From The Colorado Sun (Mark Jaffe):
The specter of climate change underscores the importance of gauging how well Colorado’s mountains can wring moisture from those enigmatic flakes
As the world’s climate warms, forced by the buildup of man-made greenhouse gases in the atmosphere, the key questions for the arid West will be how much snow falls on the region’s mountains and how much usable water it will yield.
“As climate changes, we need a more accurate understanding of the water in the snowpack,” said Deems, 46, who got hooked on studying snow as a University of Colorado undergraduate, in part because, as a backcountry skier, he was interested in avalanches. “So far it has been difficult to measure snowpack and get good information on how much water is going to come out of it.”
Snow is elusive. It comes in storms that mix and mingle with mountains and forest. Even after the storms leave, winds continue to move it around the landscape. “Pretty rapidly it becomes a difficult problem to simulate the snow accumulation process,” Deems said.
“Snow is close to its melting point, so it can change its character over a short distance or time,” he said. “It’s weird stuff.”
The big Earth system models (ESMs) used to plot global climate change have challenges simulating snow. Deems said he hopes the data generated from remote-sensing exercises, like the airborne observatory, will eventually “nudge the ESMs in the right direction.”
The airborne observatory is the result of a collaboration between the snow and ice data center, which is affiliated with NASA and CU Boulder. Deems has been with the project since it began in 2012.
Lidar – light detection and ranging radar – measures the snow-water equivalent and the mass spectrometer measures the albedo, the snowpack’s reflective capacity, an indicator of melting.
The airborne observatory is part of NASA’s larger SnowEx project, which aims to estimate how much water is stored in Earth’s snow-covered regions.
The snow-pit measurements were to help calibrate – “ground truthing” scientists call it – the remote sensing data from the airplane above.
Snow presents a water-gauging enigma
What scientists know from on-the-ground observation and model simulations is that in the last few decades, there has been less snow in the West, and it is likely there will be even less in the future.
In ascertaining what is going to happen to the region’s water, understanding snow is the most bedeviling element.
When it rains, it rains. But snow only comes when both the precipitation and temperature are just right. At warmer temperatures, the air holds more water and the snowfall is bigger, but a few degrees more and the snow turns to rain.
Since 1980, the area covered by snowpack has declined 20% in the low to middle elevations across the basins of the Colorado, Missouri and Columbia rivers, according to a study led by U.S. Geological Service (USGS) researcher Gregory Pederson. This contributed to low stream flows and a more active fire season.
A tree-ring paleo study by Pederson found that in the last 1,000 years, there were only two periods of sustained low snowpack that compared to current levels — from 1300 to 1330 and from 1511 to 1530.
But in each of those cases, there was a regional split, tied to forces such as the El Niño, the periodic warming of Pacific Ocean waters, which led to drying in either the northern Rockies or the southern Rockies while the other area was wet. Now the dearth of snow stretches across the West.
“Over the past millennium, late 20th-century snowpack reductions are almost unprecedented in magnitude across the northern Rocky Mountains,” according to a research paper by the Pederson team.
An analysis by researchers at Oregon State University and the University of California, Los Angeles found that between 2005 and 2016, snowpack in the Sierra Nevada and Cascades declined at 90% of the snow-monitoring sites with long records, with a third posting drops that were “significant.”
To see what the future may hold, a team led by John Abatzoglou, a University of Idaho geography professor, regionalized temperature and precipitation projections from 20 of the global ESMs and added data from the U.S. Department of Agriculture’s Snow Telemetry System (SNOTEL).
There are about 730 SNOTEL sites across the West and Alaska. Each has a “snow pillow,” akin to a waterbed filled with antifreeze that measures the density of the snow and its water content.
In warmer areas, such as the Cascades, snowpack reductions of 35% to 70% were projected by mid-century, according to the Abatzoglou models. In the coldest locations, including the middle and southern Rockies, snowpack was seen declining 5% to 20%.
But the model also showed that in a few of the coldest, high-elevation stations, including some in Colorado, the impacts of warming would be offset by increased precipitation. Still, overall the loss of snow would lead to an 18% decline in the flow of the Upper Colorado River.
The added moisture in the air would lead to more of the snowpack coming from a handful of very heavy storms, while there would be fewer snowfalls over the snow season. That season will narrow to December through February.
The shoulder months of November and March, which have been in part of the snow season, will see a mix of rain and snow – except a few areas where snow will continue to dominate, such as central Colorado and the Unita and Bighorn ranges, according to the Abatzoglou analysis.
At “broad scales,” the models indicate a 30% decrease in areas across the West with temperatures favorable for snow, one Abatzoglou study calculated.
Still, figuring out what is going to happen in the mountains is tricky, Abatzoglou said in an interview. “We’ve under sampled our mountains. We don’t nearly have as good a record,” he said. “The models are coarse.”
“We’ve seen decreases in snowpack, but how is snow going to be moving forward?” he asked. One scenario sees years with large snowfalls and years with “snow droughts.”
Another University of Idaho study found that as temperatures increase, snow droughts will become six times more likely in the second half of the century.
“When water falls, it is so much more valuable when it falls over the mountains as snow,” Abatzoglou said. “It contributes substantially to ecosystems and regional water supplies.”
As the snowfall becomes more limited and variable, understanding the spatial patterns of the snowpack becomes all the more important. “Where are the refuges that will be able to hang on to snow?” Abatzoglou said. “If we get a little more on spatial estimates, we can get a better handle on water supply.”
Digging by hand for details
Getting a handle was precisely what Deems and Elder were doing with their avalanche shovels. There are four SNOTEL sites in the Blue River drainage of Lake Dillon, Denver Water’s largest reservoir. Deems and Elder were above those SNOTEL sites.
It turned out when the Airborne Snow Observatory scans were twinned with the on-the-ground measurements, more than half of the season’s inflow to Lake Dillon was in the upper reaches of the mountains, even after the SNOTEL sites had melted out.
There was an estimated 114,000 acre-feet of water above the SNOTEL sites, enough water to supply nearly 300,000 families of four for a year.
“Those SNOTEL sites don’t measure anything above 10,000 feet in our watershed,” Elder said. “We have a large area above that, and we don’t get good information on it.”
“Ninety percent of water management is dealing with the extremes, the wet years and the dry years, which are getting drier, that is where climate change plays havoc,” said Douglas Kenney, director of the Western Water Policy Program at the CU Law School…
The goal is a policy that is adaptive and flexible, Kaatz said, because there is no crystal ball with which to tell the future. “We are aware of the limitations of what climate science can tell us,” she said. “We just don’t know what is really going to happen. That’s why planning for a range of scenarios makes sense.”
There is one certainty, she said. “The future is going to be warmer, hotter. Climate change is appearing here and now, and it is not going away.”