Cleaner air in the future will exacerbate the risk of compound flood-heatwave extremes in the Northern Hemisphere

The reductions in non-methane short-lived climate forcer (SLCF) emissions to improve air quality can contribute to additional warming of the climate system in the short term, thereby increasing the risk of extreme climate events. In this study, we quantitatively investigated the changes in future (2031-2050 versus 1995-2014) compound flood-heat wave extremes (CFHEs) and the resulting population exposure in the Northern Hemisphere (NH) due to non-methane SLCF reductions by using multi-model ensemble simulations under two future scenarios from the Aerosol and Chemistry Model Intercomparison Project. These two future scenarios share the same greenhouse gas (GHG) emissions but have weak (SSP3-7.0) versus strong (SSP3-7.0-lowNTCF) levels of air quality control measures. The results showed that reductions in non-methane SLCF emissions by 2050 will result in a 6% increase in grid exposure to CFHEs in the NH relative to the period 1995-2014. During the period 2031-2050, the frequency of CFHEs across the NH will increase by 2.9 events/decade due to non-methane SLCF reductions. The increases in CFHE frequency will be more pronounced in East Asia, South Asia, Siberia, and northern and eastern North America, reaching 4.8±2.0, 4.2±1.6, 3.5±0.9, 4.1±0.5, and 3.1±1.3 events/decade, respectively. At the regional scale, the sensitivity of CFHE frequency to non-methane SLCF mitigation was 1.2-1.9 times higher than that to GHG forcing. Future reductions in non-methane SLCFs will result in NH-averaged increases in population exposure to CFHEs of (5.0 ± 2.0) × 105 person-events in the period 2031-2050, contributing to increases in population exposure of 13.2-25.9% in different subregions. This study emphasizes the importance of considering the impacts of cleaner air in future responses to compound extreme climate events and corresponding societal planning.