Extreme heat is no longer arriving only as isolated spells affecting individual cities or countries. Some events now stretch across continents, overlap with one another and persist somewhere on Earth for months or even years.

A Reuters investigation of global temperatures since 1960 found that exceptionally large heat events have increased from an average of about five a year in the 1960s to around 102 annually in the current decade a more than twentyfold increase.

Reuters has introduced the term “heatstorm” to describe these unusually widespread events. It is not an official meteorological classification, but a framework developed by the news organisation after consulting climate researchers and analysing more than six decades of global temperature data.

The findings offer another way of understanding a warming planet: not simply through changes in global average temperature, but through the expanding amount of the Earth experiencing unusual heat at the same time.

What is a ‘heatstorm’?

There is no single international definition of a heatwave.

For its analysis, Reuters defined a heatwave as at least five consecutive days in which daily high temperatures reach the hottest 10 per cent of the historical range for that location and time of year.

The largest of these events become “heatstorms” when they cover at least one per cent of the Earth's surface at their peak, an area roughly half the size of Canada.

Using that measure, Reuters found that heatstorms were relatively rare six decades ago.

Today, they have become much more common.

Ordinary heatwaves have also increased, but the analysis found that the geographical area affected by heatwaves and heatstorms has expanded dramatically. The average share of the planet likely to be covered by such events has grown more than twelvefold since the 1960s.

Over the same period, atmospheric carbon dioxide concentrations have increased by more than 30 per cent.

Extreme heat is also lasting longer

The change is not only about how many heatstorms occur.

They are increasingly overlapping.

Reuters describes periods when heatstorms occur consecutively or simultaneously as “heatstorm cascades.”

Such cascades lasted an average of roughly two months in the 1960s. By the early 2000s, their average duration had grown to around nine months.

Then something unusual happened.

A cascade began on July 6, 2008.

It has not ended.

For more than 18 years, Reuters found, at least one heatstorm has been active somewhere on the planet every day, and often several have existed simultaneously.

The pattern illustrates how extreme heat is becoming less episodic at the global level. One region may cool while another enters a period of exceptional warmth, allowing large heat events to overlap across the planet.

One heatstorm touched most of the Earth

The largest event identified in the analysis, named Heat Storm Vera by Reuters, began in July 2023 and lasted 367 days.

Over its lifetime, Vera passed across nearly 85 per cent of the Earth's surface, although different regions experienced the heat at different times.

Its impacts appeared in very different forms.

Reuters documented extreme heat affecting students in the Philippines, drought and wildfires in Louisiana, cocoa production in Ivory Coast and wildlife in Brazil.

The event demonstrates why geographical scale matters.

What may appear locally as separate heatwaves can sometimes be part of a much larger pattern of unusual temperatures unfolding across different regions.

And Vera has already been followed by another major event.

Heat Storm Raimundo, which began on February 28, 2026, had touched almost three-quarters of the Earth's surface by the time of the Reuters analysis.

At its peak on August 28, it covered 33.8 per cent of the globe simultaneously, surpassing Vera's previous record for peak geographical extent.

The event contributed to severe European summer heat and conditions associated with major wildfires in Spain and France.

A different way to see global warming

Global warming is usually communicated through averages: fractions of a degree added to the planet's temperature over decades.

Those numbers are scientifically important, but they can feel distant from everyday experience.

Extreme heat is different.

It closes schools. It puts outdoor workers at risk. It raises electricity demand. It stresses crops and livestock. It can worsen drought and wildfire conditions and place additional pressure on hospitals.

The Reuters analysis does not replace established scientific measures of climate change, nor does the term “heatstorm” represent a new meteorological category.

Instead, it offers a way of visualising what rising global temperatures increasingly look like on the ground.

A warmer world does not mean every place is unusually hot every day.

It means the conditions for extreme heat are shifting.

Six decades ago, exceptionally large heat events were unusual.

Today, somewhere on Earth, one has been continuously under way for more than 18 years.