Analysis - Avalanches

Climate change impacts on NATURAL EVENTS

3.3 AVALANCHES

Results and interpretation

Observation and analysis methods

Switzerland:
The occurrence of destructive snow avalanches in Switzerland between 1947-1993 shows no obvious trend. The situation in the surroundings of Davos shows a similar feature. However, the magnitude of the large events decrease systematically. This can be explained by the defence structures.

Swiss Alps:
During the past three centuries, periods of high avalanche activity have generally coincided with cold, damp winters. The climate has not changed enough during the 20th century to produce any palpable effects on avalanche activity. Unlike glaciers, avalanche react more to sudden extrem fluctuations than to long-term climate change. Hence they cannot be used as indicators of climate change during the past centuries.

The informations comes from records and articles covering the whole Switzerland. The data on avalanche activity have been compared with meteorological data from Davos; Bever and Andermatt.

Maurienne massif:
For both low- and high-frequency avalanche tracks, frequencies correlated closely with the number of times in the winter that precipitation was recorded during at least 3 successive days. The relationship was even stronger when calculated separately for each of the two groups of avalanche tracks.

Alps:
• Avalanches are events often related to extreme meteorological phenomena (heavy precipitation, fast temperature increase…) which make difficult the detection of trends.
• Attempts have been done to link snow cover (which shows clear trends) with the avalanche activity, but they generally do not give significant results. Snow cover data can not solely explain avalanche activity.
• Evolution of meteorological parameters explains only partially the avalanche activity (for certain avalanche types).
• Besides, this activity can be considerably perturbed by construction of avalanche barrier structures or by vegetation development. Thus the study of the climatic influence is often impossible.

Eastern Switzerland:
If both the snow depth and the 3-day new snow exceed 75 cm, the probability of a disastrous avalanche day increase significantly. There is strong evidence that the mean potential avalanche activity (PAA), at least for the Davos area, has remained the same for the past 96 years (1896-1993). Periods of more or less intensive PAA could be detected, but they show no systematic trend or periodicity.

To infer the avalanche activity before 1947, the avalanche activity was modelled using meteorological data. The correlation of meteorological factors with destructive avalanches was used to find a threshold value where numerous destructive avalanches occur.

Maurienne massif:
The probability of occurrence of snowmelt avalanche increased strongly when precipitation exceeded 40 mm. The effect of temperature became clear when precipitation was above 50 mm. Finally, total precipitation on the day of a given event and the day before were most significant, especially when precipitation totalled 60 mm.

Swiss Alps:
Early moistening of the snow cover as a result of a warmer atmosphere will indeed create avalanche of heavy or wet snow. Taken over the whole year, there should be the same number of destructive avalanche events involving heavy snow. Considered as an average and ignoring any fluctuations, a slight climate change will not affect avalanche activity fundamentally. A 1-2°C variation and a 10-20% variation in precipitation will not influence the occurence of extreme meteorological conditions leading to avalanche.

Alps:
The tree and shrubbery vegetation cover and more especially the tree cover play an important rule in the snow cover stabilisation, mainly with the anchorage due to the trunks and the interception by the branches. The forests cover disappearing leads also to a change in the snow evolution and of the snow cover mechanical characteristics, and therefore on the snow cover stability. Some snow flows could be expected in such cases, depending on the importance of the vegetation cover disappearing.

French and Swiss Alps:
The impact of the temperature increase on the avalanche activity seems to be much more reduced than on the snow cover.

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AVALANCHE INTENSITY

Results and interpretation

Observation and analysis methods

Swiss Alps:
The strong intensity avalanche events occur at irregular interval, with an average of an event each 2.5 years for the 1947-1993 period. Events of high to disastrous intensity occur at much longer intervals, several decade or even a whole century. No marked change in the avalanche situation in the northern Alps over the 1881-1992 period. Slight general increase of the avalanche situation in the southern Alps since 1945.

The informations comes from records and articles covering the whole Switzerland.

Alps:
20th century : no clear trend concerning damaging avalanches observed, no clear change in avalanche activity.

Alps:
Events like the 1999 snowy and avalanche winter can be considered as rare today but also “normal” and possible in the future.

French and Swiss Alps:
While the climatic changes of the last decades are often seen as the natural catastrophe increase origin, there is no reason to think that in the future there would be a change in the avalanche activity, especially on the catastrophic situations.

