Declining Snowpack over Western US Mapped at a Finer Scale

Authored by uanews.arizona.edu and submitted by mvea

Researchers have now mapped exactly where in the Western U.S. snow mass has declined since 1982.

A University of Arizona-led research team mapped the changes in snow mass from 1982 to 2016 onto a grid of squares 2.5-miles on a side over the entire contiguous U.S.

A person could practically find the trend for their neighborhood, said first author Xubin Zeng, a UA professor of hydrology and atmospheric sciences. Grid size for previous studies was about 40 miles on a side, he said.

"This is the first time anyone has assessed the trend over the U.S. at the 2.5-mile by 2.5-mile pixel level over the 35-year period from 1982 to 2016," Zeng said. "The annual maximum snow mass over the Western U.S. is decreasing."

In the Eastern U.S., the researchers found very little decrease in snow mass.

Even in snowy regions of the West, most of the squares did not have a significant decrease in snow. However, some parts of the Western U.S. have had a 41 percent reduction in the yearly maximum mass of snow since 1982.

UA co-author Patrick Broxton said, "The big decreases are more often in the mountainous areas that are important for water supplies in the West."

Snow mass is how much water it contains, which is important in regions where winter snows and subsequent snow melt contribute substantially to water resources. Snow melt contributes to groundwater and to surface water sources such as the Colorado River.

Snow is also important for winter sports and the associated tourism, which is a multi-billion-dollar industry in the U.S.

If all the squares in the Western U.S. that had a 41 percent reduction in snow mass were added up, the combined area would be equal in size to South Carolina, said Zeng, who holds the Agnese N. Haury Chair in Environment. He and his team looked at the interannual and multidecadal changes in snow mass for the contiguous U.S.

Zeng’s team also found over the period 1982-2016, the snow season shrank by 34 days on average for squares that, if combined, would equal the size of Virginia.

"The shortening of the snow season can be a late start or early ending or both," Zeng said. "Over the Western U.S. an early ending is the primary reason. In contrast, in the Eastern U.S. the primary driver is a late beginning."

Temperature and precipitation during the snow season also have different effects in the West compared with the East, the researchers found.

In the West, the multidecadal changes in snow mass are driven by the average temperature and accumulated precipitation for the season. The changes in the Eastern U.S. are driven primarily by temperature.

The paper, "Snowpack Change from 1982 to 2016 Over Conterminous United States," by Zeng, Broxton and their co-author Nick Dawson of the Idaho Power Company in Boise, Idaho, was published in Geophysical Research Letters on Dec. 12.

Previous estimates of interannual-to-multidecadal changes in snow mass used on-the-ground, or point, measurements of snow height and snow mass at specific stations throughout the contiguous U.S.

One such network of data is the National Weather Service Cooperative Observer Program (COOP), in which more than 10,000 volunteers take daily weather observations at specific sites throughout the U.S.

The other is the U.S. Department of Agriculture’s Snowpack Telemetry, or SNOTEL, network, an automated system that collects snowpack and other climatic data in the mountains of the Western U.S. However, for many locations, such measurements are unavailable.

Zeng and his colleagues used an innovative method to combine data collected by COOP and SNOTEL with a third data set called PRISM that gives temperature and precipitation data over all of the lower 48 states and is also based on on-the-ground measurements.

The result is a new data set that provides daily information about snow mass and snow depth from 1982 to the present for the entire contiguous U.S.

Developing the new dataset has allowed the UA-led research team to examine the changes in temperature, precipitation and snow mass from 1982 to 2016 for every 2.5-mile by 2.5-mile square in the contiguous U.S, as well as to study how snow can affect weather and climate.

"Snow is so reflective that it reflects a lot of the sunlight away from the ground. That affects air temperature and heat and moisture exchanges between the ground and the atmosphere," said Broxton, an associate research scientist in the UA School of Natural Resources and the Environment.

Zeng is now working with NASA to figure out a way to use satellite measurements to estimate snow mass and snow depth.

NASA and the UA's Agnese Nelms Haury Program in Environment and Social Justice funded the research.

connaught_plac3 on December 26th, 2018 at 01:45 UTC »

I work with one of the top avalanche guys in the US. He has been taking readings for the USFS (Forest Service) for almost 40 years.

He says 30 years ago, the same snow pack would last months longer into the summer. If we have 100", it is now gone by June; it used to stick around into July or August in places. Ski resorts have shortened their ski season by a day or two each year in recent decades, some places have gone from 180 days of skiing to 120 during my lifetime.

So who is to blame?

He says China. The world has cleaned up a bunch since the industrial revolution, but China still spews clouds of pollution that travel across the ocean and land on other continents. His evidence is his luminosity readings: snow was significantly brighter back when I was a kid and he was just starting. The snow doesn't reflect as much sun today as it has more particulate matter in it. Snow seems white, it is simply not *as* white as it used to be. The loss in reflectivity causes it to absorb more sun and melt faster, so the same snow pack is gone much sooner, regardless of global temps.

Darker snow equals faster melting; which equals less snow pack; which equals less snow to reflect the sun; which means the earth absorbs more of the rays of the sun and the world gets hotter. Greenhouse gasses get all the press, but particulate matter causes damage unique to itself.

IHaTeD2 on December 26th, 2018 at 01:22 UTC »

Are there global maps for this? I'm really curious if there's data that matches my personal / anecdotal experience. I am living in Germany and in the area I am living in I remember 30+ centimeters of snowfall quite often from when I was a child (~25 years ago). I remember my mother & grandmother saying how it was way more than that up to the point where you couldn't just walk through it when they were younger. Nowadays you might get a couple centimeters for maybe a couple days when you're lucky. It barely comes anything down anymore, most of the time it is just raining. And when come comes down it just melts into brown slush because the ground is still too warm to keep it frozen for long. Proper winter activities are just not possible anymore, I don't even have any proper winter clothing nowadays like I did as a kid. I don't think it even snowed once yet in my city this winter (lots of rain though).

whats-ittoya on December 26th, 2018 at 00:46 UTC »

Any idea why they started looking at it starting in 1982? There are weather records going back a hundred years so a study that looks at 34 years seems odd.