Human-induced climate change is causing a global rise in temperatures, leading to more frequent and intense heatwaves with severe impacts. Extreme heat was already responsible for an average of 500,000 excess deaths annually in the last two decades, disproportionately affecting the most vulnerable. High humidity exacerbates heat impacts by hindering the evaporation of sweat, which is the body’s mechanism to cool off. To understand these high heat stress conditions, scientists use a measurement called wet-bulb temperature, which combines temperature and humidity. If the wet-bulb temperature exceeds 35°C (95°F) for more than six hours, the average person's body will be unable to cool itself off by evaporating sweat and maintain a stable core body temperature. This can result in organ failure and brain damage if the situation is not improved.
Currently, wet-bulb temperatures have crossed this critical threshold in at least two weather stations, one in the Persian Gulf and one in the Indus River Basin. Jacobabad, Pakistan, known as one of the hottest cities on Earth, has experienced this occurrence at least twice since 2010. Although these instances have been limited to only a few hours each, their frequency is increasing. For example, during a 2023 heatwave in India, wet-bulb temperatures went above 34°C. Research indicates that by 2070, parts of South Asia and the Middle East will regularly surpass this threshold. Currently, around 30 per cent of the global population is exposed to deadly climate conditions for at least 20 days per year, and this number could rise to over 70 per cent by 2100.
excess deaths annually attributed to heat from 2000-2019
maximum wet-bulb temperature from which humans can survive
global population exposed to deadly climate conditions at least 20 days per year
Importantly, 35°C wet-bulb temperature is the upper limit of what humans can survive. Impacts can be felt at much lower temperatures, varying greatly depending on who people are, where they live and the work they do. For example, the 2021 heatwave that registered over 600 heat-related deaths in British Columbia reached a wet-bulb temperature of just 25°C. Older adults, young children and individuals with specific medical conditions are more susceptible to the effects of extreme heat. Occupations such as construction work, farming or working in hot kitchens expose individuals to additional heat from the environment or physical activity. Similarly, socioeconomic conditions can affect vulnerability: in the same city on the same day, the difference between an air-conditioned room and living quarters under a tin roof can be fatal.
Solutions to this problem are not easy, and while cutting greenhouse gas emissions is key, it will not be enough. The reality is that we are quickly approaching a tipping point past which people will not survive. Much of the discussion on the topic has focused on how people will move away from unbearably hot areas. However, many people will not be able to escape these conditions and will be trapped due to work or social obligations, financial or political limitations or disabilities. Consequently, adaptation solutions must also be implemented where people live now, including changing our environments, homes and behaviours. It is crucial for people, communities and governments to prepare and provide support to help take care of individuals who are trapped in the unliveable.
Tipping point: Being exposed to above 35°C wet-bulb temperature for longer than six hours will result in a healthy, young, resting adult in the shade and wind suffering extreme health consequences. This threshold becomes far lower as other factors are considered, such as age, medical conditions or activity level.
Shared Root Causes
Human-induced greenhouse gas emissionsShared with Uninsurable future, Mountain glacier melting, Accelerating extinctions
Continued anthropogenic greenhouse gas emissions increase global temperatures, accelerating unfavourable conditions for physical, natural and human systems
Living and working in at-risk areasShared with Uninsurable future
People move to or remain in places that expose them to increased risk due to social or economic pressures
Migration/displacementShared with Uninsurable future, Groundwater depletion, Mountain glacier melting
Some people may choose or are pushed to move temporarily or permanently to new, ideally less risky, locations
Increasing heat and humidity not only impacts humans, but many other species who may be unable to survive extreme conditions