German Alps:
One should expect an avalanche increase during winter because of the extreme precipitation increase, faster wind speeds tending to create snow accumulation.

French Alps:
An increased frequency of localised precipitation events at high altitude can let suppose an incidence on the triggering of heavy-snow avalanches. More ice blocks and rocks in avalanches from glacial watershed (because of the glacier retreat, previously not mobilisable materials become mobilisable) is an eventuality.

According to M.Badré and D.Laurens IGE report (01/23/06): “Concerning avalanches, among which the triggering and propagation undergo complex physical processes, new situations become possible. The trajectory of corridor avalanches should not be appreciably modified by these evolutions, except in case of topographic modifications, resulting from ice melt in transit and triggering zones.”

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AVALANCHE FREQUENCY

Results and interpretation

Observation and analysis methods

Central Europe:
High avalanche activity periods have been identified during the last 500 years; they correspond to wet and cold winter high frequency, especially around 1720, 1775 and 1817. Different meteorological situations can be identified during the exact event’s days: North-West perturbations, South flows, wet snow and rain fall coming from South-West flows on the southern Alpine flanks, etc.

Historic documents analysis. Some criteria have been used to compare past events to recent avalanche events. An avalanche list, with associated damages has been provided for Switzerland and neighbouring countries.

Ecrins massif:
Lichenometry shows a diachronicity in sedimentation on avalanche boulder tongues during the LIA. According to the lichen patterns, an irregular decrease of avalanche run-out distances has occurred for these last centuries. However, a distinction must be made according to the elevation.

Before 1650 the frequency of avalanches was sufficiently high to prevent lichens from colonizing the deposits. Between 1650 and 1730, avalanche activity decreased, allowing free development of lichens on the surface of snow avalanche fans. Between 1730 and 1830, the activity seems to have increased, only occasional lichens located on protected sides of blocks surviving this erosive activity. From 1830 to 1850, according to the mean of the 5 largest lichens, the frequency reaching the distal zone was extremely low. It may reflect a decrease in the quantity of debris transported by avalanches, reducing their erosive power. Between 1830 and present, 2 contrasting periods can be distinguished. Because lichen size decreases from the base to the median part of all deposits, the magnitude of avalanches has decreased to 30% between 1830 and 1950 (lower frequency). From 1950, magnitude is low but frequency remains high in the upper part of the deposits.

At low elevation, the variation of the avalanche activity during the LIA is different. The lack of a clear hiatus in the distribution of the lichens suggests that there was a maximum in avalanche activity around 1600-1650 (according to the lichen growth curve extrapolation). Since around 1700, colonization in the distal zone is observed, which is due to a decrease in activity of avalanche process. The phase observed around 1830, during which maximum run-out distances at high elevation occurred, also exists but the activity was less intense.

Measurements of Rhizocarpon sp.lichens were perfomed during two fields campaigns on 25 deposits for which geometrical and sedimentological characteistics have been studied previously.

The location of lichens on boulders, in relation to the supposed trajectories of avalanches was also noted. Between 15 and 70 measurements (one measuremet on each block) were made within each sample area. To compare the lichen patterns between deposits, the lenght deposit was measured over a constant segment of 10 m. To date the avalanche activty, a lichenometric growth curve was used.

Eastern Switzerland :
The occurrence of destructive snow avalanches in Switzerland between 1947-1993 shows no obvious trend.

Direct correlation of long-term, daily meteorological data to avalanches events

Swiss Alps:
Winters with devastating avalanches are homogeneously divided; there is no increase of their frequency during the 20th century.

Swiss Alps:
During the past three centuries, periods of high avalanche activity have generally coincided with cold, damp winters. High-intensity events occur at very irregular intervals, with an average of occurrence of 1 event / 2.5 years for the 1947-1993 period. Events of high to disastrous intensity occur at much longer intervals, several decades or even a whole century. No marked change in the avalanche situation in the northern Alps over the 1881-1992 period. Slight general increase of the avalanche situation in the Southern Alps since 1945. No general trend in avalanche activity (increase or reduction) can be found in the data relating to major events since the 15th century or systematic surveys of destructive avalanches in the past 50 years.

The informations comes from records and articles covering the whole Switzerland.

French and Swiss Alps:
The catastrophic avalanche activity (the last that occurred in the Alps was during the February 1999) is mainly the consequences of extreme snow fall, with occurrence probability remained stable during the 20th century (around one catastrophic situation each 10 years for the whole territory).

