The occurrence of rockfall has been observed at high resolution and high frequency in Switzerland during the period 2017–2021. These observations were made at a height in the Alps near the lower limit of permafrost occurrence. All major observed rockfall events occurred in summer, and many of them in the second half of summer and in the (late) afternoon. Rapid permafrost degradation is viewed as a triggering factor of these events, causing progressive failure of the rock wall. This indicates that climate change may already be an important factor in triggering rockfall. The cementing effect of the ice-bonded discontinuities in the rock decreases progressively during the summer at higher temperatures. The observed temporal clustering of the rockfall events in late summer has an important implication for hazard management, potentially endangering hikers making use of the hiking trail in summer (49).

Since the mid-1980s there have been five major rock slides of more than 1 million m3 in the Alps: Veltlin in 1987, Randa in 1991, Mont Blanc-Brenvaflanke in 1997, Thurwiserspitze/Ortler in 2004, and Eiger in 2006. The slide
path sometimes reached far below the forest line (Veltlin, Randa, Mont Blanc) and all except Eiger affected tourism areas (roads, ski slopes, hiking trails). The relationship to glaciers and permafrost has been proven for three of these cases (Mont Blanc, Ortler, Eiger), in the other two it is probable (Veltlin) or likely, but uncertain (Randa) (16).

In the summer of 2003, the 0°C limit rose above 4500 meters elevation for 10 days. This unusual duration increased impacts, especially on shady rock walls at high altitudes, where thaw penetrated considerably deeper than in previous (warm) years and some areas may have been exposed to thawing for the first time. … Thaw penetration below the surface was up to 2 meters deeper than in previous years. These extreme conditions caused widespread but relatively small rock falls, such as the one which forced the evacuation of 90 persons on the Matterhorn. However, deep penetration of permafrost warming and thawing over longer time periods (decades, centuries) might cause larger landslides and rock falls at high altitudes (3).

During the last century, Alpine permafrost in Europe has warmed by 0.5 to 0.8⁰C in the upper tens of meters (5). The stability of steep rock slopes can be reduced due to melting of ice-filled crevices and subsequent build-up of hydrostatic pressure (6). Even prior to thaw, frozen rock joints have been demonstrated to destabilize with rising temperatures, entering a zone of minimal stability between -1.5 and 0⁰C (7). The exceptional rock fall activity during the summer 2003 is likely an indication of this rapid destabilization that takes place as an almost immediate reaction to extreme warming.

In northern slopes, the depth of thaw is mainly controlled by the influence of air temperature (mostly via long-wave radiation) on surface temperatures, whereas southern slopes additionally receive high amounts of shortwave radiation. As a consequence, southern slopes exhibit greater inter-annual variability of thaw depth, larger pre-2003 maxima and, therefore, a smaller 2003 anomaly. … The observed domination of events in northern slopes can be explained by the strong effect of 2003 as well as the greater extent of perennially frozen northern slopes (1).

Large landslides near glaciers

Ice volume reduction of retreating glaciers leads to debuttressing of oversteepened valley flanks, causing landslides (39) and catastrophic rock falls (40). Failure of such landslides poses significant risks to surrounding settlements and critical infrastructure, because of the formation, and possible breakout of landslide dams, as well as enhanced sediment production, which can lead to debris and mud flows (38). The Great Aletsch Glacier in southwest Switzerland is one of these glaciers where large landslides directly respond to glacier ice loss. Since 1870, the glacier retreated 3 km to its present-day position with a length of about 22 km. For one of these landslides it was found that changes in glacier ice height spatially correlate to rock failures and deformation features at the toe of the landslide. Over a large area the mountain slope reacts to glacier retreat once debuttressing through glacier ice height reduction reaches a critical threshold (38). Outburst floods from landslide-dammed lakes are reported worldwide in areas that undergo deglaciation (41). Knowledge on landslides near the Great Aletsch Glacier suggests that these floods may occur more frequent as a result of on-going rapid deglaciation and destabilization of critical rock slopes (38). 

