Leveraging emergent constraints to reduce uncertainty in future compound drought and heatwave events across mainland China

Recent increase in compound drought and heatwave (CDHW) events has resulted in serious socio-economic impacts globally as well as in China and attracted growing concern. However, the underlying uncertainty in Earth system models may reduce the confidence levels in extreme events projections. Previous studies demonstrated the effectiveness of the emergent constraint (EC) method in addressing this issue, while they primarily focused on the treatment of climate variables or single extremes. The potential of EC in reducing the uncertainty of compound extremes has not been sufficiently evaluated yet. Here, we construct the EC relationships between historical daily maximum temperature and future (2021–2100) changes of CDHW events characteristics (duration, severity, and magnitude) using 24 CMIP6 models under four Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5). The application in mainland China presented that EC decrease the variance of CDHW events characteristics by 35 % (duration), 25 % (severity) and 21 % (magnitude) on average compared to raw projections. Specially, EC performs best under SSP3-7.0 scenario, with variance reduced by 42 %, 32 % and 29 % for duration, severity, and magnitude, respectively. By the end of 21st century, constrained growth (relative to 1981–2010) in CDHW events duration, severity, and magnitude are projected to reach 8.55 (±3.86)-31.58 (±7.46) days, 0.72 (±0.25)-2.37 (±0.57) ℃ and 1.46 (±0.56)-6.20 (±1.65) ℃, respectively. The constrained results imply that CDHW events duration, severity and magnitude are decreased by 3.58 %-6.00 %, 2.50 %-4.88 % and 2.48 %-4.95 % than currently expected, respectively, providing valuable insights for mitigation strategies and risk assessments of compound extremes.