Alps:
20th century : no clear trend concerning damaging avalanches observed, no clear change in avalanche activity.

Maurienne massif:
High frequency avalanches
When the mean air temperature is close to 1°C during the day of the event, the probability of avalanche occurrence is above 0.3 when precipitation exceeded 25 mm the day before of the event, and over 0.5 when precipitation reached 35 mm. When precipitation exceeded 70 mm, the role of temperature was negligible.
The annual probability of high avalanche activity increased strongly with the occurrence of 3 successive days of high precipitation per winter, especially when 6 such triplets were counted per winter.

Low frequency avalanches
The probability of avalanche occurrence seemed not to be influenced by the meteorological conditions during the day of the event. The probability of avalanche occurrence was above 0.4 when precipitation exceeded 40 mm the day before the event. Moreover, the precipitation effect was not constant. The triggering probability increased strongly when precipitation ranged between 30 and 60 mm the day before the event.
The annual probability of high avalanche activity was around 0.5 when the occurrence of 3 successive days with high precipitations exceeded 8 per winter.

No correlation with any specific circulation patterns or with the NAO Index was found and no cyclical trend in the temporal distribution of avalanche occurrences was evidenced in this part of the Alps.

The probability model used has evaluated uncertainties associated with discrepancies between the climate at meteorological stations and at avalanche sites. The effects of climatic variables on avalanche initiation were simulated using a logistic regression model.

The weather classification at 500, 700, and 850 hp was also used to evaluate the relationship between avalanche occurrence and any special synoptic situation. The NAO Hurell Index (pressure ratio between Reykjavik and Lisbon) was used to test the teleconnection hypothesis.

Alps:
Avalanches are events often related to extreme meteorological phenomena (heavy precipitation, fast temperature increase…) which make difficult the detection of trends. The considered data set are generally not sufficiently important to be statistically representative.

The best known avalanche activity is often the “catastrophic” avalanches one. But these avalanches represent only a small part of the total number of avalanches.

Finally, the avalanche observation is very partial and biased (limited to certain periods and certain geographical sectors). There is no exhaustive database allowing a quantification of the avalanche activity and the detection of climatic trend.

French Alps:
According to the simulated avalanche hazard for the 1984-1998 period, the number of days with a moderate or high avalanche hazard varies between 107 (Mt Blanc) and 28 (Ubaye) per year. The delineation follows the spatial distribution of precipitation. The lowest massifs of the Pre-Alps show fewer days with moderate or high avalanche index than their neighbours.

3 types of avalanche-hazard have been determined by MEPRA: new-snow (precipitation or fragmented particules), wet-snow (wet grain) and mixed (both types are present). In the north, new-snow avalanche hazard is predominant, while wet-snow avalanche hazard is predominant in the south, in conjunction with warmer and sunnier conditions.

Days with mixed avalanches occur when the snow/rain limit is relatively high (1500 m or above). They are minimum in the southeast, where this type of situation is rarely encountered. The interannual variability of the avalanche hazard is very high and the number of days with high or moderate avalanche hazard can double from a year to another. The lowest values are obtained during the driest winters and in this case, wet-snow avalanche hazard is at least equal to new-snow avalanche hazard. Precipitation is a key factor for natural avalanche hazard: the correlation between the number of days with moderate or high avalanche hazard and winter precipitation at 1500 m is significant (0.75), while correlation with winter temperature is low (0.45).

According to the climate change scenarios, the number of days with moderate or high avalanche hazard decreases in all massifs. Variations are higher in the north, where reference values are high. The highest relative variations are encountered in the Vercors, Chartreuse and Bauges massifs, where snow height and duration diminish drastically because of their lower elevation. In the other regions, the number of days with moderate or high avalanche hazard is diminished by 5-9 days/year. Partial scenarios indicate that this number increases with P and decreases with T. For example, in the Mt Blanc massif: presently 107 days a-1; PT 96 days a-1; P 120 days a-1; T 84 days a-1. The evolution of the mean avalanche-hazard index between November and June in the same massif is maximum in February and secondary in May. In the PT scenario, the partial index for wet snow and mixed avalanches increases systematically except in May and June (decrease in snow-cover duration). In contrast, the partial index for new-snow avalanches decreases. The full index decreases throughout the winter (except in March), but variations cannot be considered important when looking at the daily variability of this index. The maximum decrease can be seen in May and June because of the wet snow avalanches.