Along with its discharge the stream’s sediment transport capacity increased, but sediment export did not respond in the same way. The impact of stream discharge increase on sediment transport is not straightforward. Sediment transport depends on the volumes of sediment upstream that are available for transport. If this sediment cannot reach the stream, than discharge may increase but sediment transport increase is lagging behind. This ineffectiveness of available sediment upstream reaching the stream may reduce the extent to which climate forcing of landscape response can be detected in valley-bottom sediment delivery and deposits. Rather, increasing sediment export is a function of extreme events that can move sediments to the streams (35).

As for the future, water yield is likely to increase further, at least until it becomes limited by available ice to melt, and with it sediment transport capacity in the drainage network. According to research (37) maximum annual runoff in Alpine watersheds may be reached by 2050, followed by a tendency to decreasing and/or more variable water yield, although it is still unclear how this timing applies to small, heavily debris-covered glaciers. 

Projections of climate change between 2010 and 2100 have been translated into future debris flow activity for two Alpine catchments, one in France (Barcelonnette Basin) and one in Italy (Fella River catchment) (29). These projections are based on a number of regional climate models (driven by general circulation models) and both an intermediate and high-end scenario of climate change (the so-called RCP4.5 and RCP8.5 scenarios, respectively). From the model outcomes two meteorological proxies for debris flow activity were derived and translated into future debris flow activity: one using 1-day rainfall amounts and one being representative of short-lasting convective rainfall systems. The 1-day rain-proxy was chosen as precipitation is a dominant trigger of debris flows. As rain gauges are very local and often non-catchment representative, the second proxy was used to mimic the mesoscale atmospheric conditions leading to heavy local convective precipitation. All of these projections show either an increase or little change in the number of days with debris flows between now and 2100 (29). 

The overall frequency of debris flows may decrease in absolute terms, but the magnitude of events may increase. This was concluded from an analysis of debris flow events in the past 150 years and using this information for future projections on climate change (regional climate model runs driven by the A1B emission scenario) (26). According to this research, the overall absolute number of days with conditions favorable for the release of debris flows will likely decrease in the second half of the century. A prolongation of the debris-flow season is projected, however, in line with what has been observed as a result of climate warming since 1864 (27). Events might become possible in March, especially in the late 21st century, and in November and December (26).

Rising temperatures and melting permafrost will destabilise mountain walls and increase the frequency of rock falls, threatening mountain valleys (3,5). Besides, higher temperatures might lead to changes in the frequency and intensity of snow and ice avalanches (2).

The vulnerability of settlements and infrastructure to natural hazards, such as flash-floods, avalanches, landslides, rock fall and mudflows will increase due to heavy rain- and snowfalls and the upward shift of the permafrost line. Retreat of permafrost also destabilises the infrastructure in high altitudes like lodges, lifts and top stations (3).

Past and modeled future climatic change is likely more pronounced in mountain areas than the global or hemispheric average (8). … The extreme summer of 2003 and its impact on mountain permafrost may be seen as a first manifestation of the projections of climate change. Wide-spread rock fall and geotechnical problems with human infrastructure are likely to be recurrent consequences of warming permafrost in rock walls due to predicted climatic changes (1).

In wintertime, the seasonal freezing level (altitude, where surface air temperature is 0°C) has risen by about 200 m per degree of warming from approximately 600 m in the 1960s to approximately 900 m in the 1990s (14). If warming in winter continues as expected, the freezing level will further rise by about 180 m until 2050 in case of moderate warming (+0.9°C), by about 360 m in case of medium warming (+1.8°C), and by about 680 m in case of strong warming (+3.4°C) (15). The freezing level roughly corresponds to the height of the snow line (the lower limit of the snow cap). The zone of warm permafrost (mean annual rock temperature approximately -2 to 0°C), which is more susceptible to slope failures than cold permafrost, may rise in elevation a few hundred meters during the next 100 years (18). This in turn may shift the zone of enhanced instability and landslide initiation toward higher-elevation slopes that in many regions are steeper, and therefore predisposed to failure.

