Mountain Snow Declines Past and Future

The Cascades have a dubious distinction among western snow-bearing mountains: even a little warming can take a big chunk out of a season’s snowpack. This sensitivity to temperature was vividly, if painfully, illustrated during the record warm, record low snowpack of the 2014/2015 snow season. It’s also the subject of two recent and noteworthy climate studies.

In their paper in The Cryosphere, Eric Sproles and colleagues from Oregon State University examined two winters with very low snow: the winter of 2013/2014 and the winter 2014/2015.

Focusing on the McKenzie River Basin in Oregon and employing a high-resolution snow model, the researchers compared what snowpack in the region looked like during the historic baseline of 1989–2009 to fictional versions of those same years that were 2 degrees Celsius (3.6 degrees Fahrenheit) warmer, a simple way of looking at warming. The researchers then compared these results to the winters of 2013/2014 and 2014/2015.

Sproles and colleagues found that in a warmer future, snowpack conditions similar to those that occurred during the winter of 2013/2014 will be fairly typical. However, even in a 2 degrees Celsius (3.6 degrees Fahrenheit) warmer future, the researchers note, the low snowpack of 2014/2015 will remain exceptional.

In fact, Sproles and colleagues estimate a 92 percent chance of annual exceedance for a 2014/2015 like year; in other words, in a warmer future, snowpack levels similar to those experienced in 2014/2015 could be expected to reoccur roughly once every 12 years.

Note: this does not mean that a 2014/2015-type snowpack year will occur like clockwork once every 12 years, or that two or more 2014/2015-type snowpack years can’t occur back to back, but instead that 2014/2015-type snowpack years are likely to occur on average once every 12 years when measured over a 100-year period.

However, it’s worth noting that this is only for a future that is 2 degrees Celsius (3.6 degrees Fahrenheit) warmer than the baseline years 1989–2009. Future warming trends could be higher. CIRC and OCCRI research suggests the Pacific Northwest could be anywhere from 1 to 8 degrees Celsius (2 degrees to 15 degrees Fahrenheit) warmer by the year 2100.

Qualitatively speaking, Sproles and colleagues’ results are not surprising. Temperatures during 2013/2014 period were only 0.9 degrees Celsius (1.6 degrees Fahrenheit) warmer than the baseline years (1989–2009), whereas temperatures during the 2014/2015 period were notably warmer, a whole 2.7 degrees Celsius (4.9 degrees Fahrenheit), than the baseline, or close to what CIRC and OCCRI research suggests could be the norm for the Pacific Northwest by mid-century.

In Sproles and colleagues’ computer models, elevation profiles of simulated snowpack tell more of the story. In 2015, the models estimate, almost no snow remained below 1,500 meters (4,900 feet) by April 1st. Warm winters like 2014/2015 tend to remove most or all of the snow at the lower elevations while sparing snow at the higher elevations. In the future, snowpack in watersheds like the McKenzie River Basin is expected to be hit hard by rising temperatures. Currently the McKenzie River Basin accumulates roughly 93 percent of its snow at elevations between 1,000 and 2,000 meters (3,280 and 6,560 feet), the very elevations hit hard during the winter of 2014/2015.

A similar theme emerges in a complementary second paper published in Climatic Change and written by CIRC colleagues Bart Nijssen, John Abatzoglou, and former CIRCian Dennis Lettenmaier. Northwest Climate Science Center (NW CSC) graduate fellow Diana Gergel led the paper.

In their paper, the CIRC and NW CSC researchers examined past snow records and future snow projections across the entire western US. To do this, Gergel and colleagues employed the VIC hydrologic model to simulate change over the western US. The researchers fed in high-resolution daily weather data from the past, and 10 possible futures from different climate models.

The computer simulations tell a sad tale of low snowpack for the Pacific Northwest’s mountains under climate change. The Cascade Mountains will be one of the hardest hit. By the year 2100, warming under the simulated high-emissions RCP 8.5 scenario led to a reduction in total April 1 snowpack of 65 percent for the Cascades. Only the bone-dry White Mountains of Arizona (which in climate projections suffer the double whammy of warming and reduced winter precipitation) are projected to experience a greater reduction (98 percent) among the western mountain ranges covered by the study.

(For more on emissions scenarios, see our webpage Human Choice, Warming, & Emissions: The Representative Concentration Pathways.)

Since the VIC model produces a full suite of hydrologic variables, Gergel and colleagues also looked at soil moisture and fire risk. They discovered that in non-mountainous areas, summer soil moisture is projected to go down a bit in some places and even to go up a bit in others. However, the researchers note, in all mountainous regions soil moisture is projected to dry up a lot. This bodes ill for fire risk. As multiple studies have already clearly documented, years with low mountain snowpack and soil moisture frequently have more and bigger fires.

Studies: The CryosphereClimatic Change

Citations: Sproles, Eric A., Travis R. Roth, and Anne W. Nolin. “A spatial-probabilistic assessment of the extraordinarily low snowpacks of 2014 and 2015 in the Oregon Cascades.” The Cryosphere, 11, 331–341. (2017). DOI:10.5194/tc-11-331-2017

Gergel, Diana R., Bart Nijssen, John T. Abatzoglou, Dennis P. Lettenmaier, and Matt R. Stumbaugh. “Effects of climate change on snowpack and fire potential in the western USA.” Climatic Change 141, no. 2 (2017): 287-299.

Photo Caption: SR 20 – North Cascades Highway – Washington Pass, Dec. 1, 2008. (Photo Credit: Washington State Department of Transportation, some rights reserved.)

 

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