Translating climate projections into phase-specific heat-risk information for rice production in Southeast Asia

Southeast Asia is central to global rice security; however, rising heat extremes increasingly threaten the stability of major rice-production systems. Here, we combined a high-resolution downscaled six-model Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble at 8-km resolution with CD35 , defined as the maximum consecutive duration of days with daily maximum temperature (T max) at or above 35 °C within rice-growth phases, normalized by phase duration, to quantify sustained heat exposure across the principal rice-growing systems of Thailand, Vietnam, and Indonesia under SSP1-2.6 , SSP2-4.5 , and SSP5-8.5 . Risk was evaluated for the historical baseline (1995–2014) , near future (2031–2050) , and far future (2080–2099) using country-specific crop calendars and phenological stages. Projected heat exposure increased across all three countries, with the strongest intensification during the midseason. Thailand showed the earliest and most severe shift, reaching a high to extreme CD35-based risk across most seasons by 2031–2050 under SSP2-4.5 and becoming uniformly extreme by 2080–2099 under SSP5-8.5 . By the late century under SSP5-8.5, midseason CD35 reached up to 60% in the Mekong Delta, 40% – 55% in northeastern Thailand, and 50% – 65% in the major rice-growing areas of Java, indicating that threshold-exceeding heat is becoming concentrated in the core production systems of the region . Together, these findings identify midseason as the critical window for future rice heat stress and provide a scalable framework to support climate services, adaptation planning, and prioritization of heat-risk management under future warming.