The evolution of extreme events follows the evolution of mean parameters and the number of days with indexes higher than 7 decreases significantly in scenarios PT and T.

The SAFRAN/Crocus/MEPRA (SCM) software has been used to assess the present avalanche activity and its modifications in a changed climate.

The MEPRA estimation of the avalanche hazard is available at a 3 hour time-step, with a vertical discretization of 300 m (1500-3000 m asl) and for six aspects (north, west, south, east, southeast and southwest). For each analysis, the MEPRA system deduces from the Crocus snowpack simulations additional characteristics (shear strength, ram resistance and grain types). After classifying the ram and stratigraphical profile, this model studies the natural mechanical stability of the snowpack.

French Alps:
For the low and high frequency avalanches, the occurrence probability is closely correlated to the number of time in the winter that the precipitations have been observed during three consecutive days. The relation was even stronger when the calculi have been done for each type of avalanche.

Swiss Alps:
Considered as an average and ignoring any fluctuations, a slight climate change will not affect avalanche activity fundamentally. A variation of the mean temperature of 1 to 2°C and of 10 to 20% of the precipitations should have no influence on the avalanche activity.

Swiss Alps:
The avalanche danger should not experience important modifications. It is likely that the positive effects of avalanche protection measures (in the Viège valley, Swiss Valais) would counterbalance the possible impacts of climate change.

Swiss Alps:
The warm winter spells may have impacts, especially if the temperatures exceed the freezing point, inducing a melting of snow enhanced early runoff into river basins that originates in the mountains, avalanche risk for the exposed slopes.

German Alps:
One should expect an avalanche increase during winter because of the extreme precipitation increase, faster wind speeds tending to create snow accumulation.

French Alps:
An increased frequency of localised precipitation events at high altitude can let suppose an incidence on the triggering of heavy-snow avalanches.

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AVALANCHE SEASONALITY

Results and interpretation

Observation and analysis methods

Ecrins massif:
No correlation with any specific circulation patterns or with the NAO Index was found and no cyclical trend in the temporal distribution of avalanche occurrences was evidenced in this part of the Alps.

Swiss Alps:
A rise in winter temperature with a consequent shortening of the period of snow cover will probably have the effect of shortening the season when avalanches take place.

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AVALANCHE LOCATION

Results and interpretation

Observation and analysis methods

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EXPERIENCED FEED BACK

Experiences feed-back

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RECOMMENDATIONS

The policy makers are not really aware of the climate change induced necessities which requires urgent actions to identify and implement Society protection measures; such as […] measures to assure the water resource during summertime by creating artificial lakes, more efficient water use and protecting the mountain forest in a way to increase the buffer effect of soils and improve the avalanche protection.

