High-resolution dynamically downscaled projections of future extreme temperatures, heatwaves and exposure in Southeast Asia.

Extreme heat and heatwaves driven by global warming pose escalating risks globally, particularly in Southeast Asia (SEA), home to 680 million people, with a high concentration in urban areas. This study made use of CMIP6-based convection-permitting dynamically downscaled simulations at 8 km resolution over SEA under the three Shared Socioeconomic Pathways (SSP) scenarios: SSP1-2.6, SSP2-4.5, and SSP5-8.5. Projected changes in daily maximum temperatures and key heat metrics including the frequency of hotter days, heatwaves, and cumulative heat intensity and exposure time were analyzed across the region, as well as for individual countries and cities, for the near future (2040-2059) and far future (2080-2099) relative to the historical baseline (1995-2014). Results indicate a substantial rise in daily maximum temperatures over SEA, with average increases of 1.0-2.1 °C by 2040-2059 and 1.1-4.0 °C by 2080-2099 across the three scenarios. The Mekong Delta, eastern Sumatra and southern Borneo are identified as hotspots with pronounced temperature increase. More frequent and prolonged heatwaves are also projected over SEA, with heatwave frequency and duration doubling around 2025 and 2040, respectively, across the three scenarios, and increasing fivefold around 2045 and 2070, respectively, under SSP5-8.5. Specifically, the Maritime Continent is projected to face a notably higher frequency of hotter days, establishing a new heat norm by the end of the century. Under SSP2-4.5 and SSP5-8.5, approximately 4 and 9 months/year, respectively, will be as hot as or hotter than the historical 5 % hottest days. Cities along the Strait of Malacca and on Java are expected to experience extreme heat with heightened cumulative intensity and longer durations. Mitigating emissions along a low-carbon pathway would provide substantial benefits for the Maritime Continent in the second half of the century, not only for human health but also for agriculture and ecosystems. This study provides the highest resolution and most updated projections of extreme heat over SEA to help inform targeted climate adaption strategies in this highly vulnerable region.