Mega-fires can adversely impact air quality in the United States and the impacts are likely to become more serious in the future due to the possibility of more frequent and intense mega-fires in response to the projected climate change. This study investigated U.S. mega-fires and fuel conditions and their environmental impacts under the changing climate. A comprehensive approach of data analysis, algorithm development, and numerical modeling was used to understand present mega-fires, project their future trends, and simulate fuel loading and smoke transport. Dynamical downscaling of regional climate change was used to obtain present and future fire potential indices and atmospheric conditions. Ensemble was made based on the results obtained under multiple combinations of global-regional climate model simulations. The major accomplishments included building mega-fire-climate relationships using historical data, revealing the spatial and temporal features of climate conditions for fires, developing mega-fire occurrence probability projection models using extreme value theory, projecting future mega-fire trends using the dynamically downscaled regional climate change scenarios, projecting future fuel loading using a dynamic global vegetation model with a developed algorithm for filling the time gap with the climate change scenarios, projecting the impacts of climate change on escape risk and burn windows of prescribed burning, and estimating emission changes in response to changing fire and fuel and the smoke impacts on air quality.

The research with this project led to some major findings on climate, fuel, and fire interactions. Close relationship was found between mega-fires and the Keetch-Byram Drought Index, which was substantially different between the inactive and active mega-fire episodes transited in the mid-1990s. Future fire potential would become more severe by one grade in many regions. The fire concurrent probability, however, would become smaller in summer for all regions except North Central and Northeast. The mega-fire projection model is capable of simulating the distribution of fire occurrence with abnormal KBDI category. The mega-fires are projected to increase by nearly 60% by middle this century, despite large variability among regional climate change scenarios. An increase in fuel load was projected for the future, mainly in the western U.S. An algorithm was developed to fill the time gap with the regional climate change scenarios to improve ecosystem modeling of fuel load. The prescribed burn windows are expected mostly to become shorter in the future in the eastern U.S., the Pacific coast and southwestern border areas due to increasing escape fire risks. The PM2.5 emissions from mega-fires increased several times from the inactive fire episode of 1980-1996 to the active fire episode of 1997-2013 and are expected to increase remarkably in the future, leading to more deteriorative air quality conditions in some large U.S. cities.

The datasets for present and future mega-fire events, climatology of current and future fire indices, future change in fuel loading and particle concentrations are provided.