Earth is heating beyond human limits, scientists warn of deadly thresholds

By Dr. Chinta SidharthanReviewed by Susha Cheriyedath, M.Sc.Feb 5 2025

Extreme heat events are surpassing human survival thresholds, with rising temperatures putting billions at risk, particularly in vulnerable regions. Scientists warn that without urgent action, the frequency and severity of deadly heat waves will escalate.

Review: Mortality impacts of the most extreme heat events. Image Credit: by-studio / Shutterstock

As global temperatures rise, more regions are approaching dangerous heat thresholds that surpass human tolerance. In a recent review published in the journal Nature Reviews Earth & Environment, a team of scientists from the United Kingdom and the United States discussed the increasing frequency of life-threatening heat events and identified regions at risk of exceeding both uncompensable (beyond which human core body temperature rises uncontrollably) and unsurvivable physiological survival limits. Understanding these shifts is crucial to mitigating heat-related mortality and guiding adaptation strategies in vulnerable communities.

The threat of rising temperatures

Extreme heat has long been recognized as a major public health threat, with historical events such as the 2003 European heatwave causing tens of thousands of deaths. Heat stress occurs when the body can no longer regulate its temperature, leading to severe health risks, particularly for older adults, outdoor workers, and low-income populations.

Atmospheric dynamics, land surface conditions, and global warming all play a role in shaping heat extremes. Additionally, high-pressure systems, or anticyclones, contribute to the persistence of heat waves, trapping hot air and creating prolonged periods of extreme heat. Furthermore, climate models predict a significant increase in extreme heat events, exacerbated by urbanization, deforestation, and rising greenhouse gas emissions.

Historical data shows that some parts of the world are already reaching critical heat thresholds, with wet-bulb temperatures — a combined measure of heat and humidity — occasionally surpassing safe limits for human survival. Particularly vulnerable regions include the Persian Gulf, the Indo-Gangetic Plain, and parts of North Africa, where extreme humid heat events have already been recorded. Despite these alarming trends, gaps remain in understanding the upper limits of heat tolerance. Scientists are now investigating not only the frequency of uncompensable heat, where the body's cooling mechanisms fail but also unsurvivable heat, where the core body temperature rises uncontrollably within six hours, leading to fatal outcomes.

Impacts of extreme heat

The researchers observed that extreme heat is already claiming lives at an alarming rate. Since 2000, more than 260,000 deaths have been linked to heat waves, with the most extreme heat wave events concentrated in Europe, North America, and South Asia. Mortality rates have been the highest among older adults, young children, and individuals with pre-existing health conditions.

Furthermore, as global temperatures rise, heat-related deaths are expected to increase, particularly in densely populated and low-income regions where access to cooling is limited. Physiologically, humans struggle to cope with excessive heat, particularly when temperatures surpass uncompensable thresholds, which are levels where sweating and other cooling mechanisms fail.

The researchers identified wet-bulb temperatures of 19 °C–32 °C as critical for younger adults and 20 °C–34 °C for older adults, beyond which the body can no longer regulate heat effectively. Beyond these thresholds, core body temperature rises uncontrollably, leading to heat stroke and organ failure. The study also noted that some past heat events, including those in Karachi (2015) and Lagos (2024), may have briefly reached or exceeded these thresholds, posing extreme risks to affected populations.

The review also highlighted that heat-related mortality is not only a direct consequence of high temperatures but also an indirect result of cascading impacts. Crop failures, infrastructure strain, and increased energy demands contribute to social and economic instability, while vulnerable populations face disproportionate risks, including outdoor workers and residents of poorly ventilated housing.

Furthermore, the study warned that if global warming reaches 2 °C above pre-industrial levels, the land area experiencing uncompensable heat could triple, with extreme conditions affecting up to 35% of the global land area for older adults. Without intervention, billions of people could be exposed to life-threatening heat conditions in the coming decades.

Mitigation measures

Reducing the deadly effects of extreme heat requires immediate adaptation measures. The study outlined both individual and community-level strategies to minimize mortality risks. Workplace adaptations were also suggested, including implementing heat safety guidelines, such as shifting work hours to cooler parts of the day and providing rest breaks.

The researchers also proposed urban planning solutions, including green spaces, reflective surfaces, and improved ventilation, to help mitigate heat build-up in cities, where temperatures are often higher due to the urban heat island effect.

At the community level, heat health warning systems could play a crucial role in reducing fatalities. The researchers suggested that governments and health agencies can issue alerts when heat conditions become dangerous, enabling individuals to take protective measures. Investments in climate-resilient infrastructure, including better insulation and cooling technologies, are also critical.

However, the study cautioned that reliance on air conditioning alone is not a viable solution. While air conditioning can provide relief, it is energy-intensive, costly, and inaccessible to many vulnerable populations. Additionally, widespread use of cooling systems can overload power grids, particularly during extreme heat events, potentially leading to blackouts that leave entire communities exposed.

The study indicated that long-term strategies must ultimately focus on mitigating climate change. It emphasized that without significant reductions in greenhouse gas emissions, adaptation measures will become increasingly difficult to implement. A global effort to curb warming is essential to protecting lives from the intensifying threats of extreme heat.

Conclusions

To summarize, the review called attention to the growing threat of extreme heat. The researchers stressed that without urgent action, heat waves will continue to claim lives at an increasing rate. Adaptation strategies can help, but mitigating climate change remains the most effective long-term solution. Additionally, as climate models project more frequent and intense heat extremes, ensuring access to cooling and protective measures is crucial, particularly in regions already experiencing dangerous wet-bulb temperatures.