indigenous midlatitude vegetation is able to withstand large temperature variations during winter and spring. The safety margin between last frost events, budding, and leaf emergence was found to be uniform across elevations and taxa, except for beech trees. The probability of freezing damage to natural vegetation is almost nil, but late frost risk constitutes a real threat to fruit growers. The ratio of phenological and last frost trends is ambiguous. An increase or decrease in frost risk depends on regions, elevations, and species. Vegetation at high altitudes is exposed to a harsh climate with a long-lasting snow cover, low temperatures, and a short growing season. Snowmelt is a necessary but insufficient requirement for the start of the growing season, which has to be supplemented by plant-specific temperature sums to activate the growth of most alpine and subalpine species. The seasonal cycle has to be completed within a short time. Advances in remote sensing technology have provided access to high-resolution landscape scale phenological information. Especially in remote areas, like the Alps, in situ observations could be supplemented by satellite observations. Observations from both methods, I -situ and remote sensing, have been applied to describe spring vegetation dynamics, but the correlation between these data sets have typically been weak because of differences in temporal and spatial scales and resolutions. A successfully combined description of the seasonal vegetation cycle is still lacking. The area of the European Alps offers a wealth of long chronicles, containing historical phenological observations some of which have been extracted and digitized. Grape harvest dates belong to the most readily available historical phenological observations, which have helped reconstruct summer temperatures as far back as the 15th century.

Chinese Meteorology During World War II

Meteorology and military activities in China were closely interrelated during World War II. When the Second Sino-Japanese War broke out in 1937, the Nationalist government, under ferocious assault by the Japanese military, withdrew deep into the Chinese interior. Meteorological research organizations and the air force also relocated to Sichuan, the latter setting up weather stations in the southwest and the northwest and reorganizing the armed forces’ meteorological intelligence system while the former made use of the resulting meteorological data to research various weather phenomena in western China, thereby shifting the focus of meteorology in China away from the coastal regions. However, by the start of World War II, aviation had already become an important means of waging war, and high-altitude weather data was highly sought after as military intelligence. Consequently, after instigating the war, Japan extended its meteorological stations in northwest China, engaged in high-altitude surveying and observation, and created an information system between the Japanese home territory and colonies. Japanese analysis of the resulting weather data maintained the safety of flight routes and was used for formulating military strategy. The Chinese government, in contrast, having recently relocated and with a weak air force, lacked the power to expand research on aeronautical meteorology during the initial phase of the war. It was not until after becoming allied with the United States in December 1941 that the government was able, with American technical support, to begin expanding meteorological observation posts and conducting high-altitude surveying and observation. Moreover, the inauguration of flights over the aerial supply route known as the Hump resulted in the discovery of the jet stream over the towering mountain ranges of southwestern China. World War II opened up the Chinese interior for meteorological research and, as a result of military applications, brought about greater understanding of high-altitude meteorology.

Saharan Dust Records and Its Impact in the European Alps

Mineral dust is one of the main natural sources of atmospheric particulate matter, with the Sahara being one of the most important source regions for the occurrence and deposition of mineral dust in Europe. The occurrence of dust events in the European Alps is documented via measurements of airborne dust and its deposits onto the glaciers. Dust events occur mainly in spring, summer, and early autumn. Dust layers are investigated in ice cores spanning the last millennium as well as in annual snow packs. They strongly affect the overall flux of dust-related compounds (e.g., calcium and magnesium), provide an alkaline input to wet deposition chemistry, and change the microbial abundance and diversity of the snow pack. Still airborne mineral dust particles can act as ice nuclei and cloud condensation nuclei, influencing the formation of cloud droplets and hence cloud formation and precipitation. Dust deposits on the snow lead to a darkening of the surface, referred to as “surface albedo reduction,” which influences the timing of the snowmelt and reduces the annual mass balance of glaciers, showing a direct link to glacier retreat as observed presently in a warming climate.

Zhu Kezhen 竺可楨 (1890–1974)

Zhu Kezhen (1890–1974), also known as Chu Coching, was a Harvard-educated meteorologist who worked in the field of climate sciences in China from 1918 to 1974. He was highly regarded under vastly different political regimes. His concerns regarding the development of observatory networks, educational practices, and the establishment of research topics reflect the development of the field in China, which only began at the very end of the 19th century. Zhu Kezhen was influenced by the meteorological and climate knowledge imparted to him by his academic teachers in the United States and appropriated Ellsworth Huntington’s ideas on climate determinism, which shaped some of his fundamental concerns. One of his main achievements was to make use of a wide array of observational and other data in order to contribute to the “localization” of climate science. In fact, employing data culled from traditional sources and making use of and expanding the phenological knowledge of traditional Chinese rural society allowed him to approach climate science in a way that was not easily possible in the West. Zhu’s research into historical climate change in China embodied many aspects of his approach to the localization of science in China, but changes in the international scientific network (from an American-European to a Soviet-dominated network) and the political turmoil in the People’s Republic of China greatly impaired his work. Zhu’s research remains highly influential and has exerted considerable influence on environmental and climate history.

