Increasing heat is making more of the planet uninhabitable.

Key messages

• Rising heat and humidity due to climate change are pushing more people outside the climatic conditions ideal for human physiology.  Currently, around 600 million people live outside habitable climatic conditions, and with each degree of future warming, an estimated 10% of the global population will join them.
• Some regions, concentrated in the Global South, are more exposed to extreme heat and humidity than others. These populations also tend to lack the adaptive infrastructure (e.g., air conditioning) to support continued habitability.  
• Heat action plans and early warning systems will become increasingly important as a preparedness strategy for countries and across the most affected regions. Protecting particularly vulnerable groups (outdoor workers, older people, pregnant women, children, among others) will require specific measures.

Humans have flourished within a surprisingly narrow range of climatic conditions, but today increasing temperatures and humidity driven by human-induced climate change threaten to shift the regions that fall within this ideal range. Archaeological records and climate reconstructions, from as far back as 6,000 years ago, reveal that this most habitable range of climatic conditions, the “human climate niche”, has a mean annual temperature of ~13°C and precipitation of ~1,000mm per year. A recent study estimates that due to the ~1°C level of warming already observed, over 600 million now find themselves living in areas outside the human climate niche. At the same time, each degree of future warming would further alter habitability conditions such that the number of people living outside of the “human climate niche” will increase by 10% (Figure 3). 

Higher average temperatures, which are also associated with increases in the length and intensity of  heatwaves, can lead to heat-related illnesses such as strokes, severe headaches, damage to vital organs, decreased metabolic activity, mental disorders, obstetric complications (see Insight 4), kidney and urinary tract complications, and death. Heatwaves can be especially dangerous when combined with high humidity. Recent studies emphasise that the physical ability to tolerate heat is lower as humidity increases. This implies that a far wider range of regions (and therefore their inhabitants) will be exposed to heat stress than what was previously expected. Besides the direct impacts on human health, increased heat exposure also reduces habitability through disruptions to agriculture. As extreme weather events, including heatwaves, occur more frequently, food insecurity is substantially impacted. El Niño events (see Insight 5) exacerbate heatwaves, and, for example, in southern Africa and Australia, this intense heat greatly affects crop and livestock production, such as occurred during the 2015–2016 and 2023–2024 seasons. 

In the future, many regions will experience an increased frequency, duration and magnitude of heatwaves in addition to higher average temperatures. South Asia and the Persian Gulf are already experiencing deadly heat as the world approaches 1.5°C of warming. Global analyses predict that in general, heat extremes will be concentrated in low-latitude regions, which disproportionately include many Global South countries (Figure 3). The impacts of heat will not just be unevenly distributed globally, but also within the local populations of affected regions. Outdoor workers, older people, young children, people with pre-existing illnesses, and those with cognitive or physical impairments are particularly at risk in extremely hot and humid conditions. Different communities experience heat impacts differently; cities, for example, experience higher temperatures compared to rural surroundings because of the urban heat island effect. 

Humans have adopted a wide range of individual, social and structural adaptations that enable them to thrive outside of the ideal climate niche. These adaptation measures will be critical as more regions fall outside the ideal habitability range. With an understanding of the effects of increasing heat and humidity on habitability, proactive adaptation strategies based on future climate projections can be undertaken. Measures such as improved access to air-conditioned and urban green spaces (see Insight 7) can greatly reduce long-term heat exposure, while heat action plans and robust health systems are an essential part of enhancing preparedness to extreme heat events.

Humans can thrive despite increased heat stress, but the necessary adaptations are currently very unevenly distributed. Lower-income countries in the Global South, which are expected to face the brunt of increased heat, are ill-prepared. Poverty within these countries remains a strong predictor of both heat exposure and an inability to sufficiently adapt. Therefore on current trajectories the impact of increased heat and the shifting of the human climate niche will fall disproportionately on already vulnerable communities. This goes beyond deaths, and also encompasses increased pressures on health services, reduced productivity and the general suffering that the projected heat will bring.