Carbon loss and radiative forcings of gaseous emissions from tundra wildfires during the Yukon-Kuskokwim River Delta 2015 fire season

With anthropogenic climate change, wildfires are increasing in frequency, and fires are a significant source of greenhouse gas emissions to the atmosphere. Gaseous emissions from fires have a strong net positive radiative forcing. Their magnitudes vary spatially due to combustion levels and emissions factors and temporally as a function of atmospheric lifetime of the emitted species. Region-specific methods for estimating combustion levels and radiative forcings of gaseous emissions in tundra ecosystems need to be developed for stricter calculation of wildfire effects on the climate system. We developed a novel method for measuring combustion levels and modelling the radiative forcings of greenhouse gases due to tundra fires, and applied this method to calculate radiative forcing of the 2015 wildfires on the Yukon Kuskokwim Delta (YKD), AK. We used two vegetative markers that survived the fire event— Sphagnum fuscum and Dicranum spp —to determine pre-fire height of the soil. Combustion levels estimated from each species were insignificant. We ground truth remotely sensed data of fire severity (Landsat differenced Normalized Burn Ratio) to scale our measured combustion levels to the 2015 YKD wildfires and estimated 1.69 Tg of carbon were released in the 2015 YKD wildfires. Using our estimates of biomass and organic matter combustion, we calculated the radiative forcings of gaseous emissions from the 2015 fire season, including carbon dioxide, ozone, nitrous oxide, methane, ozone precursors, and aerosols. We found that CO2 had the highest radiative forcing and that radiative forcing was highly dependent on the future ambient concentration of gas species (RCP 4.5 v. RCP 8.5). Our radiative forcings framework is only region-specific to tundra in its emissions factors and adjustment of this parameter permits its use in other biomes. With these two methods we are able to more accurately calculate effects of wildfires on climate warming. Our results highlight the climate impact of tundra wildfires, which provides a positive feedback to climate warming and increased fire frequency.