From Lethal Heat to Invisible Deaths: Physiological Impact Attribution of Extreme Humid Heat in Karachi, Pakistan

Karachi, Pakistan’s largest coastal city with a population of 25 million, falls within the hottest zone in the world when the combination of heat and humidity (lethal heat) is considered. In 2015, Karachi underwent a fatal heatwave, resulting in 1300 deaths. While in 2024, Karachi suffered from another devastating spell of extreme heat, where the official number of fatalities stayed around 55. However, alternative evidence suggests that the actual number of deaths was far more than officially reported, strengthening the issue of ‘invisible deaths’ in developing countries as suggested in the earlier literature. This is a critical issue in efforts to address the impacts of climate change, considering that ‘you cannot cure the disease unless you know its intensity’.

We compared the weather conditions of 2015 and 2024 heatwaves at hourly temporal resolution based on a seminal physiological approach for assessing human livability to conduct sustained levels of work. Our results indicate that the lethal heat conditions, for multiple hours of each heat spell day, went beyond the levels where sustained basic activities for older adults (above 65 years) at a very light intensity, such as slow-based walking and house chores, were not possible. Considering that such fatal heat episodes are accompanied by disruptions in the power supply system, such conditions prove to be fatal for older adults. Similarly, the outdoor conditions also reached the levels for both the heatwaves for younger adults (18-40 years) where sustained livelihood generating activities such as lifting, fishing, street hocking, activities in agriculture and building sector etc were not possible for multiple hours of each day, putting serious limitations especially for the daily wagers responsible for putting the bread on the table. Our analysis further reveals that weather conditions during the 2024 heatwave were more severe than those during the 2015 heatwave, strengthening the findings of studies that suggest deaths during the 2024 heatwave were far more than the ones officially reported. 

We further carried out impact attribution analysis to underscore the role of climate change in exacerbating the 2024 Karachi heatwave. Using station data and ERA-Land for observations, and the data of 10 CMIP6 climate models at sub-daily (6-hourly) temporal resolution, application of probabilistic attribution methods shows that climate change has a discernible role in amplifying the impacts of the 2024 Karachi heatwave based on physiological thresholds. Climate Change decreases the livability limit for older adults and young adults by approximately 0.3-0.5 MET (Maximum Metabolic Rate) in indoor and outdoor settings, respectively. 

Our study presents a novel approach to advance the field of heat impact climate attribution. Our results are also useful for the policy makers, stakeholders, and implementers working in the fields of climate litigation, loss and damage, weather forecasting, and disaster risks, among others.