World of Science | More climate change, fewer avalanches?

We are trying to liven up the world of science section a little more this season. You can realistically expect irregular contributions on various topics that PG authors find interesting at the moment. If you have any special topic requests or suggestions, please let us know at editorial team(at)powderguide.com!

Mountain ranges: less snow, fewer avalanches

In recent years, climate change has also increasingly become the focus of attention in the context of avalanches. Basically, where there is no snow, there are no avalanches. It therefore stands to reason that rising temperatures go hand in hand with a decrease in the frequency of avalanches. Time and again, scientific studies show precisely this correlation using local or regional data sets. For example, a recent publication from the Vosges mountains, where, according to the researchers, rising winter temperatures have led to fewer avalanches. The avalanches are also no longer as large and occur within a shorter time window (i.e. the snow season is getting shorter). The study suggests that the low mountain ranges are already showing changes that are also to be expected at higher altitudes sooner or later, and that adaptation measures can be oriented accordingly. (Giacona, F., Eckert, N., Corona, C., Mainieri, R., Morin, S., Stoffel, M., ... & Naaim, M. (2021). Upslope migration of snow avalanches in a warming climate. Proceedings of the National Academy of Sciences, 118(44).)

What in principle sounds like a simple time series analysis is not necessarily so. In order to determine whether the frequency of avalanches changes in the long term, you first need to know how many avalanches there have been in the past and how many there are now. As observations and records of avalanches usually do not go back very far, such projects often fail simply due to a lack of data. The achievement of the Vosges study is largely due to the fact that a rich treasure trove of historical records from various sources was compiled and homogenized in order to create a time series of avalanche activity in the region spanning over 200 years.

Vosges Mountains: Homogenized time series of avalanche activity with climate parameters (1774-2013), from Giacona et al, PNAS (2021) — Vosges: Homogenized time series of avalanche activity with climate parameters (1774-2013), from Giacona et al, PNAS (2021)

Giacona et al., 2021

Rockies: weather fluctuations and climate trends

Where there is a lack of human-recorded avalanche observations, tree ring analyses are a popular tool for at least partially closing data gaps (e.g. Peitzsch et al., 2021). Trees growing in avalanche paths are a kind of contemporary witness to past avalanches. Smaller avalanche damage can be seen directly in the tree rings and if all the trees were mown down at the same time, this indicates particularly large, destructive avalanches. Many climatological trend analyses of avalanche frequency are therefore based on the chronological data (= tree rings). This is also the case in a recent study from the northern Rocky Mountains (USA): on the one hand, a slight decrease in avalanche frequency was found in the avalanche areas investigated (time series from 1950 to 2017), but on the other hand there was also a high degree of variability from one season to the next. In addition to any climatological trends, there are therefore clear, short-term fluctuations from year to year.

The long-term decrease in avalanche frequency correlates with a decrease in the amount of snow. The short-term fluctuations, on the other hand, are more complicated to explain. A good half of the variability in the time series results from different amounts of snow and snow water equivalent depending on the year, as well as the PDO phase (Pacific Decadal Oscillation - more or less periodic patterns in sea surface temperatures that affect the weather in North America, especially on the northwest coast). In short: there are more avalanches in snowy winters with a lot of low-pressure influence. Rising temperatures play a comparatively small role and explain only about 14% of the variability. The study expects this to change in the future and increasing temperatures to become more relevant.

It is also conceivable that large, dry avalanches will become rarer, while wet avalanches will occur more frequently due to higher temperatures and the influence of rain at high altitudes. The authors speculate whether an accumulation of large avalanches in winters with little snow - actually atypical - towards the end of the time series indicates that the character of the avalanches is also changing, in the sense of: More wet snow, perhaps also more old snow problems due to rain crusts. However, it is still too early to make valid statements here on a climatic time scale. In addition, the tree ring method cannot distinguish between wet and dry avalanches. As is so often the case, one winter does not make a trend. 5 winters are slowly becoming somewhat conspicuous, but should not yet be seen as proof, but rather as a consolidation of evidence. (Peitzsch, E.H., Pederson, G.T., Birkeland, K.W. et al. Climate drivers of large magnitude snow avalanche years in the U.S. northern Rocky Mountains. Sci Rep 11, 10032 (2021))

Trees in avalanche paths can provide information about past avalanche activity. — Trees in avalanche paths can provide information about past avalanche activity
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LH

France: Changes in land use

The analyses from the Vosges and the Rocky Mountains are two examples that show how different data sets can be combined in order to quantify the effects of climatic changes on a specific parameter - in this case avalanche activity - through statistical evaluations. A third study from the French Alps follows a similar approach, but extends the investigation: changes in land use are also analyzed and included. In this case too, tree rings are used to create time series for certain avalanche tracks. The data goes back to around 1750. There are different trends depending on the exposure, presumably because a very massive avalanche cycle in the 1920s or 30s destroyed most of the trees in the avalanche paths exposed to the south, whereas this was not the case in the northern exposures. It is particularly interesting that a decrease in avalanche activity since the 1930s in certain avalanche zones in this study is not explained by rising temperatures, but by reforestation and changes in grazing. According to the authors of the study, small-scale differences in particular are much more strongly influenced by such factors than by overarching climate trends. (Mainieri, R., Favillier, A., Lopez-Saez, J., Eckert, N., Zgheib, T., Morel, P., ... & Corona, C. (2020). Impacts of land-cover changes on snow avalanche activity in the French Alps. Anthropocene, 30, 100244)

Summary

If one wanted to draw an overarching conclusion from these three local, or in the case of the Vosges, regional case studies, it would perhaps be the following:

The example from the Vosges shows that climate change at lower altitudes is clearly reflected in the time series of avalanche frequency: Less snow, fewer avalanches. The study from the Rockies (colder, higher) also finds corresponding trends, although these (still?) play a subordinate role compared to seasonal fluctuations in the weather. In the Alps, which have been populated for centuries, fluctuations in avalanche frequency can vary greatly from place to place and many effects come together that can overlap or even balance each other out, from land and forest use to weather conditions and climate change. The relative importance of the different factors will probably shift at least in part in the future. If you want to understand the relationships on a small scale, interdisciplinary studies are absolutely necessary.

In order to answer the more complicated, but from a skier's point of view probably more interesting question of whether not only the frequency of avalanches but also their characteristics are changing (proportionally more wet snow, possibly also more/other old snow problems?), other research approaches are needed. We are rummaging through the literature for a future edition of WdW...