More snow is melting during the winter across the west, a worrying trend that could affect everything skiing conditions to the danger of fire and agriculture, according to a new analysis by CU Boulder 40 years of data.
The researchers found that, since the late 1970s, the winter-spring border has slowly disappeared, with one-third of the 1,065 snow-measuring stations the Mexican border to the Alaskan Arctic registering the increase in snow melting in winter.
While seasons with significant increases in thaw registered them mainly in November and March, the researchers found that the thaw is increasing in all months of the cold season - October to March.
Their new findings, published in Nature Climate Change, have important implications for water resource planning and may indicate fewer days of pure dust and more crusted snow for skiers.
"Particularly in cold mountain environments, snow accumulates during the winter - it grows and grows - and reaches a point it reaches maximum depth, before the melt starts in the spring," said Keith Musselman, lead author of the study and associate research at the Institute for Arctic and Alpine Research (INSTAAR) in CU Boulder.
But the new research found that the melt before April 1 increased by almost half of the more than 600 stations in western North America, by an average of 3.5% per decade.
"Historically, water managers use the date of April 1 to distinguish between winter and spring, but that distinction is becoming more and more confusing as the thaw increases during the winter," said Noah Molotch, co-author of the study, professor geography associate and INSTAAR colleague.
Snow is the main source of water and stream flow in western North America and provides water for 1 billion people worldwide. In the west, snowy mountains act like water towers, reserving water until it melts, making it available to the lowest elevations that need it during the summer, like a natural drip irrigation system.
"This slow dripping of melt water that reliably occurs during the dry season is something we build all of our water infrastructure in the west," said Musselman. "We depend a lot on the water that flows down our rivers and streams in the hot season of July and August."
More snowmelt in the winter is effectively changing the time for water to enter the system, activating the natural drip irrigation system more often in the winter, away the summer, he said.
This is a major concern for water resource management and drought forecasting in the west, which largely depends on the levels of snow accumulated in late winter in March and April. This change in water delivery time can also affect forest fire seasons and agricultural irrigation needs.
Wetter soils in winter also have ecological implications. One, wet soils are no longer able to absorb additional water during spring melting or storms, which can increase flooding. Wetter winter soils also keep microbes awake and thawed for a period that would otherwise have been asleep. This affects the time of nutrient availability, the quality of the water and can increase carbon dioxide emissions.
An underused data source
Across the western United States, hundreds of thin, fluid-filled metal cushions are carefully placed on the floor and out of sight of outdoor enthusiasts. These sensors are part of an extensive network of long-lasting manual and automatic snow observation stations, of which you may even have used data when checking the amount of snow on your favorite snowshoe or cross-country ski trail.
This new study is the first to compile data for all 1,065 automated stations in western North America, providing valuable statistical information on how mountain snow is changing.
And when using automated, continuously logged snow stations instead of manual and monthly observations, the new research shows that winter melting trends are widespread - three times the number of snowfall seasons, according to Musselman .
The snow layer is usually measured by calculating how much water will be produced when it melts, known as snow-water equivalent (SWE), which is affected by the amount of snow that falls the sky in a given season. But as snow melting in winter is more influenced by temperature than precipitation, it is a better indicator of climate warming over time.
“These automated stations can be really useful for to understand the potential impacts of climate change on our resources, ”said Musselman. "Your observations are consistent with what our climate models are suggesting that will continue to happen."