Type of knowledge	Results and interpretation	Observation and analysis methods	References
Reconstructions
Observations	Switzerland: The occurrence of destructive snow avalanches in Switzerland between 1947-1993 shows no obvious trend. The situation in the surroundings of Davos shows a similar feature. However, the magnitude of the large events decrease systematically. This can be explained by the defence structures.		Schneebeli & al. 1997 - A
	Swiss Alps: During the past three centuries, periods of high avalanche activity have generally coincided with cold, damp winters. The climate has not changed enough during the 20th century to produce any palpable effects on avalanche activity. Unlike glaciers, avalanche react more to sudden extrem fluctuations than to long-term climate change. Hence they cannot be used as indicators of climate change during the past centuries.	The informations comes from records and articles covering the whole Switzerland. The data on avalanche activity have been compared with meteorological data from Davos; Bever and Andermatt.	Bader & Kunz 2000a - R: PNR31
	Maurienne massif: For both low- and high-frequency avalanche tracks, frequencies correlated closely with the number of times in the winter that precipitation was recorded during at least 3 successive days. The relationship was even stronger when calculated separately for each of the two groups of avalanche tracks.	The probability model used has evaluated uncertainties associated with discrepancies between the climate at meteorological stations and at avalanche sites. The effects of climatic variables on avalanche initiation were simulated using a logistic regression model.	Jomelli & al. 2007c - A
	Alps: • Avalanches are events often related to extreme meteorological phenomena (heavy precipitation, fast temperature increase…) which make difficult the detection of trends. • Attempts have been done to link snow cover (which shows clear trends) with the avalanche activity, but they generally do not give significant results. Snow cover data can not solely explain avalanche activity. • Evolution of meteorological parameters explains only partially the avalanche activity (for certain avalanche types). • Besides, this activity can be considerably perturbed by construction of avalanche barrier structures or by vegetation development. Thus the study of the climatic influence is often impossible.		Etchevers 2007 - C1
Modeling	Eastern Switzerland: If both the snow depth and the 3-day new snow exceed 75 cm, the probability of a disastrous avalanche day increase significantly. There is strong evidence that the mean potential avalanche activity (PAA), at least for the Davos area, has remained the same for the past 96 years (1896-1993). Periods of more or less intensive PAA could be detected, but they show no systematic trend or periodicity.	To infer the avalanche activity before 1947, the avalanche activity was modelled using meteorological data. The correlation of meteorological factors with destructive avalanches was used to find a threshold value where numerous destructive avalanches occur.	Schneebeli & al. 1997 - A
Modeling	Maurienne massif: The probability of occurrence of snowmelt avalanche increased strongly when precipitation exceeded 40 mm. The effect of temperature became clear when precipitation was above 50 mm. Finally, total precipitation on the day of a given event and the day before were most significant, especially when precipitation totalled 60 mm.	The probability model used has evaluated uncertainties associated with discrepancies between the climate at meteorological stations and at avalanche sites. The effects of climatic variables on avalanche initiation were simulated using a logistic regression model.	Jomelli & al. 2007c - A
Hypothesis	Swiss Alps: Early moistening of the snow cover as a result of a warmer atmosphere will indeed create avalanche of heavy or wet snow. Taken over the whole year, there should be the same number of destructive avalanche events involving heavy snow. Considered as an average and ignoring any fluctuations, a slight climate change will not affect avalanche activity fundamentally. A 1-2°C variation and a 10-20% variation in precipitation will not influence the occurence of extreme meteorological conditions leading to avalanche.		Bader & Kunz 2000a - R: PNR31
	Alps: The tree and shrubbery vegetation cover and more especially the tree cover play an important rule in the snow cover stabilisation, mainly with the anchorage due to the trunks and the interception by the branches. The forests cover disappearing leads also to a change in the snow evolution and of the snow cover mechanical characteristics, and therefore on the snow cover stability. Some snow flows could be expected in such cases, depending on the importance of the vegetation cover disappearing.		Demirdijan 2004 - A
	French and Swiss Alps: The impact of the temperature increase on the avalanche activity seems to be much more reduced than on the snow cover.		Ancey 2005 - E

Type of knowledge	Results and interpretation	Observation and analysis methods	References
Reconstructions
Observations	Swiss Alps: The strong intensity avalanche events occur at irregular interval, with an average of an event each 2.5 years for the 1947-1993 period. Events of high to disastrous intensity occur at much longer intervals, several decade or even a whole century. No marked change in the avalanche situation in the northern Alps over the 1881-1992 period. Slight general increase of the avalanche situation in the southern Alps since 1945.	The informations comes from records and articles covering the whole Switzerland.	Bader & Kunz 2000a - R: PNR31
Observations	Alps: 20th century : no clear trend concerning damaging avalanches observed, no clear change in avalanche activity.		Umweltdachverband 2006 - R*
Modeling
Hypothesis	Alps: Events like the 1999 snowy and avalanche winter can be considered as rare today but also “normal” and possible in the future.		ProClim 1999 - E
	French and Swiss Alps: While the climatic changes of the last decades are often seen as the natural catastrophe increase origin, there is no reason to think that in the future there would be a change in the avalanche activity, especially on the catastrophic situations.		Ancey 2005 - E
	German Alps: One should expect an avalanche increase during winter because of the extreme precipitation increase, faster wind speeds tending to create snow accumulation.		Seiler 2006 - P*
	French Alps: An increased frequency of localised precipitation events at high altitude can let suppose an incidence on the triggering of heavy-snow avalanches. More ice blocks and rocks in avalanches from glacial watershed (because of the glacier retreat, previously not mobilisable materials become mobilisable) is an eventuality. According to M.Badré and D.Laurens IGE report (01/23/06): “Concerning avalanches, among which the triggering and propagation undergo complex physical processes, new situations become possible. The trajectory of corridor avalanches should not be appreciably modified by these evolutions, except in case of topographic modifications, resulting from ice melt in transit and triggering zones.”		RTM-Requillart 2007 - C1

