Snowflake by snowflake, mountain snowpacks build an important water resource for communities and ecosystems. And though intense “heat domes”—such as the one that struck the Pacific Northwest in 2021—can trigger early melting, researchers have now shown that even short-term, moderate temperature spikes can prematurely melt significant amounts of snow. Given that such springtime heat waves have been increasing in frequency since the early 1990s, many ecosystems will likely be affected by changes in the availability of water and snow in the future, the team concluded.
Deadly Heat
In late June 2021, a so-called heat dome blanketed the Pacific Northwest in record-setting heat. Temperatures approached 50°C in some places, and hundreds of people died from heat-related illnesses.
“We went in wanting to look at just what the heat dome did.”
That extreme event, though tragic, was also a catalyst for many researchers. Luke Reyes, an Earth scientist at Washington State University in Vancouver, and his thesis adviser, Marc Kramer, were interested in understanding how the heat dome affected snowpack in parts of the Pacific Northwest. “We went in wanting to look at just what the heat dome did,” Reyes said.
The researchers focused on roughly 51,000 square kilometers (20,000 square miles) of Oregon and Washington State at a mean elevation of about 1,600 meters (5,000 feet). That decision was borne partly out of necessity, Reyes said. “When the heat dome rolled around, there wasn’t too much snowpack at low elevations.”
Reyes and Kramer amassed daily snow measurements from a model—the Snow Data Assimilation System (SNODAS)—that had been validated using weather station observations. But rather than analyze snowpack depth, the researchers focused on so-called snow water equivalent. That metric refers to the depth of the pool of water that would persist if the snow that’s present completely melted. Snow water equivalent takes into account the density of snow, which can vary by up to an order of magnitude, Reyes said. It’s therefore a more accurate reflection of the amount of water truly stored in snowpack, he said.
Emptying Lake Shasta
“What’s going on with these other dips?”
The researchers compared their snow water equivalent data with daily measurements of air temperature taken from a climate model. Not surprisingly, they found that the 2021 heat dome had a significant effect: Snow water equivalent after the event was roughly 2.5-fold lower (74 mm; 3 inches) than it was before (180 mm; 7 inches). Over just 5 days, more than 540 million cubic meters (19 billion cubic feet) of water flowed out of high-elevation snowpack—that’s roughly equivalent to the volume of Lake Shasta in Northern California.
But the data reflected other discrete decreases in snow water equivalent as well, Reyes and Kramer found. “We looked at the snowmelt overall for that season and saw there were actually a couple of times that snowmelt rapidly accelerated,” Reyes said. “What’s going on with these other dips?” he remembered thinking.
Again and Again
Reyes and Kramer realized that those events corresponded to other heat waves that had happened earlier in the year. “They weren’t as extreme as the heat dome, but they were still evidently enough to cause rapid snowmelt,” Reyes said. The researchers tabulated four such heat waves from April through June with temperature anomalies ranging from 3.8°C to 7.0°C (the heat dome had an average temperature anomaly of 13.4°C). Each of those springtime heat waves removed roughly 90 mm (3.5 inches) of snow water equivalent, the team found. Therefore, the cumulative impact of those heat waves far outweighed that of the heat dome, Reyes and Kramer concluded.
“That’s a really important finding,” said Rachel White, an atmospheric scientist at the University of British Columbia in Vancouver not involved in the research. Relatively little is known about the impact of early-season heat waves on snowpack, she said. “When we study heat waves, most of the research is about summertime heat waves.”
The researchers also expanded their search for springtime heat waves beyond just 2021. “We zoomed out from there to look at other years,” Reyes said. The team found an increasing trend in the frequency of 5.0°C+ temperature anomalies occurring in April, May, and June since 1993. Even years that were expected to be characteristically cooler—so-called La Niña years—were exhibiting more heat waves, Reyes and Kramer noted. It’s therefore likely that early-season snowpack loss will accelerate in the future, the team concluded. These results were published in npj Climate and Atmospheric Science.
Such a shift could have significant impacts not only on local plants and animals that rely on persistent snow cover but also on lower-elevation ecosystems. That’s because snowpack is somewhat like a battery, said White. “It stores the precipitation from winter and then releases it in summer so that we have streamflow and water within an ecosystem.”
There’s now good evidence that heat waves are increasing in frequency and intensity, she added, but getting at the root cause of exactly why requires a better understanding of circulation patterns high in the atmosphere. And that’ll require novel new data sets, said White. “We’ve been measuring surface temperatures for over 100 years in some places. We haven’t been measuring what’s happening 10 kilometers above the Earth’s surface.”
—Katherine Kornei (@KatherineKornei), Contributing Writer