Human-caused climate change added an extra 30 days of extreme heat for more than four billion people worldwide over the course of a year, a new report has found.

The report, a joint effort by the World Weather Attribution, Climate Central and the Red Cross Red Crescent Climate Center, assessed the human impact on global heatwaves over the past 12 months. It was released ahead of the Red Cross’ Heat Action Day on June 2, meant to raise awareness of extreme heat events. This year’s theme is centered on recognizing and responding to heat stroke.

The report found that in 195 countries and territories, climate change has at least doubled the number of extreme heat days compared to a world without climate change. The Caribbean country of Aruba saw the highest number of extreme heat days at 187, compared to an estimated 45 days without climate change. Out of the 12 countries and territories that were most impacted by extreme heat — which saw an average of more than 137 extreme heat days above zero-emission scenarios — 11 were in the Caribbean, and one, Micronesia, is in Oceania.

Climate change added an extra month of extreme heat for 4 billion people – attribution analysis by @wwattribution.bsky.social, Climate Central and Red Cross Red Crescent Climate Centre finds

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— Grantham Institute – Climate Change and the Environment (@granthamicl.bsky.social) May 30, 2025 at 7:50 AM

“2024 was the hottest year on record, surpassing even 2023 which was the hottest before that, and when we came into 2025, we started with the hottest January ever on record and there was record breaking low sea ice in the northern hemisphere in winter,” Mariam Zachariah, World Weather Attribution researcher at the Centre for Environmental Policy, Imperial College London, told reporters at an online press conference.

“This is no surprise,” she added. “As we all know, this is a direct consequence of continued fossil fuel use and increasing emissions in the atmosphere. As a consequence, we are seeing many heatwaves which are now very frequent. They are becoming stronger, and they’re also persistent.”

The report analyzed “extreme heat days” globally over the past 12 months, from May 1, 2024 until May 1, 2025. These days were defined as having seen temperatures hotter than 90% of observational temperatures recorded in a given area between 1991 and 2020. They then simulated how many of these extreme heat days would have occurred in a world with zero carbon emissions.

“We’re able to compare this world we have actually lived in over the last 12 months with the world that would have been without climate change to get a sense of how many additional days above this 90th percentile temperature climate change has caused,” Kristina Dahl, vice president for science at Climate Central, told reporters.

The study determined that more than four billion people, or roughly half of the world’s population, saw at least 30 additional days of extreme heat since this time last year.

“If you look at this map, which shows the number of days above that 90th percentile temperature added by climate change, you can see that the scale goes up to 120 days, and many of these countries, particularly around the tropics, have seen somewhere between 100 to 120 days of additional extreme heat due to climate change,” Dahl said.

In addition to the extreme heat days, the researchers analyzed 67 specific “significant” extreme heat events, which needed to have either record-breaking or “unreasonably high” temperatures, or result in at least 10 deaths or significant disruptions to essential services such as transportation, manufacturing and energy.

Additionally, the event also had to satisfy one of the following three conditions:

1. “The heat identified is occurring during the first 3-6 weeks of the hot season (due to heightened vulnerability of early season extreme temperatures).”
2. Heat “occurring in a densely populated area (≥200 people/km^2).”
3. Heat “occurring in a highly vulnerable area and/or one with a high lack of coping capacity.”

Each of the 67 events monitored was determined to have been influenced by climate change, and was found to have impacted “232 different countries and territories across all inhabited continents,” according to the report.

Out of these 67 events, the team studied four in depth: the extreme heatwave that hit Central Asia in March 2025, the heatwave in South Sudan in February 2025, the deadly heatwave that hit the Mediterranean in July 2024 and the June 2024 heatwave in Mexico and surrounding regions, including the American Southwest and Central America. Of these, the researchers found the Central Asian, South Sudanese and Mediterranean heatwaves would have been impossible without climate change.

“The impacts are often not reported immediately after the heatwave. So it is a silent killer, and even though something happens as a result of heat waves, it can exasperate underlying conditions, and consequently the numbers can be misreported, or it can be underreported,” Zachariah said.

Extreme heat can have devastating effects on human health, according to Lisa Patel, executive director of the Medical Consortium on Climate and Health and clinical associate professor of Pediatrics at Stanford School of Medicine.

“There have been many studies that have been done on what are the limits? We, as humans, were adopted for a certain climate down to the molecular level in terms of our proteins and how our body operates,” Patel told reporters.

“Some of those studies show that we can handle up to 115 degrees with minimal humidity at rest, drinking water continuously, if you were otherwise healthy, and we are topping those temperatures all over the world. And consider how many of those factors have to be in place, and how often most of those factors are not in place in terms of what human beings can handle,” she said.

Patel noted that we humans have one physiological response to heat: sweating.

“When it gets very hot outside, our blood starts to get hotter inside,” she explained. “We start pumping that blood out to the periphery. And that’s essentially for evaporative cooling… It does not work as well if it’s very humid outside, because if there’s already a lot of moisture in the air, you can’t sweat because there’s nowhere for that water to go. But also, if it gets so hot outside that at some point your body’s lost its moisture, you can’t sweat anymore, or there’s just no way for your body to gain any additional cooling, that heated blood goes back internally that starts to set off a cascade of different problems.

“Because all your body knows to do is to pump that blood to the periphery, you start losing blood to places like your brain, your liver, your lungs. That’s why you, for example, get dizzy. People start to get confused, for example. If this goes on and on, you can end up with organ damage and heat stroke, and death can ultimately result.”

This is why heat stroke is a “minute’s emergency,” and why it’s crucial to recognize signs of heat exhaustion before that happens, she said.

A precise death toll from extreme heat events is hard to estimate because, as the study points out, “Many heat-related deaths are misattributed to comorbid conditions, such as cardiovascular and pulmonary conditions, or renal failure, obscuring the role of elevated temperatures as an aggravating factor.”

These events also harm ecological health significantly, Karina Izquierdo, urban advisor for Latin America and the Caribbean at the Red Cross, Red Crescent Climate Center told reporters.

“Environmentally, extreme heat increases wildfire risks, worsens air pollution and disrupts ecosystems, causing biodiversity loss and soil degradation,” she said.

In addition, extreme heat events can put enormous strain on systems and infrastructure, including healthcare, energy, housing, governance, transportation and much more.

“Health services are strained by increased illness and mortality, and mental health issues may be exacerbated as well,” Izquierdo added. “These challenges are often made worse by infrastructure failures like blackouts and transport disruptions, which limit access to essential services when it comes to workers, especially those that are outdoors or in poorly ventilated environments or without cooling devices.

“Heat also drives up demand for services like water and electricity, increasing the risk of shortages. Meanwhile, indoor environments shaped by construction materials, limited cooling access and vulnerable urban areas like informal settlements can intensify the exposure and add to the challenges that different groups face during a heatwave,” Izquierdo said.

She added that simple and low-cost solutions are available to reduce these risks, like shared cooling spaces or keeping key locations like hospitals, schools, homes and public transportation cool. Using wet towels, taking cool showers, keeping properly hydrated and not over-exerting during the day can also help, she said, along with adaptation efforts like reflective painting on roofs, which can reduce heat in homes by several degrees Celsius, or if scaled up, can make an even larger city-wide impact and reduce urban heat island effects. But these efforts need to be paired with carbon mitigation strategies as well, she said.

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