Type of knowledge	Results and interpretation	Observation and analysis methods	References
Reconstructions	Central Europe: High avalanche activity periods have been identified during the last 500 years; they correspond to wet and cold winter high frequency, especially around 1720, 1775 and 1817. Different meteorological situations can be identified during the exact event’s days: North-West perturbations, South flows, wet snow and rain fall coming from South-West flows on the southern Alpine flanks, etc.	Historic documents analysis. Some criteria have been used to compare past events to recent avalanche events. An avalanche list, with associated damages has been provided for Switzerland and neighbouring countries.	Paul 2002 - A
Reconstructions	Ecrins massif: Lichenometry shows a diachronicity in sedimentation on avalanche boulder tongues during the LIA. According to the lichen patterns, an irregular decrease of avalanche run-out distances has occurred for these last centuries. However, a distinction must be made according to the elevation. Before 1650 the frequency of avalanches was sufficiently high to prevent lichens from colonizing the deposits. Between 1650 and 1730, avalanche activity decreased, allowing free development of lichens on the surface of snow avalanche fans. Between 1730 and 1830, the activity seems to have increased, only occasional lichens located on protected sides of blocks surviving this erosive activity. From 1830 to 1850, according to the mean of the 5 largest lichens, the frequency reaching the distal zone was extremely low. It may reflect a decrease in the quantity of debris transported by avalanches, reducing their erosive power. Between 1830 and present, 2 contrasting periods can be distinguished. Because lichen size decreases from the base to the median part of all deposits, the magnitude of avalanches has decreased to 30% between 1830 and 1950 (lower frequency). From 1950, magnitude is low but frequency remains high in the upper part of the deposits. At low elevation, the variation of the avalanche activity during the LIA is different. The lack of a clear hiatus in the distribution of the lichens suggests that there was a maximum in avalanche activity around 1600-1650 (according to the lichen growth curve extrapolation). Since around 1700, colonization in the distal zone is observed, which is due to a decrease in activity of avalanche process. The phase observed around 1830, during which maximum run-out distances at high elevation occurred, also exists but the activity was less intense.	Measurements of Rhizocarpon sp.lichens were perfomed during two fields campaigns on 25 deposits for which geometrical and sedimentological characteistics have been studied previously. The location of lichens on boulders, in relation to the supposed trajectories of avalanches was also noted. Between 15 and 70 measurements (one measuremet on each block) were made within each sample area. To compare the lichen patterns between deposits, the lenght deposit was measured over a constant segment of 10 m. To date the avalanche activty, a lichenometric growth curve was used.	Jomelli & Pech 2004 - A
Observations	Eastern Switzerland : The occurrence of destructive snow avalanches in Switzerland between 1947-1993 shows no obvious trend.	Direct correlation of long-term, daily meteorological data to avalanches events	Schneebeli & al. 1997 - A
	Swiss Alps: Winters with devastating avalanches are homogeneously divided; there is no increase of their frequency during the 20th century.		OcCC 2003 - R
	Swiss Alps: During the past three centuries, periods of high avalanche activity have generally coincided with cold, damp winters. High-intensity events occur at very irregular intervals, with an average of occurrence of 1 event / 2.5 years for the 1947-1993 period. Events of high to disastrous intensity occur at much longer intervals, several decades or even a whole century. No marked change in the avalanche situation in the northern Alps over the 1881-1992 period. Slight general increase of the avalanche situation in the Southern Alps since 1945. No general trend in avalanche activity (increase or reduction) can be found in the data relating to major events since the 15th century or systematic surveys of destructive avalanches in the past 50 years.	The informations comes from records and articles covering the whole Switzerland.	Bader & Kunz 2000a - R: PNR31
	French and Swiss Alps: The catastrophic avalanche activity (the last that occurred in the Alps was during the February 1999) is mainly the consequences of extreme snow fall, with occurrence probability remained stable during the 20th century (around one catastrophic situation each 10 years for the whole territory).		Ancey 2005 - E
	Alps: 20th century : no clear trend concerning damaging avalanches observed, no clear change in avalanche activity.		Umweltdachverband 2006 - R*
