Pacific Northwest snowpack threatened by increasing spring heatwaves

Even in the very rainy Pacific Northwest, more frequent heat waves threaten a key water supply.

A Washington State University study that aimed to examine snowmelt under a single extreme event, the 2021 “heat dome,” instead revealed an alarming, longer-term upward trend in waves of successive heat melting the snowpack earlier in the year.

The findings have implications for many parts of the world that rely on snow-capped mountains to provide water in summer as heat waves increase globally.

“Short-term events like heat waves have had an underestimated impact on accelerating snowmelt and, cumulatively, they can amplify each other,” said Luke Reyes, a doctoral student in the School of environment at WSU and lead author of the study. Published in npj Climate and atmospheric science.

Heat domes, rare events that occur when the atmosphere traps warm ocean air, caused record temperatures near 122° Fahrenheit in the Pacific Northwest in late June 2021. Yet researchers have discovered that by the time the dome arrived, much of the region’s snowpack had already melted.

Their analysis of high-resolution snowpack and temperature data found that snow at high altitudes began to melt during a series of heat waves in April, May and early June, when temperatures were 7. 2 to 12.6° Fahrenheit above normal.

Even more worrying, when researchers looked at temperature records from 1940 to 2021, they found that these spring heat waves had doubled in frequency, intensity, or both since the mid-1990s.

“The data suggests that we don’t necessarily need to worry about a very rare event like heat dome, but that heat waves are becoming much more widespread and are more likely to lead to significant loss of snowpack at the future.” said co-author Marc Kramer, associate professor of environmental chemistry at WSU.

The effect of short-term heat waves on snowpack has been understudied because, historically, researchers looked at April 1 snowpack levels and average monthly temperatures to estimate the impacts of climate change on snow loss. snow cover. These averages can show single-digit temperature increases, but mask the impact of heat spikes that may last only a few days.

Additionally, for many years it was thought that the mountain snowpack was resistant to short-term high temperatures in spring because it remained cold enough at high altitudes. The WSU study found that this buffering power appears to have diminished in the face of more frequent and intense heat waves.

In 2021, the combined result of heat waves and the heat dome caused the snowpack to melt approximately three weeks earlier than usual, with most of the snow cover having disappeared by the end of June. Normally, melting snow provides water to the Pacific Northwest through August.

This extremely early melting occurred even though 2021 was a La Niña year, a global weather phenomenon that typically results in a thicker snowpack. In fact, the Pacific Northwest’s snowpack this spring was 135 percent of normal for the highest elevation snow zone and its 18-year record examined by the study. But by the end of June, that was no longer the case.

This rapid melting does not bode well for the coming year, 2024, which is expected to experience drier weather due to El Niño.

“We will be facing an El Niño phenomenon next year and the next year, and there could be amplification effects,” Kramer said. “If we have less snow to begin with, the snowpack will be even more vulnerable to these thermal anomalies early in the season.”

The study is part of the Kramer lab’s broader research into the impacts of heat waves on ecosystems and agriculture.