Peer Review: Impact of Solar Geoengineering on Wildfires in the 21st Century in CESM2/WACCM6

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> We quantify future changes of wildfire burned area and carbon emissions in the 21st century under four Shared Socioeconomic Pathways (SSPs) scenarios and two SSP5-8.5-based solar geoengineering scenarios with a target surface temperature defined by SSP2-4.5: solar irradiance reduction (G6solar) and stratospheric sulfate aerosol injections (G6sulfur), and explore the mechanisms that drive solar geoengineering impacts on fires. This study is based on fully coupled climate-chemistry simulations with simulated occurrence of fires (area burnt and carbon emissions) using the Whole Atmosphere Community Climate Model Version 6 (WACCM6) as the atmospheric component of the Community Earth System Model Version 2 (CESM2). Globally, total wildfire burned area is projected to increase over the 21st century under scenarios without geoengineering and decrease under the two geoengineering scenarios. By the end of the century, the two geoengineering scenarios have lower burned area and fire carbon emissions than not only their base-climate scenario SSP5-8.5 but also the targeted-climate scenario SSP2-4.5. Geoengineering reduces wildfire occurrence through decreasing surface temperature and wind speed and increasing relative humidity and soil water, with the exception of boreal regions where geoengineering increases the occurrence of wildfires due to a decrease in relative humidity and soil water compared to present day. This leads to a global reduction in burned area and fire carbon emissions by the end of the century. However, geoengineering also yields reductions in precipitation compared to a warming climate, which offsets some of the fire reduction. Overall, the impacts of the different driving factors are larger on burned area than fire carbon emissions. In general, the stratospheric sulfate aerosol approach has a stronger fire-reducing effect than the solar irradiance reduction approach.