Over roughly six and a half decades, heat waves have progressed from sporadic, isolated episodes to large-scale, cascading climate events capable of spanning vast areas of the planet for months or even years on end.
The largest heat wave of the 1960s affected about 6% of the planet as it stretched for 46 days across the Atlantic Ocean, parts of Latin America, and West Africa.
The largest heat wave of this decade swept nearly 85% of the globe during its 367-day duration, reaching polar latitudes and affecting virtually all humans on Earth.
Continuing to call these far-reaching phenomena “heat waves” would be like calling a monsoon a passing shower. To understand the new reality of these immense expanses of heat, Reuters consulted two dozen meteorologists and climate researchers—and analyzed hourly temperatures for every day since 1960 for more than a million grid squares covering the globe. From these results, we developed a new concept: the “heat storm.”
In simple terms, heat storms are gigantic heat waves. Reuters uses a conservative parameter to define a common heat wave: five or more consecutive days of abnormally high temperatures. Then we define the largest heat waves — those that, at their peak, cover at least 1% of the globe, approximately half the size of Canada — as heat storms.
Applying this definition, Reuters identified a surprising transformation. Heat storms were rare in the 1960s, when there was an average of about five active storms per year. In this decade, heat storms invaded the planet, averaging about 102 per year.
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One of them caused the deadly heat this summer in Europe, with the highest June temperatures ever recorded in Bordeaux, Budapest, and Bilbao, the latter at 42.7°C. As it battered Europe for most of June and July, the heat storm bent steel railway rails, caused what European governments estimated as more than 10,000 additional deaths in a single week, and led Belgian zookeepers to cool their tigers, feeding them raw meat wrapped in ice.
The largest heat wave on record — the one that lasted 367 days between 2023 and 2024 and swept nearly 85% of the globe — raised temperatures in parts of the Philippines to 38°C, causing hardship for students in schools in the country, most of them without air conditioning. In Manila, mathematics teacher Joel Arrojado recalled a student lying face down on his desk, crying. When Arrojado persuaded the boy to raise his face, he saw his face and chest covered in blood. The boy, Arrojado said, had suffered a spontaneous nasal bleed, a problem common in extreme heat. As more blood gushed from his nostrils, the child begged him to call his mother.
While the same heat wave ravaged southern Louisiana for months, it intensified a historic drought. The state burned, suffering what the governor called “the worst wildfire we’ve seen since, at least, World War II” — and about 600 other fires in August alone. Two months later, the smoke from a smoldering swamp fire transformed into a dazzling “superfog” that caused a pile-up on a highway involving at least 158 cars, killing seven people.
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The same heat storm helped fuel a cyclone in the Mediterranean in 2023, which unleashed torrential rains over Libya. In the dead of night, two dams broke near the coastal town of Derna, ripping apart entire neighborhoods. A week later, on her knees, Sabreen Blil dug through the rubble of her brother’s house with her bare hands, hoping to unearth the dead. As she cried, she recited their names. “Taym, Yazan, Luqman, Salmah, Tumador, Hakim and his wife. My God. My family, where are you?” she lamented. “Oh God. Even if it’s only one — my God — let me find at least one body.”
During the annual Hajj pilgrimage in Saudi Arabia, the heat wave raised temperatures at the Grand Mosque in Mecca to 51.8°C, killing more than 1,300 pilgrims. Bodies lay in the streets. Meanwhile, as Europe’s largest wildfire on record spread across Greece during that same heat wave, the flames killed migrants crossing the dense Dadia forest. Their bodies remained in the woods for days; even the vultures, which might have fed on the corpses, had fled the flames.
These distant, seemingly unrelated disasters shared a common thread: a single heat storm.
“Heat storms cause a ‘great disruption to global civilization,’” said James Renwick, a physical geography professor at Victoria University of Wellington in New Zealand. “Heat is the deadliest killer of climate change,” he said, noting that it can lead to “hunger, famine, death and conflicts over resources.”
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A NEW WAY TO UNDERSTAND CLIMATE CHANGE
Heat storms constitute a new way of conceptualizing how Earth’s climate is changing as humanity emits greenhouse gases into the atmosphere.
The finding by climatologists that the global average temperature in 2025 was 1.47°C above preindustrial levels is useful for scientists and policymakers, but for many people it is so abstract that it seems meaningless. What people really understand are heat waves — in Spanish, an “ola de calor”; in Arabic, a “mawjat hararia”; in Mandarin, a “rè làng.”
However, there is no universal definition of a heat wave. For example, authorities disagree on duration: the U.S. National Weather Service defines a heat wave as weather that is “unusually and uncomfortable hot,” typically lasting “two or more days,” while meteorological agencies in Australia, France, Spain, and the United Kingdom set the limit at three days. Some of these countries also have different rules about which temperatures qualify as a heat wave.
In addition, what constitutes a heat wave in Anchorage, Alaska, would not be considered a heat wave in Kuwait City. The climate in these places differs markedly, and a heat wave is an unusual period of hot weather for a given location.
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These conflicting definitions made it virtually impossible to assess whether heat waves are more common globally today than in the past. But Reuters’ system provides the precision needed to do so.
