Black Carbon from Burned Forests Dominated Radiative Forcing of Recent Western U.S. Snowpack

Abstract:

The vast majority of surface water resources in the semi-arid western U.S. begin as seasonal mountain snowpacks. Solar radiation is the primary driver of snowmelt, making these snow water resources especially sensitive to even small increases of light absorbing particles such as black carbon (BC) from wildfires and mineral dust deposited on snow. Using satellite observations, we document a 5-day earlier snow disappearance persisting for >10 years following fire within the perimeters of 850 burned forests. Using depth-integrated snow samples analyzed for BC and dust and modeling, we show that BC dominated overall radiative forcing in 2018 at 50 of the 51 alpine and sub-alpine sites widely distributed across the intermountain west. BC contributed an average of three times more radiative forcing than dust and up to 17 times more at specific locations. Evaluation of 2015 to 2017 archived snow samples yielded similar results. Our findings show that snowpacks at sites in proximity to burned forests had the highest BC fluxes and were significantly correlated with the area of nearby extensive recent forest fire occurrence, suggesting that forest fires may have immediate and lasting impact on seasonal snowmelt through transport and deposition of BC outside of burned forest areas. Area burned in western forests increased by 9% each year during recent decades, suggesting that BC-driven forcing exceeded dust-driven forcing after about 2006. This trend will continue as wildfire activity further increases under a warming climate.