Sorely reducing emissions of non-methane short-lived climate forcers will worsen compound flood-heatwave extremes in the Northern Hemisphere

Non-methane short-lived climate forcer (SLCF) or near-term climate forcer (NTCF) emissions, as a significant driver of climate change, can be reduced to improve air quality. These reductions may contribute to additional warming of the climate system in the short term, thereby strongly affecting the likelihood of climate extremes. However, there has been no quantitative assessment of the impact of non-methane SLCF mitigation on compound flood–heatwave extremes (CFHEs). This study quantitatively investigates the changes in future (2031–2050 versus 1995–2014) CFHEs and the resulting population exposure in the Northern Hemisphere (NH) due to non-methane SLCF reductions. We used multi-model ensemble simulations under two future scenarios from the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) in the Coupled Model Intercomparison Project Phase 6 (CMIP6). The two future scenarios share the same greenhouse gas (GHG) emissions but have weak (Shared Socioeconomic Pathway (SSP) 3–7.0) versus strong (SSP3-7.0-lowNTCF) levels of air quality control measures. The results show that future non-methane SLCF reductions during 2031–2050 results in about a 7.3% ± 2.3% increase in grid exposure to CFHEs in the NH relative to the period 1995–2014. The frequency, intensity, and duration of CFHEs increase by varying degrees. During the period 2031–2050, the frequency of CFHEs across the NH increases by 2.9 ± 0.9 events per decade due to non-methane SLCF reductions. The increases in CFHE frequency are more pronounced in East Asia, South Asia, Siberia, and northern and eastern North America. In East and South Asia, the intensities of both heatwaves and floods corresponding to CFHEs increase markedly, where heatwave magnitude (HWM) increases by 0.3 ± 0.2 K in East Asia and weighted average precipitation (WAP) increases by 18.3% ± 15.3% and 12.0% ± 4.5% in East Asia and South Asia, respectively. In other regions, rising temperatures dominate the increase in CFHEs. With regard to the duration of CFHEs, future reductions in non-methane SLCFs increases the duration of CFHEs in the NH by 0.3 ± 0.1 d. Regionally, the sensitivity of CFHE frequency to global warming caused by non-methane SLCF mitigation is 1.2–1.9 times higher than that caused by GHG forcing. Non-methane SLCFs results in NH-averaged increases in population exposure to CFHEs of (5.0 ± 2.0) × 10⁵ person·event in the period 2031–2050. This study emphasizes the importance of considering the impacts of cleaner air in future responses to compound extremes and corresponding societal planning.