	Maurienne massif: High frequency avalanches When the mean air temperature is close to 1°C during the day of the event, the probability of avalanche occurrence is above 0.3 when precipitation exceeded 25 mm the day before of the event, and over 0.5 when precipitation reached 35 mm. When precipitation exceeded 70 mm, the role of temperature was negligible. The annual probability of high avalanche activity increased strongly with the occurrence of 3 successive days of high precipitation per winter, especially when 6 such triplets were counted per winter. Low frequency avalanches The probability of avalanche occurrence seemed not to be influenced by the meteorological conditions during the day of the event. The probability of avalanche occurrence was above 0.4 when precipitation exceeded 40 mm the day before the event. Moreover, the precipitation effect was not constant. The triggering probability increased strongly when precipitation ranged between 30 and 60 mm the day before the event. The annual probability of high avalanche activity was around 0.5 when the occurrence of 3 successive days with high precipitations exceeded 8 per winter. No correlation with any specific circulation patterns or with the NAO Index was found and no cyclical trend in the temporal distribution of avalanche occurrences was evidenced in this part of the Alps.	The probability model used has evaluated uncertainties associated with discrepancies between the climate at meteorological stations and at avalanche sites. The effects of climatic variables on avalanche initiation were simulated using a logistic regression model. The weather classification at 500, 700, and 850 hp was also used to evaluate the relationship between avalanche occurrence and any special synoptic situation. The NAO Hurell Index (pressure ratio between Reykjavik and Lisbon) was used to test the teleconnection hypothesis.	Jomelli & al. 2007c - A
	Alps: Avalanches are events often related to extreme meteorological phenomena (heavy precipitation, fast temperature increase…) which make difficult the detection of trends. The considered data set are generally not sufficiently important to be statistically representative. The best known avalanche activity is often the “catastrophic” avalanches one. But these avalanches represent only a small part of the total number of avalanches. Finally, the avalanche observation is very partial and biased (limited to certain periods and certain geographical sectors). There is no exhaustive database allowing a quantification of the avalanche activity and the detection of climatic trend.		Etchevers 2007 - C1
Modeling	French Alps: According to the simulated avalanche hazard for the 1984-1998 period, the number of days with a moderate or high avalanche hazard varies between 107 (Mt Blanc) and 28 (Ubaye) per year. The delineation follows the spatial distribution of precipitation. The lowest massifs of the Pre-Alps show fewer days with moderate or high avalanche index than their neighbours. 3 types of avalanche-hazard have been determined by MEPRA: new-snow (precipitation or fragmented particules), wet-snow (wet grain) and mixed (both types are present). In the north, new-snow avalanche hazard is predominant, while wet-snow avalanche hazard is predominant in the south, in conjunction with warmer and sunnier conditions. Days with mixed avalanches occur when the snow/rain limit is relatively high (1500 m or above). They are minimum in the southeast, where this type of situation is rarely encountered. The interannual variability of the avalanche hazard is very high and the number of days with high or moderate avalanche hazard can double from a year to another. The lowest values are obtained during the driest winters and in this case, wet-snow avalanche hazard is at least equal to new-snow avalanche hazard. Precipitation is a key factor for natural avalanche hazard: the correlation between the number of days with moderate or high avalanche hazard and winter precipitation at 1500 m is significant (0.75), while correlation with winter temperature is low (0.45). According to the climate change scenarios, the number of days with moderate or high avalanche hazard decreases in all massifs. Variations are higher in the north, where reference values are high. The highest relative variations are encountered in the Vercors, Chartreuse and Bauges massifs, where snow height and duration diminish drastically because of their lower elevation. In the other regions, the number of days with moderate or high avalanche hazard is diminished by 5-9 days/year. Partial scenarios indicate that this number increases with P and decreases with T. For example, in the Mt Blanc massif: presently 107 days a-1; PT 96 days a-1; P 120 days a-1; T 84 days a-1. The evolution of the mean avalanche-hazard index between November and June in the same massif is maximum in February and secondary in May. In the PT scenario, the partial index for wet snow and mixed avalanches increases systematically except in May and June (decrease in snow-cover duration). In contrast, the partial index for new-snow avalanches decreases. The full index decreases throughout the winter (except in March), but variations cannot be considered important when looking at the daily variability of this index. The maximum decrease can be seen in May and June because of the wet snow avalanches. The evolution of extreme events follows the evolution of mean parameters and the number of days with indexes higher than 7 decreases significantly in scenarios PT and T.	