We began by determining the historical temperature range for each day of the year, from 1961 to 1990, for each of the 1.04 million grid squares worldwide. The temperature data come from the European Centre for Medium-Range Weather Forecasts, and our detailed methodology is available here. A heat wave occurs when the daily maximum temperature at a given location reaches at least the 90th percentile of the historical series — that is, the hottest 10% — for five consecutive days.
Thus, a July heat wave in Istanbul corresponds to five or more consecutive days with temperatures of at least 25.6°C, while in Phoenix, Arizona, it requires reaching 40.3°C.
In Reuters’ definition, a heat wave can occur in any season; thus, in Istanbul, a January heat wave would be five consecutive days with temperatures of at least 11.2°C, and in Phoenix the threshold would be 20.8°C.
To measure the geographic extent of heat waves, we mapped neighboring grid squares affected by the heat wave that form a cohesive area. As very extensive heat waves have a cascading effect on the planet’s warming, Reuters created an additional definition: if, at any moment, a heat wave covers 1% of the planet, we designate it as a heat storm and assign it a name, just as is done with tropical storms and hurricanes.
Giving names to these events is “a powerful way to help people understand that something is happening,” said Anthony Leiserowitz, director of Yale University’s Program on Climate Change Communication, to Reuters. However, “we do not currently do this with heat waves,” he said, although it is estimated that heat kills nearly half a million people per year. Defining and naming heat storms is an important innovation, he added, and could help people track impacts and consequences.
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As decades pass, heat storms and heat waves have not only caused individual catastrophes. They have also contributed to systemic and lasting effects.
They exacerbate droughts and intensify fires — from Chile to Europe, the largest wildfires on record occurred during heat storms — and, if occurring during pregnancy, studies indicate that women are more likely to suffer from outcomes ranging from stillbirth to preeclampsia, one of the leading causes of death among pregnant and postpartum women.
Higher temperatures may bring benefits, at least for some regions. Siberia now produces almost as much wheat as Russia’s traditional breadbasket near the Black Sea. This is partly due to the fact that the rise in heat storms and heat waves helped open northern latitudes for agriculture, while contributing to droughts around the Black Sea, reducing production in the region.
18 YEARS OF CONSTANT HEAT STORMS
Dozens of thousands of common heat waves rise and fall each year. Like heat storms, they too have increased in number, more than doubling from the 1960s to this decade. The vast majority, however, are small, never covering even 0.01% of the planet — roughly the size of a square you could drive across in under two and a half hours. Only a tiny fraction of heat waves become large enough to become heat storms, which now total about 102 per year.
However, heat storms now account for the majority of the Earth’s exceptionally hot days, those in the top 10% of the historical record. In the 1960s, heat storms represented, on average, 3% per year for any point on the planet. (The remainder was made up of common heat waves and short heat spikes that did not last long enough to become a heat wave.)
In this decade, heat storms accounted for nearly 43% of those days.
As greenhouse gas emissions increased, the same occurred with heat storms and common heat waves. From the 1960s to this decade, atmospheric carbon dioxide rose by more than 30%. In that period, the planet’s average share susceptible to being covered by heat storms and heat waves rose more than twelvefold. El Niño — the periodic episodes of exceptionally warm waters in the Pacific Ocean that drive major climate patterns — acted as an accelerator, intensifying heat storms and heat waves.
Reuters also examined the opposite of heat waves and heat storms — cold waves and cold storms. They declined. From the 1960s to this decade, the total area affected by a cold wave or cold storm on a typical winter day shrank by nearly 80%.
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Despite its gigantic scale, the heat storm phenomenon has largely gone unnoticed by the public. Until recent decades, heat storms were rarer and smaller in magnitude, making them easier to ignore.
But perhaps as important as that is the fact that they had not been defined and named as precisely as typhoons and hurricanes. Yes, people in Montreal, Perth, and Hong Kong knew they were experiencing a string of hot days — but they did not know they were all part of a single heat storm.
As part of this series examining heat storms and their effects, Reuters integrated an online Heat Storm Tracker into our regular weather coverage. With this tool, people can see what had previously gone unnoticed and, using that knowledge, assess the impact of heat storms as they sweep across regions.
The rapid evolution of heat has not only made individual heat storms larger. Now, in sharp contrast to what happened just 20 years ago, massive heat storms frequently meet, merge, and separate from one another. What has become common are heat storms that occur consecutively, simultaneously, or in both forms – the “heat storm cascades.”
The duration of these cascades increased from an average of about two months in the 1960s to nine months in the early 2000s. Then, on July 6, 2008, a cascade began — and did not stop. For more than 18 years uninterrupted, 6,681 days, at least one heat storm has been ravaging the planet, and often more than one.
Many of them were colossal. In the nearly half-century from 1960 to the start of the current 18-year cascade, only five heat storms had reached a maximum size greater than one-tenth of the planet, roughly the area of Europe and Asia combined. The ongoing cascade included 56 of these megat Storms.
The largest of them was the one we named Heat Storm Vera, the storm that killed pilgrims during the Hajj, intensified wildfires in Louisiana, and reached nearly 85% of the planet over its 367 days of duration.