Natural hazards in the Alpine countries have resulted in economic losses of EUR 57 billion over the 1982-2005 period (insured losses EUR 10,5 billion). This is without counting the large investments made by Alpine countries in protection and prevention measures (9).

Avalanches have claimed an average of 100 victims per year in the Alps over the last 30 years (11). Victims are primarily skiers. There is no clear trend in the frequency of avalanches (9). A change in avalanche hazards in connection with climate change is uncertain. In general, it is assumed that the possible change would follow the snow cover evolution. A decrease in avalanche hazards is likely at low and medium altitudes. Yet, heavy precipitation events might counterbalance this trend by triggering general avalanche situations (17).

Landslides and mudflows are expected to increase due to more intense precipitations during summertime. On the other hand, the possible future decrease of total summer precipitation may have a positive effect by reducing the deep and shallow landslides activity (17). Frequency of rock falls is also expected to increase due to changes in the spring thawing process (9,13).

The projected glacier retreat in the 21st century may form new potentially unstable lakes. Probable sites of new lakes have been identified for some alpine glaciers (19). Rock slope and moraine failures may trigger damaging surge waves and outburst floods from these lakes. In the short-term, increased summer floods are expected from glacier melt. In the long-term, reduced summer river flows are expected (9).

Snow avalanches in the Swiss Alps

The assumption that the diminishing snow cover in the mountains during the winter season will lead to less avalanches is too simplistic, according to a recent study for the Swiss Alps (51). Mere knowledge about changes in duration and depth of the seasonal snow cover is not sufficient to determine the effect of climate change on snow avalanche occurrence, the authors of this study argue. They highlight the importance of distinguishing between dry- and wet-snow avalanches.

The stability of a snowpack not only depends on its thickness, but also on the vertical layering of the snowpack under the influence of factors such as precipitation, temperature, wind, and radiation throughout a winter season. This layering is a key factor in the formation of snow avalanches (51).

For wet- and dry-snow avalanches, the underlying physical processes are different. Dry-snow avalanches mostly release during or shortly after a snowstorm, or by skiers. Wet-snow avalanches, on the other hand, form when liquid water from melting or rain-on-snow events infiltrates the snowpack and reduces the strength of existing weak layers. Climate change will have a different impact on these processes, and therefore on the occurrence of wet- and dry-snow avalanches (51).

The authors made future projections of changes in avalanche activity by feeding climate model projections into a model that simulates the vertical snow stratigraphy over time. The study shows that the occurrence of dry-snow avalanches will indeed decline this century, but this decline will be partially compensated for by an increase in wet-snow avalanche activity. Currently, the number of avalanche days in the Swiss Alps is 45–75 per year. This number may decrease by up to 65% by the end of the century under a high-end scenario of climate change (RCP8.5). On the other hand, under the same scenario, the number of wet-snow avalanche days may increase by up to 30%, as the snowpack is more frequently wetted and weakened by meltwater or rain (51).

As a result, the net effect the authors anticipate across a range of climate change scenarios is a reduction in the total avalanche activity ranging from under 10% to as much as 60% by the end of the century (51).

These findings are supported by observations. Rising temperatures have led to a decrease in the number and size of avalanches (52), and to an increase in the proportion of wet-snow avalanches (53).

There is medium confidence that high-mountain debris flows will begin earlier in the year because of earlier snowmelt, and that continued mountain permafrost degradation and glacier retreat will further decrease the stability of rock slopes. There is low confidence regarding future locations and timing of large rock avalanches, as these depend on local geological conditions and other non-climatic factors (20). Research has not yet provided any clear indication of a change in the frequency of debris flows due to recent deglaciation. In the French Alps, for instance, no significant change in debris flow frequency has been observed since the 1950s in terrain above elevations of 2,200 m (22). Processes not, or not directly, driven by climate, such as sediment yield, can also be important for changes in the magnitude or frequency of alpine debris flows (23).

