Who is responsible for heat that kills?

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by Ian Angus

Discussions of climate change that focus on the 1.5°C or 2.0°C targets can be misleading. An increase of less than two degrees seems small compared to the normal temperature variations we experience from season to season or even from night to day.

The problem, of course, is that 1.5°C and 2.0°C are averages that conceal extremes. Graphs of current and projected temperatures consistently display what statisticians call “fat tails”—meaning that high temperatures are more likely than standard bell curves suggest.

The danger to human life lies not so much in heat as in heat waves, extreme events that have more than doubled in frequency and length in recent decades.

In 2003, a heat wave in Europe killed over 70,000 people. Another, in Russia in 2010, killed 56,000. In 2015, a heat wave in India and Pakistan killed 5,500. During six days of the 2024 Haj pilgrimage to Mecca, heat stress killed more than 1,300 pilgrims. Between 2000 and 2019, more people were killed by heat waves than by floods, hurricanes, earthquakes and wildfires combined.

A study in The Lancet comments:

“Extreme heat events are becoming permanent features of summer seasons worldwide, causing many excess deaths. Heat-related morbidity and mortality are projected to increase further as climate change progresses, with greater risk associated with higher degrees of global warming. Particularly in tropical regions, increased warming might mean that physiological limits related to heat tolerance (survival) will be reached regularly and more often in coming decades.”^[1]

Humans are warm blooded, which means that internal metabolic processes keep our body temperature stable—no matter how cold or hot the surrounding environment is, your system fights to keep your core temperature very close to 37°C (98.6°F). The hypothalamus, a small organ near the center of your brain, monitors thermoreceptors located throughout your body. If your temperature drops below normal, it releases hormones that reduce blood flow near your skin, so heat can’t escape, and it may cause muscle contractions (shivering) that warm your body. If you are too hot, it instructs your blood vessels to open wider, releasing heat through your skin. It may cause sweating, which cools you by evaporation.

But there are limits beyond which even a very healthy young body cannot adapt. At a certain point—typically 41°C, but it varies with age and physical condition—your internal heat control system will stop working. “When the body’s enzymes begin to denature, in essence being ‘cooked,’ these vital chemical messengers that maintain our metabolic processes cease to function, liver and kidney failure follow, and the brain and central nervous system malfunction prior to shutdown. Extreme heat kills.”^[2]

A widely-cited study led by Dr. Camilo Mora of the University of Hawai’i found “Twenty-Seven Ways a Heat Wave Can Kill You” — 27 different physiological mechanisms triggered by heat that can cause organ failure and ultimately death.

“The described deadly heat pathways can be triggered anytime that climatic conditions result in hyperthermia, highlighting that everyone can be at risk. …  The health impacts of heat waves could be reduced through social adaptations that limit heat exposure (eg, alert systems, air conditioning, and greening cities). Although such protective measures have been effectively used in the past, they may not be affordable for all of humanity, and even among those who can afford them, a warming world will recurrently “imprison people” indoors and may turn infrastructure failures (eg, power outages) into catastrophic events.”^[3]

We have evolved to survive in the relatively narrow band of atmospheric temperatures that have prevailed on Earth in the last 300,000 years or so. During that time, and through the last 12,000 years—known to geologists as the Holocene Epoch—more deaths were caused by exposure to cold weather than hot, simply because long cold winters have been more common than very hot summers. Now, global warming is shifting the balance towards higher temperatures, making it more likely that people will face dangerous heat. Extreme temperatures that only occurred once in 50 years in preindustrial times will occur 9 times as often if the global average temperature rises 1.5°C; 14 times as often if it rises 2.0°C; and 40 times as often if it rises 4.0C°.^[4]

Heat waves are increasing in frequency, duration, and intensity, and affecting more people every year.

“Globally, climate change added on average 41 additional days of dangerous heat in 2024 that threatened people’s health…. The countries that experienced the highest number of dangerous heat days are overwhelmingly small island and developing states, who are highly vulnerable and considered to be on the frontlines of climate change.”^[5]

Infants and older adults face the greatest risks.

“Adults older than 65 years and infants younger than 1 year, for whom extreme heat can be particularly life-threatening, are now exposed to twice as many heatwave days as they would have experienced in 1986–2005…. Over 60% of the days that reached health-threatening high temperatures in 2020 were made more than twice as likely to occur due to anthropogenic climate change and heat related deaths of people older than 65 years increased by 85% compared with 1990–2000.”^[6]

It’s usually estimated that close to half a million people have died of heat related causes every year since 2000,^[7] but since heat mortality statistics for many countries range from inadequate to nonexistent, it’s likely that the real death toll is much higher.

