Studies: Climate change affecting Aspen’s frost-free days, snowpack melting

Scott Condon
The Aspen Times

U.S. Environmental Protection Agency Administrator Gina McCarthy will visit Aspen today to discuss climate change on the heels of scientific findings that show 2014 was the warmest year on record for the planet.

Last year wasn’t the hottest on record for Aspen, according to daily maximum and minimum temperature data stretching back to 1940, but data shows a definitive long-term trend toward significant warming, according to Jaime Cundiff, forest program director for the Aspen Center for Environmental Studies.

The number of annual frost-free days has soared since the 1950s and the number of consecutive frost-free days also is trending higher, Cundiff said. Snowpack levels vary from year to year with no definitive trend, but evidence shows that the snowpack is melting out sooner in the spring, she said. And the number of winter days with temperatures at or below zero is falling, creating more mild winters.

All of those factors have consequences for the environment, Cundiff said. Frost-free days, for example, can affect the budburst of vegetation and the overall species composition of an area.

ACES is analyzing weather and snowpack data for Aspen, among other mounds of data, as part of its Forest Health Index — an ongoing project to monitor a wide range of interlocking environmental conditions that affect forest health.

As part of the study, ACES found that the number of frost-free days in Aspen increased from an average of 73 per year in the 1950s to 99 in the 1990s and 107 in the 2000s.

Aspen also is experiencing milder winters. In the 1940s, there was an average of 35 days per year at or below 0 degrees during the 1940s. That fell to an average of 15 days per year during the 2000s.

“Extreme low temperatures are understood to play an important role in forest ecology,” the Forest Health Index said. “For instance, hard freezes are believed to help control insect-related disturbances such as the Mountain Pine Beetle infestation. Changes in extreme cold also serve as a broader indicator of change to the region’s climate.”

The index also found, in general, that the Aspen-area snowpack starts melting a week earlier in the spring and reaches the half-way melt-out period about one week sooner in the summer than it did in earlier decades.

A second report, Climate Change and Aspen, prepared by the Aspen Global Change Institute for the city of Aspen, noted many of the same environmental trends.

“For Aspen, climate change will likely include longer summertime warm periods, earlier onset of spring snowmelt, more precipitation arriving as rain rather than snow, and longer dry periods with heavier precipitation events in between,” said the report, released in December. “These types of changes could exacerbate already risky wildfire conditions, place extra pressure on already stretched water providers and users, provide additional challenges to ski area operators and other winter and summer recreation providers, as well as result in other impacts to every sector important to the Aspen community.”

Cundiff said the Forest Health Index places more stock in extremes, such as number of days with high and low temperatures rather than average annual temperatures. Statistics for any given year aren’t really useful. It’s trends that the index is seeking.

Nevertheless, she analyzed maximum and minimum daily temperatures for Aspen since 1940 and found that the average annual temperature between 1940 and 2014 was 41.44 degrees. The average for 2014 alone was 42.71 degrees. However, 2014 didn’t stand out. Several other years were at or above that temperature, she said. So, climate change didn’t make 2014 the hottest on record for Aspen, but trends show global warming is stressing Aspen’s environment.

EPA’s McCarthy will make a number of appearances in Aspen today. She will speak to an Aspen Middle School class, visit the Winter X Games, meet for lunch with Aspen Skiing Co. officials and give a press conference — all designed to build support for regulations the EPA is drafting to battle climate change.