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.
Ground surface temperature reconstruction for the last 500 years obtained from permafrost temperatures observed in the SHARE STELVIO Borehole, Italian Alps