There has been a significant decline in snowpack levels in the U.S. over the last four decades. Warmer temperatures are creating an earlier snowmelt and a longer drier season, which is increasing the length and intensity of the fire season.
In addition to reducing water supply, the reduction in snowpack is also increasing fire danger, and wildfire itself is impacting the snowpack.
Luke Ryan spoke to Professor Anne Nolin with the University of Nevada Reno who co-authored a paper on the connection between wildfire and diminishing snowpack.
Anne Nolin: We have over a thousand stations across the western United States where the Natural Resources Conservation Service has been measuring snowpack since the late 70s and early 80s. And at 93% of these stations, we see declines in snowpacks, and the average decline is about 23%. We also see an earlier snowmelt, on average, again, by about 18 days of earlier snow disappearance across the West.
Luke Ryan: So what does this decline in snowpack mean for wildfire season?
Nolin: So if we have declining snowpacks, we have an earlier snow disappearance date, and that means that our dry season is going to start earlier. We might have an earlier, longer fire season. We see this relationship, statistically, between an earlier snow disappearance date and the length, intensity, and severity of the fire season and the number of fires during that fire season.
So now we've got a fire season that has significantly increased, (like) of course, the Marshall Fire in late December, two years ago. Snow serves as a reservoir for moisture and accumulates during the winter. And it doesn't melt really until we get to spring and early summer. And it provides a moisture subsidy for forests and ecosystems.
Now if that snow melt occurs earlier, then you're going to have a longer dry season. And our mountain forests have been experiencing more and more of these long dry seasons. Therefore, they're experiencing more moisture stress, and they're more vulnerable to fires because they're drier, and these dry fuels are really a critical ingredient for wildfire.
Ryan: What about down here in the plains, where we have a lot of grasses, and you're getting a lot of snowmelt and precipitation early on?
Nolin: So, it's different in the mountains where you've got forests, and where you might have earlier snowmelt, creating vulnerable conditions by late summer. But when you have earlier snowmelt in the grasslands in the spring, you're exposing those to sunlight and those grasses can grow up earlier.
But again, with less snow and a longer dry season, you can have a lot of grasses that dry out early and you might have a really challenging grassland fire season. We're seeing some of this in Texas right now, in the panhandle of Texas and into Oklahoma.
Ryan: Can you walk us through the negative feedback loop that arises after these fires burn through a forest or a grassland?
Nolin: After a fire, we see that the burned woody debris and black carbon that shed from those burned remaining trees blacken the snowpack's surface, and the lack of a forest canopy no longer shading the snowpack allows it to. absorb all that extra sunlight. So, we've kind of pre-conditioned the snow to have more incoming sunlight, earlier snowmelt, and that lasts for 10, 15 years after a fire, according to my colleague, Dr. Kelly Gleason at Portland State University. We hypothesize that the regrowing vegetation and the hydrology will be changing in ways that we might not expect for years after a fire.
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That story was shared with us via Rocky Mountain Community Radio, a network of public media stations in Colorado, Wyoming, Utah and New Mexico.