Why are heatwaves getting worse?

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Heatwaves have been breaking records around the world, with China enduring its highest-ever temperature, heat advisories being issued across vast swathes of the United States, and fierce heatwaves in southern Europe endangering lives.

Sweltering heat in Europe this summer has presented severe threats to people's health in Italy, Spain and Greece, where wildfires on the islands of Rhodes and Corfu have led to thousands of residents and tourists being evacuated.

With July 2023 set to be the hottest month on record, United Nations Secretary-General António Guterres said "the era of global warming has ended" and "the era of global boiling has arrived."

This extreme heat in Europe and North America - with temperatures topping 45 degrees Celsius (113°F) - would have been "virtually impossible" without the impacts of climate change, according to new analysis released in late July by World Weather Attribution, an international group of scientists.

"None of them is actually an extreme event in the sense that it's rare," said Friederike Otto, a climate scientist at Imperial College London and co-founder of the group. "They are, in today's climate, not rare events."

The research said such severe heatwaves could now be expected about every 15 years in North America, 10 years in southern Europe, and five years in China - where climate change made the recent heatwave at least 50 times more likely.

"It could well be that this will be a cool summer in the future if we don't stop burning fossil fuels," Otto told journalists.

The continued release of planet-heating emissions - largely from the use of coal, oil and gas - will push global temperatures into "uncharted territory" in the coming years, scientists have said.

"The extreme weather – an increasingly frequent occurrence in our warming climate – is having a major impact on human health, ecosystems, economies, agriculture, energy and water supplies," Petteri Taalas, secretary-general of the World Meteorological Organization, said last week.

Here's how climate change is contributing to new global heat extremes - and human risks:

Is climate change the main driver of new heat records?

Yes. Climate change is fuelling a range of extreme weather around the world, from flooding and storms to droughts, but the change it is most clearly producing is more extreme heat.

A record-breaking heatwave across India, Bangladesh, Laos and Thailand in April 2023, for instance, was at least 30 times more likely because of climate change, World Weather Attribution scientists said in May.

Continued use of oil, coal and gas to power homes, cars and the world's economy results in the release of greenhouse gases that blanket the planet, trapping ever more of the sun's energy in the atmosphere rather than letting much of it escape.

About 90% of that excess energy - or heat - has so far been absorbed by the world's oceans, moderating temperature increases.

But ocean surface temperatures are now at their highest level ever recorded, say scientists.

They fear seas may be reaching the limits of their heat-absorbing abilities, which could mean more heat stuck in the atmosphere - and soaring thermometers.

Why are heatwaves dangerous?

Many people look forward to hot summer temperatures - especially holidaymakers - and photographs on sweltering days still often show people at the beach or splashing in fountains.

But heat can be deadly, and many people are unprepared for the level of heatwaves that are now appearing and that are predicted in the future, scientists say.

Extreme heat stress has already doubled in the last 40 years, according to the U.S. space agency NASA. 

Especially in already humid places, when heat and humidity combine to produce a so-called "wet bulb" temperature above 35 degrees Celsius (95 degrees Fahrenheit), the human body can no longer effectively get rid of enough heat, scientists say.

In such extreme heat conditions - which are becoming much more frequent around the world - those exposed can die without swift access to air conditioning, fans or other cooling.

Residents of already hot countries, from India to Iran, are at particular risk - but so are people in normally cooler places such as the U.S. Pacific Northwest, who may not understand the risks or have access to cooling technology.

In 2021, a powerful heatwave killed at least 112 people in the U.S. state of Washington, with 1,400 dying across the broader region, including in Canada.

In Europe, a series of heatwaves in 2022 led to more than 60,000 heat-related deaths, with the highest mortality rates in Italy, Greece and Spain, according to estimates released last week by European health institutes.

How can risks from worsening heatwaves be reduced?

Building awareness that heatwaves are becoming increasingly deadly is a first key step, doctors and scientists say.

Heat researchers have proposed giving heatwaves names, as already happens with hurricanes and other powerful storms, to emphasise the level of threat they present to people. 

Authorities in Spain and Italy have started naming heatwaves in the past year, while the Italian weather news service Meteo.it referred to this summer's prolonged period of extreme heat a "heat storm".

Ensuring that those in the path of heatwaves have access to reliable cooling is also crucial - a problem when increased demand for cooling during heatwaves can spark power outages.

Because growing use of air conditioning can drive more climate change and produce even hotter temperatures, finding low-carbon means of cooling - such as using wind and solar power to generate electricity - is crucial, scientists say.

Shifting work and school hours to cooler parts of the day and providing more breaks and water for workers can also save lives, scientists say, as can finding simple, low-cost ways to make homes and workplaces cooler, such as painting roofs white.

What happens if climate change isn't curbed?

If oil, gas and coal emissions continue and heatwaves worsen, especially hot parts of the planet could become unlivable, sparking mass migration and potentially large-scale deaths if people are unable to find respite from the heat.

Extreme heat could also spur other types of disasters, from water shortages as more water evaporates in hot conditions to worsening droughts, wildfires and losses of nature.

Heatwaves could also hurt economies as workers - especially those toiling outdoors, such as farmers - find it increasingly difficult to do their jobs, or as heat-blighted crops fail, adding to the planet's already growing numbers of hungry people.

This story was updated on July 28, 2023, to include comments by António Guterres.

(Reporting by Laurie Goering and Jack Graham, Editing by Kieran Guilbert and Megan Rowling)