The impact of stratospheric aerosol injection on extreme fire weather risk varies regionally

Wildfire risk is expected to increase in many fire-prone regions in the 21st century as temperatures warm and precipitation, relative humidity, and wind patterns shift. While stratospheric aerosol injection (SAI) has been shown to be effective in limiting global warming and preserving large-scale temperature patterns, the impact on wildfire risk is still unknown. In this study, we analyze extreme fire weather events - dry, warm, and windy conditions - as simulated in the CESM2-WACCM6 ARISE-SAI-1.5 (Assessing Responses and Impacts of Solar climate intervention on the Earth system with stratospheric aerosol injection) ensemble. ARISE-SAI-1.5 uses a moderate Shared Socioeconomic Pathway scenario (SSP2-4.5) and begins SAI in 2035 to ensure that global mean temperature and north-south and equator-to-pole temperature gradients remain close to 1.5°C above the pre-industrial levels. We find that SAI results in strong regional variations in the impact on extreme fire weather throughout the simulation period (2035-2069). Over the Mediterranean and eastern Amazon regions, SAI prevents almost all of the expected increases in extreme fire weather frequency from the first year of injection until 2069, relative to climate change simulations without SAI. However, expected increases in extreme fire weather event frequency are dampened by SAI starting only halfway through the simulation period in eastern China, and SAI brings little to no relief to increases in extreme fire weather events over northern Australia. More notably, in the western Amazon and Congo Basin, SAI leads to an additional 25% increase in extreme fire weather risk over the already large increases expected under SSP2-4.5 by the last decade of the experiment. We then examine the role of SAI on changes in temperature, relative humidity, precipitation and wind speed to uncover the drivers behind the regional and temporal variations in extreme fire weather risk. Our work highlights the importance of assessing the impact of SAI and other solar radiation management strategies on extreme fire weather events and could inform future implementations of SAI in model simulations.