And it is growing fast. A study of trends in 748 cities in 46 countries found that “even under warming levels in line with the Paris agreement (1.5–2 °C), non-extreme seasons are becoming increasingly rare for most locations while uncharted territories are first becoming the new extremes and then eventually regular.”^[8] The Lancet projects that even if the average global temperature increase is kept under 2ºC, the annual number of heat-related deaths will increase 370% by 2050.^[9]

A Rutgers University study finds that if greenhouse gas emissions remain at current levels, by 2100 heat stress from combinations of high humidity and heat will affect 1.2 billion people a year, particularly outdoor workers and the elderly. That’s more than four times the number of people affected by heat stress today and more than 12 times the number who would have been affected without global warming.^[10]

During their lifetimes, today’s children will directly experience up to seven times as many extreme heat events as those born in 1960, even if the Paris Agreement targets are met. “Under a 1.5 °C pathway, 52% of people born in 2020 will experience unprecedented lifetime exposure to heatwaves. If global warming reaches 3.5 °C by 2100, this fraction rises to 92% for heatwaves, 29% for crop failures and 14% for river floods.”^[11]

Heat domes, an intense form of heat wave that occurs when the high-altitude jet stream stalls over a region for days or weeks, have tripled in frequency since the 1950s. A 2025 study warns that current climate models don’t take this increase into account, so predictions of extreme summer heat (including those described above) may understate the danger.^[12]

Between May 2024 and May 2025, heat waves affected every country in the world, and 4 billion people—about 49% of the global population—experienced at least 30 additional days of extreme heat, compared to the 1991-2020 period.^[13] The old and the weak were most severely affected.

“Among older adults and people with pre-existing medical conditions, extreme heat increases the risk of cardiovascular strain, respiratory distress, and premature death. Low-income and marginalized communities often lack access to cooling, healthcare, and safe housing, exacerbating their exposure while limiting their ability to recover from heat-related illness and other impacts. Outdoor workers and people working indoors without cooling face heightened occupational risks, including dehydration, heat stress, and reduced productivity.”^[14]

A recent analysis found that, between 2020 and 2024, the number of days considered dangerously hot for pregnant individuals doubled in 90% of countries and 63% of cities, notably in the Caribbean, and parts of Central and South America, the Pacific Islands, Southeast Asia, and sub-Saharan Africa.^[15]

As Dr. Mora and his associates write, “our choices for deadly heat are now between more of it or a lot more of it.”^[16]

Attribution: Murderers in Boardrooms

Until recently, reports on extreme weather carried a routine disclaimer to the effect that while such events would become more common with global warming it was impossible to attribute any specific event to climate change. That particular hurricane or heat wave or drought or wildfire might just have been an example of normal climate variation. The merchants of doubt of course seized on that uncertainty to declare there was nothing to worry about.

That’s no longer the case. The discipline known as Extreme Event Attribution has made huge strides in determining whether specific events are caused climate change. Complex mathematical tools are required, but in general a detailed weather history of the area affected is prepared and input to a massive computer climate simulation that is run thousands of times, with and without climate change, to see how likely the actual event was.^[17]

The first such study, published in 2004, showed that the European heat wave of 2003 was made twice as likely by climate change. By 2024, 735 extreme weather events had been the subjects of 612 attribution studies, with these results:

Climate change impact	Events
More severe or more likely	547 (73.6%}
No influence	71 (9.6%)
Less severe or less likely	64 (8.7%)
Inconclusive	53 (7.2%)

These results include cases in which scientists found that the extreme event would have been “virtually impossible” without human influence. ^[18]

Extreme Event Attribution took a major leap forward in 2025, with a study of 213 heatwaves between 2000 and 2023. In addition to proving that the heatwaves were made much more likely by climate change, it shows how much each major fossil fuel and cement producer contributed to the rise in deadly heat.

Using the scientifically proven fact that greenhouse gas emissions are driving climate change, and public data from 180 “carbon majors” that together have accounted for 60% of cumulative anthropogenic emissions since 1850, the researchers found:

• That heatwaves became about 20 times more likely during 2000–2009, and about 200 times more likely during 2010–2019.
• That a quarter of those heatwaves (55) were made at least 10,000 times more likely by climate change —e.g. they were “virtually impossible” without it.
• That any one of the carbon majors, even the smallest, produced enough emissions to cause several of the ‘10,000 times more likely’ events.^[19]

About half of the emissions caused by the 180 carbon majors came from just 14 corporations and state-owned producers: the state-owned companies in the former Soviet Union and their successors, People’s Republic of China for coal, Saudi Aramco, Gazprom, ExxonMobil, Chevron, National Iranian Oil Company, BP, Shell, India for coal, Pemex, CHN Energy, and People’s Republic of China for cement.

