Abstract
Forest fires are increasing across the American West due to climate warming and fire suppression. Accelerated snow melt occurs in burned forests due to increased light transmission through the canopy and decreased snow albedo from deposition of light-absorbing impurities. Using satellite observations, we document up to an annual 9% growth in western forests burned since 1984, and 5 day earlier snow disappearance persisting for >10 years following fire. Here, we show that black carbon and burned woody debris darkens the snowpack and lowers snow albedo for 15 winters following fire, using measurements of snow collected from seven forested sites that burned between 2002 and 2016. We estimate a 372 to 443% increase in solar energy absorbed by snowpacks occurred beneath charred forests over the past two decades, with enhanced post-fire radiative forcing in 2018 causing earlier melt and snow disappearance in > 11% of forests in the western seasonal snow zone.
Highlights
Forest fires are increasing across the American West due to climate warming and fire suppression
To assess the large-scale impacts of post-fire radiative forcing on snow, we used 2000–2016 moderate resolution imaging spectroradiometer (MODIS) satellite measurements of snow covered area across the American West to determine relative changes in snow disappearance date (SDD) before and after fire
Our results suggest that forest fires in the seasonal snow zone have immediate and profound impacts on snowmelt throughout burned forests in the American West, resulting in accelerated snow disappearance, earlier springtime meltwater release, and likely lower stream flows during drier summer months
Summary
Forest fires are increasing across the American West due to climate warming and fire suppression. We estimate a 372 to 443% increase in solar energy absorbed by snowpacks occurred beneath charred forests over the past two decades, with enhanced post-fire radiative forcing in 2018 causing earlier melt and snow disappearance in > 11% of forests in the western seasonal snow zone. Earlier snowmelt extends the growing season resulting in amplified late summer drought[10], reduces forest productivity limiting carbon sequestration[11], and shifts phenological synchronicity with impacts to the reproductive success of many plants, pollinators, birds, and fish[12,13] Another consequence of climate warming and earlier snowmelt has been an increase in forest fire intensity, duration, extent, and frequency[14,15,16], with total area burned likely to continue increasing across the West[17,18,19,20]. Future increases in forest fires under a warming climate and associated radiative forcing potentially will have vast implications for the volume and timing of western streamflow and water resource management[30,31]
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