The Role of Black Carbon from Wildfires in Accelerating Snow and Glacier Melt in Washington State

Abstract:

In Washington, snow and glacier melt provide an important source of water resources, however spring snowpack levels are declining and glaciers are retreating. While warming temperatures are a well-recognized factor contributing to snowpack decline and glacier retreat, another cause may be the deposition of black carbon (BC) onto snow and glacier surfaces. Since 2010 we have collected snow and ice core samples to characterize the spatial and temporal variability of BC deposited in Washington snow and glacier ice. BC concentrations in the winter snowpack are relatively low, with BC concentrations increasing in spring and summer due to melt induced enrichment and increased dry deposition. BC induced melt may accelerate the timing of spring snowmelt at lower elevations, however BC induced melt is likely largest at relatively high elevations where the snowpack persists into the summer months when BC concentrations were observed to be highest. Based on our research to date, the highest BC concentrations in Washington snow and ice are linked to forest fires. A shallow ice core retrieved from Mt. Olympus demonstrated that BC deposition was a magnitude higher during the 2011 Big Hump forest fire, resulting in a threefold increase in the rate of change of river discharge due to glacier melt. An ice core from South Cascade Glacier spanning the 20^th century also suggests that the highest BC concentrations are associated with forest fires. Furthermore, burned areas can continue to provide a source of BC to the snowpack post-fire. We measured BC concentrations in snow at a study site from 2010−2013 in Washington State. The surrounding forest burned in 2012, after which BC deposited on the snowpack post-fire was at least four-fold higher than pre-fire. This research has implications for projected climate change, as forest fires are projected to increase and the seasonal snowpack is projected to decrease, both of which contribute to higher BC concentrations in the snowpack.