The SAFRAN/Crocus/MEPRA (SCM) software has been used to assess the present avalanche activity and its modifications in a changed climate. The MEPRA estimation of the avalanche hazard is available at a 3 hour time-step, with a vertical discretization of 300 m (1500-3000 m asl) and for six aspects (north, west, south, east, southeast and southwest). For each analysis, the MEPRA system deduces from the Crocus snowpack simulations additional characteristics (shear strength, ram resistance and grain types). After classifying the ram and stratigraphical profile, this model studies the natural mechanical stability of the snowpack. SCM has been used to simulate the snow coverage and the avalanche hazard of the past 15 years (1984-1998) in the French Alps. The avalanche-hazard index was calculated all year round (except July and August) using the procedure previously defined. The thresholds for moderate and high avalanche-hazard index were calibrated using all massifs and 5 winter months (December-April). Thresholds found were 0.4 and 1.6. For the lowest massifs, the index was based on a limited number of elevations. Finally, the SCM software has been used to compare the January-February 1999 avalanche episodes to past extreme events. Episodes with an avalanche-hazard index higher than 7 (between high and very high) for at least 2 days and 2 massifs have been compiled. The sensitivity of avalanche hazard to climate change was estimated with simple methods such as constant perturbations of the meteorological variables analysed by SAFRAN. Crocus and MEPRA were run perturbed data, and the results were compared to those of the reference run detailed previously. Because both a general warming and increased precipitation are expected for the next century, 3 runs (a full scenario and 2 partial ones) were made: a temperature rise of 1.8°C coupled with a precipitation increase of 10% (PT); a temperature rise of 1.8°C (T); and a precipitation rise of 10% (P). In experiments PT and T, the critical temperature (at which the precipitation turns from snow to rain) is fixed at 1.5°C.	Martin & al. 2001 - A
Modeling	French Alps: For the low and high frequency avalanches, the occurrence probability is closely correlated to the number of time in the winter that the precipitations have been observed during three consecutive days. The relation was even stronger when the calculi have been done for each type of avalanche.	The probability model used has evaluated uncertainties associated with discrepancies between the climate at meteorological stations and at avalanche sites. The effects of climatic variables on avalanche initiation were simulated using a logistic regression model.	Jomelli & al. 2007c - A
Hypothesis	Swiss Alps: Considered as an average and ignoring any fluctuations, a slight climate change will not affect avalanche activity fundamentally. A variation of the mean temperature of 1 to 2°C and of 10 to 20% of the precipitations should have no influence on the avalanche activity.		Bader & Kunz 2000a - R: PNR31
	Swiss Alps: The avalanche danger should not experience important modifications. It is likely that the positive effects of avalanche protection measures (in the Viège valley, Swiss Valais) would counterbalance the possible impacts of climate change.		Bader & Kunz 1998 in Stoffel & Monbaron 2000 - P
	Swiss Alps: The warm winter spells may have impacts, especially if the temperatures exceed the freezing point, inducing a melting of snow enhanced early runoff into river basins that originates in the mountains, avalanche risk for the exposed slopes.		Beniston 2005a - A
	French and Swiss Alps: While the climatic changes of the last decades are often seen as the natural catastrophe increase origin, there is no reason to think that in the future there would be a change in the avalanche activity, especially on the catastrophic situations.		Ancey 2005 - E
	German Alps: One should expect an avalanche increase during winter because of the extreme precipitation increase, faster wind speeds tending to create snow accumulation.		Seiler 2006 - P*
	French Alps: An increased frequency of localised precipitation events at high altitude can let suppose an incidence on the triggering of heavy-snow avalanches.		RTM-Requillart 2007 - C1

Type of knowledge	Results and interpretation	Observation and analysis methods	References
Reconstructions
Observations
Modeling	Ecrins massif: No correlation with any specific circulation patterns or with the NAO Index was found and no cyclical trend in the temporal distribution of avalanche occurrences was evidenced in this part of the Alps.		Jomelli & al. 2007c - A
Hypothesis	Swiss Alps: A rise in winter temperature with a consequent shortening of the period of snow cover will probably have the effect of shortening the season when avalanches take place.		Bader & Kunz 2000a - R: PNR31

Type of knowledge	Results and interpretation	Observation and analysis methods	References
Reconstructions
Observations
Modeling
Hypothesis

Experiences feed-back	Objectives	Learnings	References

Recommendations	Remarks	Object	References
The policy makers are not really aware of the climate change induced necessities which requires urgent actions to identify and implement Society protection measures; such as […] measures to assure the water resource during summertime by creating artificial lakes, more efficient water use and protecting the mountain forest in a way to increase the buffer effect of soils and improve the avalanche protection.			Seiler 2006 - P*