Thursday, September 29, 2016

Are Recent Bad Snow Years at Alta Due to Climate Change?

Since the 2010/11 ski season, the Wasatch Range has experienced below average snowfall years.  Data from the Utah Avalanche Center shows an average seasonal (October-April) snowfall at Alta of 497 inches since 1945/45, but over the last five seasons we've received only 330, 383, 358, 268, and 393 inches.  

I am often asked if these poor snowfall years are due to climate change.  

The answer is no.  

Many people are surprised to hear this, but here's why I do not think the recent poor snowfall years at Alta are due to climate change.  

First, the base of Alta is at 8500 feet, well above the freezing levels of most storms during the winter.  While it is possible that you can experience a brief rain-on-snow or drizzle-on-snow event at Alta, nearly all of the precipitation that falls there during the cool season falls as snow.  The low numbers over the past five years are not due to more wintertime precipitation falling as rain, but instead less precipitation.

Second, at that elevation, the fraction of precipitation that falls as snow during the cool season is relatively insensitive to another 1ºC (about 1.8ºF) of warming.  The image below from my book Secrets of the Greatest Snow on Earth and based on work by Leigh Sturges and John Horel here at the University of Utah shows the percentage of snow that currently falls as snow that would instead fall as rain if the climate were 1, 2, 3, or 4ºC or warming.  Alta is blessed with high altitude and thus snowfall at that elevation is less sensitive to the first couple of degrees of warming.  

Percentage of snow that would instead fall as rain with 1, 2, 3, and 4ºC of warming
Third, we're not really sure if the cool-season precipitation in northern Utah will increase or decrease with global warming.  Some model projections suggest we'll be drier, others wetter.  For all future emissions scenarios most climate models lean to a wetter future (the blue color bars below indicate the average winter precipitation change amongst models, the whiskers the middle 50% of the projections, and the dots the middle 80%).  Bottom line is that at present there isn't a strong reason to expect global warming to cause a decrease in wintertime precipitation, although it can't be ruled out.  

Projected seasonal and annual precipitation change over the central Wasatch and surrounding region for the 30-year periods centered on 2040 (2026–2055), 2060 (2046–2075), and 2085 (2070–2099) relative to 1976-2005.  See Fig. 6 for emissions scenarios.  Winter is December through February, Spring is March through May, Summer is June through August, and Fall is September through October.  Bars show the average change across the model runs, whiskers the range between the 25th and 75th percentile of the model runs, and dots the range between the 10th and 90th percentile of the model runs. Source:
Instead, I see the recent poor snow years at Alta as largely a reflection of climate variability instead of climate change.  Shifts between wet and dry periods are evident in both the instrumented and paleoclimate (e.g., tree ring) records.  The causes for these shifts are not well understood and I hesitate to speculate as to the causes of recent poor snow years, but my view is that the climate-variability dice simply have not rolled in our favor the past five seasons.

That being said...

This analysis is for Alta, a high elevation site in the Wasatch Range.  It does not mean that climate change is not influencing snowfall and snowpack at lower elevations (e.g., base of PCMR, Mountain Dell, etc.) or that Alta and the upper elevations of the Wasatch will not suffer pain and agony in the future.  It also doesn't mean that it isn't getting warmer in Utah (it is).  Different snow and climate measures (e.g., temperature, snowfall, snowpack, etc.) respond differently to climate change depending on elevation, aspect, and regional and local climate (for example, see Western Snow Trends and Global Warming: Part I and Part II).   When it comes to snow, we simply need to be careful about generalizing across all climates, elevations, and aspects, and cautious about conflating climate variability with climate change.  


Shortly after posting this article, I remember I've written about this topic previously.  See Is This a Long-Term Trend.

1 comment:

  1. I live near the Washington Cascades where our highest peaks 8500 ft are your base elevations lol but back in the 70s and 80s, it wouldn't seem unusual to get heavy snowfall in October or after May 1st. Now it's very rare. Snowfall might be affected more so during these transition seasons.