“There are heatwaves that the carbon majors have made at least 10,000 times more likely compared with preindustrial levels, and which would have otherwise been virtually impossible without anthropogenic influence. Even relatively minor shares in total emissions lead to very substantial increases in the frequency of these events.” ^[20]

The emissions of each of the 14 largest carbon majors were enough to cause over 50 heat waves that would have been virtually impossible without climate change. Even the smallest of the 180, Russian coal miner Elgaugol, produced enough CO2 to cause 16 otherwise impossible extreme heat events.

In a sense, this is not a surprise. The science on climate change is very robust, and scientists have long identified fossil fuel and cement production as primary sources of the emissions that are driving climate change.

Nevertheless, this study reveals the smoking gun. It proves that a few entities are directly responsible for the unprecedented increase in deadly heatwaves in our time, that even the smallest ones are pushing our climate over the brink.

There is, we can now say with certainty, a prima facie case that what Frederick Engels long ago called social murder is being perpetrated, and that the top decision makers in a handful of global organisations are guilty.

POSTSCRIPT: One day after this article was posted, an attribution study by researchers at Imperial College London estimated that 16,469 people in 854 European cities died in the summer of 2025 as a result of climate change. That’s nearly 70% of all heat-related deaths in those cities in June through August, which was the fourth hottest European summer on record. People aged 65 and over made up 85% of the excess deaths. The researchers warn the result is only a snapshot of the death toll linked to extreme heat, as the cities studied represent about 30% of Europe’s population.[21]

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Notes

[1] Kristie L. Ebi, et al., “Hot Weather and Heat Extremes: Health Risks,” The Lancet, August 21, 2021.

[2] Jay Lemery and Paul Auerbach, Enviromedics: The Impact of Climate Change on Human Health, (Rowman & Littlefield, 2017), 21.

[3] Camilo Mora et al., “Twenty-Seven Ways a Heat Wave Can Kill You: Deadly Heat in the Era of Climate Change,” Circulation: Cardiovascular Quality and Outcomes, November 9, 2017.

[4] Jonathan Gregory, Matt Palmer and Ed Hawkins, “Climate Change 2021—The Physical Science Basis, Key Points,” Climate Lab Book, February 25, 2022.

[5] World Weather Attribution, “When Risks Become Reality: Extreme Weather In 2024,” December 27, 2024.

[6] Marina Romanello et al., The Lancet Countdown on health and climate change, 2023.

[7] John Nairna and Simon J. Mason, “Extreme heat and heatwaves: hazard awareness and impact mitigation,” The Lancet Planetary Health, July 7, 2025.

[8] Samuel Lüthi et al., “Rapid increase in the risk of heat-related mortality,” Nature Communications, August 24 2023.

[9] Romanello, The Lancet Countdown 2023.

[10] Dawei Li, Jiacan Yuan and Robert Kopp, “Escalating global exposure to compound heat-humidity extremes with warming, Environmental Research Letters, 2020.

[11] Luke Grant et al., “Global emergence of unprecedented lifetime exposure to climate extremes,” Nature, May 7, 2025.

[12] Xueke Li et al., “Increased frequency of planetary wave resonance events over the past half-century,” Proceedings of the National Academy of Sciences June 16, 2025.

[13] Joseph Giguere, et al., Climate Change and the Escalation of Global Extreme Heat. (Climate Central, Red Cross Red Crescent Climate Centre, World Weather Attribution, 2025), 8.

[14] Giguere, Climate Change, 8.

[15] Climate Central, Climate change increasing pregnancy risks around the world due to extreme heat, May 14, 2025.

[16] Mora. “27 Ways.”

[17] Ayesha Tandon, Q&A: The evolving science of ‘extreme weather attribution’, Carbon Brief, November 18, 2024.

[18] Robert McSweeney and Ayesha Tandon, “Mapped: How climate change affects extreme weather around the world,” CarbonBrief, November 18, 2024.

[19] Yann Quilcaille et al., “Systematic attribution of heatwaves to the emissions of carbon majors,” Nature, September 11, 2025, 392-98; Karsten Haustein et al., “Heatwaves linked to emissions of individual fossil-fuel and cement producers,” Nature, September 10, 2025, 319-20.

[20] Quilcaille, Systematic Attribution,” 395-6.

[21 Imperial Grantham Institute, Summer heat deaths in 854 European cities more than tripled due to climate change. 2025.