Human liveability and survivability in response to outdoor heat stress in Southeast Asia under different emission pathways

Southeast Asia, characterized by climatologically high temperature and high humidity all-year round, has faced increasing challenges due to unprecedented levels of extreme heat events, which appear to be attributable to global warming. While many previous studies have attempted to measure human heat stress primarily using either temperature-centric indices or temperature-humidity combined indices, recent efforts to incorporate physiological factors into heat stress assessments have gained momentum, drawing increased attention to indices derived from biophysical models. Using bias-corrected, high-resolution regional climate projections, this study employs physiology-based liveability and survivability indices that account for diurnal variations in mean radiant temperature, while differentiating heat tolerances between young and older populations. The analysis focuses on a comparative assessment of changes in liveability and survivability in response to low (SSP1-2.6) and high (SSP5-8.5) emission scenarios to quantify the effects of emission reduction on heat vulnerability.  Under SSP5-8.5, approximately 75% of Southeast Asia will become areas restricted to light activities for the older demographic, whereas this coverage could be reduced by 33% under SSP1-2.6. In addition, physiologic survivability, calculated as the fraction of time during which survival conditions are met, declines sharply under SSP5-8.5 compared to SSP1-2.6, indicating a significant collapse of thermal safety under the unmitigated scenario. Notably, while older adults face greater vulnerability to lower liveability and non-survivable heat, younger adults may also encounter distinct challenges due to larger diurnal fluctuations in liveability and a significant reduction in liveability. Our findings underscore the necessity of age-differentiated heat risk assessments, emphasizing the importance of mitigating future emissions.

[Acknowledgment]

This research was supported by Research Grants Council of Hong Kong through Theme-based Research Scheme (T31-603/21-N) and General Research Fund (GRF16308722).