Perceptions of and Resilience to Coastal Climate Risks

Coasts are dynamic places operated on by powerful natural and human forces. They are also historically attractive places for human settlement and use, with a still constantly growing concentration of people due to increased population growth and migration toward the coast. Coastal societies historically have evolved and developed culturally embedded relationships with their environment, which have resulted in different cultural settings, influencing the way they experience and react toward climate change impacts in their lifeworld. Coastal risks are specific to different regional, natural, and societal settings and can be distinguished between slow-onset (e.g., sea level rise or ocean acidification) and sudden extreme events (e.g., tropical cyclones or storm surges). Coastal climate risks come from flooding, storms, storm surges, saltwater intrusion, invasive species, declining fish stocks or shifting species’ regimes, coral bleaching, coastal erosion, and morphological change. For centuries, coastal societies have learned to defend the coast against threats from the sea with a broad range of technical measures based on a long history of trial and error, with successes and failures. Further, for centuries, littoral societies have constructed coasts and infrastructure according to their interests and needs (e.g., engineering the coastline, installing coastal defenses, constructing harbor and landing infrastructures, and even claiming land from the sea). Risks at the coast have always been there—but are exacerbated by climate change. A more integrated and transdisciplinary approach to understanding coastal climate risks is required, in keeping with the characterization of climate change as a wicked problem. The ways in which individuals, societies, and politics respond to climate change are in many cases contingent on perceptions of its causes, consequences, and wider implications. To study climate change impacts, therefore, an improved understand is required of the place-specific perception of coast and of coastal climate risks. These perceptions, along with other influencing factors, such as economic interests and politics, will inform the societal resilience and response of a coastal community. Resilience—understood as people’s ability to respond adequately to shocks and stressors—is place-dependent and closely connected to historic experiences and learning processes in dealing with hazards as well as the existing political and institutional arrangements that underpin governance structures. Resilience does not simply reflect the expected effects of quantifiable factors such as level of assets, or even less quantifiable social processes such as people’s experience, but is also determined by more subjective dimensions related to people’s perceptions of their ability to cope, adapt, or transform in the face of adverse events. Based on the existing place-specific experience of the littoral society, with its liminal environment and development, adaptation strategies and policies for the future need to be developed between the extremes of “living with” and “making way for” coastal and climate changes. Against this background, climate change adaptation (CCA) strategies have to be integrated and merged with disaster risk reduction (DRR) challenges, based on the integration of multiple interests in a transdisciplinary way. Societal risk construction and negotiation are crucial elements of integrative risk management, requiring participative, transparent, and flexible processes for the implementation of discursive practices and—in extreme situations—the transformation of governance structures. To understand and evaluate climate change adaptation strategies and measures along the coastline, climate change impacts threatening coastal livelihoods have to be understood alongside the societal frames of CCA policies. The capacity to adapt to changing conditions is based on the ability to develop new risk cultures and the flexibility to transition by (a) developing new norms, practices, and material culture; (b) resisting the lock-ins from routines and habits; and (c) guiding changes through scrutinizing new options or creating technocultural niches that favor certain technologies over others. Adaptive capacity in coastal societies plays an important role in dealing with coastal climate risks. The focal questions are the following: Which societal frames of climate change perception precondition adaptation? Which risks are perceived? Which cultural and political barriers hinder successful adaptation? How can DRR be integrated in CCA endeavors and future climate-resilient and sustainable pathways?

History and Importance of Mountain Observatories in Alpine Climate

Mountain observatories have played an important role in developing scientific research since the 18th century. These alpine observatories have been used by numerous scientists who have carried out a wide range of investigations, and have thus been able to establish significant meteorological findings. They were established to better understand atmospheric properties, such as dynamics, and are now used for climate and environmental science in addition to astronomy. The data measured at mountain observatories provide information on the climatic conditions of certain alpine regions and show that even more high-altitude stations are needed to better understand climatic and environmental changes in the 21st century.

Analog Models for Empirical-Statistical Downscaling

Global climate models (GCM) are fundamental tools for weather forecasting and climate predictions at different time scales, from intraseasonal prediction to climate change projections. Their design allows GCMs to simulate the global climate adequately, but they are not able to skillfully simulate local/regional climates. Consequently, downscaling and bias correction methods are increasingly needed and applied for generating useful local and regional climate information from the coarse GCM resolution. Empirical-statistical downscaling (ESD) methods generate climate information at the local scale or with a greater resolution than that achieved by GCM by means of empirical or statistical relationships between large-scale atmospheric variables and the local observed climate. As a counterpart approach, dynamical downscaling is based on regional climate models that simulate regional climate processes with a greater spatial resolution, using GCM fields as initial or boundary conditions. Various ESD methods can be classified according to different criteria, depending on their approach, implementation, and application. In general terms, ESD methods can be categorized into subgroups that include transfer functions or regression models (either linear or nonlinear), weather generators, and weather typing methods and analogs. Although these methods can be grouped into different categories, they can also be combined to generate more sophisticated downscaling methods. In the last group, weather typing and analogs, the methods relate the occurrence of particular weather classes to local and regional weather conditions. In particular, the analog method is based on finding atmospheric states in the historical record that are similar to the atmospheric state on a given target day. Then, the corresponding historical local weather conditions are used to estimate local weather conditions on the target day. The analog method is a relatively simple technique that has been extensively used as a benchmark method in statistical downscaling applications. Of easy construction and applicability to any predictand variable, it has shown to perform as well as other more sophisticated methods. These attributes have inspired its application in diverse studies around the world that explore its ability to simulate different characteristics of regional climates.

Elevation-Dependent Climate Change in the European Alps

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.