Avalanches and Climate Change

Questions about possible linkages to climate change are bound to arise whenever something unusual or exceptional happens, and this year's weak snowpack and high number of avalanche fatalities is no exception.  

Let's start with what we know and what we can say with some confidence.  We are in a period of climate warming, driven by human activity, and this is changing the snow climates of the western United States.  Weather and climate variability give us ups-and-downs even in a changing climate, but we are seeing a shift to warmer mean temperatures, more winter warm spells, and fewer winter cold spells.  

Trends in snowfall and snowpack measures vary, however, depending on the measure, regional climate, and elevation.  In northern Utah, at elevations below about 6500 feet, we are seeing declines in the fraction of wintertime precipitation that falls as snow, more frequent mid-season snow-loss events, and a decrease in the amount of wintertime precipitation retained in the snowpack at the end of the snow accumulation season.  

To date, these trends weaken with elevation and are either small or non-detectable in colder, upper-elevation regions.  However, as warming continues, we will see these trends continue and become more detectable and obvious at higher elevations.  Continued warming appears to be unavoidable through about mid century.  What happens after that is strongly dependent on future greenhouse gas emissions.  

Predicting trends in future avalanche conditions is more challenging.  Snowpack characteristics depend on many factors (e.g., temperature, humidity, precipitation type, precipitation frequency, precipitation amount, long-wave radiation, short-wave radiation, vegetation, terrain, aspect, yada yada yada) and weather variations during the season.  There are many different types of avalanches, each of which is going to be affected differently and in different ways depending on the regional climate, terrain, aspect, etc.  Hard data on past trends is limited.  Current computer models do not simulate fine-scale effects of terrain on snowfall, clouds, radiation, etc., and there hasn't been a great deal of effort put into developing tools to predict how avalanche hazards will evolve in the future.  

Thus, we still rely to a large degree on expert judgement.  As an example, Wilbur and Kraus (2018) surveyed 240 North American avalanche practitioners, receiving 53 responses.  Below is a graph summarizing the responses from experts concerning their impressions for recent and predicted trends in avalanche types, elevation-dependent avalanches,  and other avalanche-relevant variables.  These practitioners reported increases in wet, glide, and upper-elevation avalanches and expected to see those trends continue in the future.  Any recent changes in dry and low-mid elevation avalanches were not statistically significant, although decreases were anticipated for the future.  

Source: Wilbur and Kraus (2018)

Future trends in avalanche conditions are of course important.  They could be considered for the design of highways and avalanche mitigation systems, buildings, development, etc.  However, it is also important to consider cultural and societal trends.  We are in the midst of an incredible upward trend in the number of backcountry recreationists in the Wasatch (and elsewhere).  This is resulting in an increase in the diversity of backcountry recreationists and shifts in backcountry social dynamic.  We are seeing many more dawn patrollers after storms, more recreationists at all times, and more competition for powder.  The weather and snowpack conditions will ebb and flow, but are we trending toward a future in which accidents become more common or less common?  This is a question for people smarter than me.