Multi-method rapid attribution shows climate change is worsening humid heat

Abstract Humidity is an important determinant of heat-related morbidity and mortality, yet few studies have quantified how anthropogenic climate change influences individual extreme humid heat events. With the frequency and severity of humid heat events on the rise–a trend that is expected to continue with further climate change–understanding this contribution is critical to adequately prepare for future climate extremes. Here, we present and demonstrate a methodology for quantifying the contribution of climate change to daily wet-bulb temperatures ( Tw ) globally. This method combines empirical data (ERA5) with forced and preindustrial simulations from nine CMIP6 climate models to assess how climate change influences the frequency and intensity of daily Tw conditions. Through four case studies of humid heat events around the globe during 2023 and 2024, we show that climate change made daily maximum wet-bulb temperature ( Tw max ), on average, 1.2 °C hotter. Moreover, the peak daily Tw max for each event was made 65–175 times more likely due to climate change. Retrospective analysis of the number of days with a daily Tw max above a severe threshold (25 °C) indicates that climate change added 22 such days on average across the globe in 2024. These results show that climate change is having a strong and detectable influence on the frequency and intensity of conditions known to be harmful to human health and wellbeing. The quantitative links between any humid heat event and climate change demonstrated by this methodology could help to inform loss and damage determinations, impact-based community heat adaptation plans and warning systems, or legal complaints aiming to hold entities accountable for climate-related damages.