Attributing extreme humid heat in South and Southeast Asia in April 2023 to climate change and its implications for vulnerable and disadvantaged communities

In April 2023, India, Bangladesh, Thailand and Lao People’s Democratic Republic (PDR) were hit by ‘humid’ heatwaves. Using peer-reviewed attribution methods, we analyse the role of human-induced climate change in altering the likelihood and magnitude of this event, over two domains- (i) southern India (excluding the semi-arid regions to the lee of the Western Ghats) and Bangladesh (hereafter, IB) and (ii) Thailand and Lao PDR (hereafter, TL). Using Heat Index (HI), a metric that reflects the ‘feels-like’ air temperature due to humidity, the events are defined as the maximum of 4-day mean HI in April (HIx4d), area-averaged over the respective domains. In the observed records, such events are not exceptional over IB in today’s climate (1-in-5-year event), but rare over TL (1-in-200-year event). By including climate models, a heatwave as extreme as the 2023 event in IB is found to be made at least 30 times more likely by human-induced climate change and 2°C hotter than a 1-in-5-year event in the pre-industrial climate. Over TL, the observed HI would have been virtually impossible to occur in the pre-industrial climate and is now 2.3°C hotter. If global warming continues, similar heatwaves are expected to become even more frequent- about 10 times more likely in TL and about 3 times more likely in IB in a 2°C warmer world, as compared to the 2023 climate. Although accustomed to humid heat, many in these regions face heightened heat-related health risks due to physiological vulnerability, occupational exposure, and societal disadvantages. Environmental factors like air pollution and urban heat effects further worsen impacts, especially for the most vulnerable. Strengthening early warning systems, improving vulnerability assessments, and ensuring inclusive, locally tailored heat action measures are critical to reducing future heat-related risks across the region.