Future projections mostly indicate an overall decrease in snow depth and snow cover duration at lower elevation, but the probability of occurrence of occasionally large snow precipitation events is projected to remain possible throughout most of the 21st century (42). An overall 20 and 30% decrease of natural avalanche activity in the French Alps is estimated for the mid and end of the 21st century, respectively, under a moderate (A1B) scenario of climate change, compared to the reference period 1960 – 1990 (47). The overall number and runout distance of snow avalanches is projected to decrease in regions and elevations experiencing significant reduction in snow cover (48)

Avalanches involving wet snow are projected to occur more frequently during the winter at all elevations due to surface melt or rain-on-snow (47, for the French Alps).

In summary, there is medium evidence and high agreement that observed changes in avalanches in mountain regions will be exacerbated in the future, with generally a decrease in hazard at lower elevation, and mixed changes at higher elevation (increase in avalanches involving wet snow, no clear direction of trend for overall avalanche activity) (42).

The Swiss resort of Pontresina sits right below a mass of warming permafrost. When the permafrost melts in the debris-covered areas, at first moment nothing happens, but strong rainfalls increase the danger of debris flows. Flooding and rock falls have drawn Switzerland's attention about the natural hazards the country faces as a result of global warming. Pontresina has taken a proactive approach to protect itself against the effects of climate change. Four years ago a dam was built. The project was expensive and no one knows if it will stand if it ever sees a landslide, but is regarded as an insurance. The dam is estimated to stop 100,000 cubic meters of stone or 250,000 cubic meters of snow (10).

Integrated cryospheric observation and modelling approaches of glaciers, permafrost and related impacts are needed. Such work is greatly facilitated by new earth observation and geoinformatics technologies. Interdisciplinary collaboration between modelling and earth observation experts, field-oriented scientists, and practitioners is best suited to ensure a sound response to new requirements for the monitoring and modelling of mountain glaciers and permafrost, and related hazards (12).

Varied species composition and sustainable use increase the resistance and stability of natural ecosystems, and best ensure human living space. Climate change can be influenced only slowly. However, we can change the way we use our living space more quickly. In so doing, we can also much more rapidly achieve a sustainable effect in order to maintain the protective functions of ecosystems (16).

• Improve public preparedness and personal responsibility by encouraging participation in emergency planning. To properly inform the public, risk management plans must address both emergency preparedness and early warning systems.
• Incorporate climate change adaptation into spatial planning. A few examples to create a sustainable regional development that is less vulnerable to natural hazards are: financial incentives, establishing hazard zones, setting appropriate construction standards of buildings and infrastructure in risk areas, keeping endangered spaces free of development, and performing hazard assessments through the use of hazard mapping.
• Involve local stakeholders in a risk dialogue. The dialogue includes meetings between important stakeholders, such as land and real estate owners, as well as those responsible for infrastructure and the public sector.
• Encourage cross-border networking on integrated risk management.
• Encourage a ‘common language’ and harmonised procedures when developing and using hazard maps.
• Increase the size of flood plains, floodwater conduits and basins. Governments need to consider multiple uses of the same land and consider strict legal binding instruments that ensure a priority for flood retention areas is given.
• Think of flood risk management in terms of an entire river basin to find solutions that are sustainable. Horizontal and vertical cooperation between all levels of government and the private sector are essential.
• When planning for natural hazard risks consider all the environmental risks within a defined area. Natural hazards—floods, droughts, landslides—generate risks that are interrelated and so should be addressed jointly.
• Use risk-management tools to explore the social and economic consequences of various adaptation measures. Risk planning tools allow for the integration of a wide range of strategies that reduce the risks of natural hazards, including spatial planning instruments, technical protection structures, specific protection measures for individual buildings and early-warning systems.
• Support the collection and interpretation of local climate change data.