Temperatures in the Northern Hemisphere are smashing one record after another, with extreme, deadly heat being reported in countries from the US and China to Japan, Italy and Spain.   

In China, local media reported new heat records of 52 degrees Celsius (125.6 Fahrenheit) in the country's northwest. Japanese authorities, meanwhile, declared a "heat stroke alert" and urged millions of people to protect themselves from scorching temperatures.  In the US, searing heat is affecting 80 million people. In Spain, a street cleaner died from heat stroke while working outside. 

If the climate warms more drastically — a potential scenario under current policies — about 3.3 billion people could face such extreme temperatures by the end of the century, according to a study published in the Nature Sustainability journal in May. 

The study, led by scientists at the UK's University of Exeter and Nanjing University in China, found that 60 million people are already exposed to dangerous heat levels, characterized by an average temperature of 29 C or higher. The world is currently at 1.1 C above pre-industrial levels.

Weather attribution scientists have found that sweltering heatwaves in the US in June were made five times more likely by climate change, while 2022's 40 C temperatures in the UK would have been practically impossible without planetary heating. Last summer, heat killed more than 60,000 people in Europe alone. 

But why is heat so dangerous to humans and how can countries prepare their populations and cities to deal with increasingly common heatwaves?

How do hot temperatures harm human health?

Extreme heat can result in a range of illnesses and death, according to the World Health Organization (WHO). These include heatstroke and hyperthermia. Temperature extremes also worsen chronic conditions and have indirect effects on disease transmission, air quality and critical infrastructure.

The elderly, infants and children, pregnant women, outdoor and manual workers, athletes and the poor are particularly vulnerable to higher temperatures.

Limiting warming to the lower Paris Agreement target of 1.5 C above pre-industrial levels would still expose 400 million people to dangerous heat levels by the end of the century, the Nature Sustainability study found.

People living in India, Sudan and Niger will all be heavily affected by even 1.5 C warming, but 2.7 C will have enormous effects on countries such as the Philippines, Pakistan and Nigeria.

Calculating the human cost of climate change

Researchers said their study breaks the trend of modeling climate impacts in economic rather than human terms. 

"It invariably distorts value away from human lives and towards centers of wealth," Ashish Ghadiali, a climate activist and co-author of the paper, told DW, adding that modeling focused on economics "places more value on a life in New York state than in Bangladesh."

Most other models also prioritize current populations over future ones, with inequality in global warming being "both globally distributed, but also intergenerational," said Ghadiali. 

"It fundamentally values my life more than my children's lives and certainly more than my grandchildren's lives," he said. 

Looking at individual country impacts on dangerous heat levels, researchers found that current emissions from 1.2 average US citizens condemn a future human to live in extreme heat. Despite having disproportionate emissions, the US population faces a much lower threat from dangerous temperatures.

How can people be protected from extreme heat?

Previous studies have shown cities are particularly vulnerable to such dangerous temperature rises, due to the "heat island effect." Buildings, roads and infrastructure absorb and radiate the sun's heat more than natural environments like forests and water bodies, raising urban temperatures by as much 15 C in some cases, compared to rural areas.

Cities around the world are introducing the new role of chief heat officer to deal with inevitable temperature increases. One of those is Cristina Huidobro, who took up the post for Chile's capital Santiago in March 2022.

"Many cities in the world face extreme heat, but the solutions and the way you approach it are very, very local," Huidobro told DW. 

Still, Huidobro said, they all broadly follow a three pronged strategy — preparedness, awareness and adaptation.

Preparedness can include categorizing heat waves in the same way as other natural disasters, or setting up an alert threshold to trigger a certain city response.

Huidobro said raising awareness of the dangers of heat are an integral part of the role.

"Taking care of yourself in an extreme heat event is really simple — drink water, seek shade and rest," she said. "Nobody has to die from extreme heat."

The third prong is adapting the city to the new reality of high temperatures, largely by creating more green spaces in the city. 

Santiago has just launched an urban reforestation project to plant 30,000 trees across the city and develop strategies that treat the trees as part of the urban infrastructure. 

"Trees, trees, trees, trees everywhere. It's bringing more green into the city," Huidobro said.

But planting trees isn't as easy as people think. 

"We're putting trees in really dense streets, like in the main avenues of the city, where you have a lot of cement. You need to dig a hole and really do some civil works." 

It's also not an instant solution to urban heat as trees need time to grow. 

"The whole idea is to try to plant the shade that we're going to have in the next 20 or 30 years," said Huidobro.

The US cities fighting extreme heat 

The US — where previous studies have found 12,000 people die prematurely from heat each year — has appointed three chief heat officers so far, in Phoenix, Miami and Los Angeles.

The Californian city of Los Angeles, which is ranked as the most susceptible to natural disasters including heat waves, recently launched a campaign to build more "resilience hubs" with shade and cooling powered by renewables in high-risk communities. It already has a network of cooling centers mainly in libraries, where people can go to beat the heat.

They are also working on an early warning system for heat waves.

Phoenix, a city in the middle of the Sonoran Desert, is working on a number of adaptations, including building cooling pavements with a special sealant that reflects the sun. The sealant makes paths several degrees cooler to the touch and keeps the night air cooler.

The city of Miami in Florida is planning major urban tree-planting campaigns, and has also spent millions of dollars on air-conditioning units for public housing residents while providing financial assistance to help cover the energy bills of low-income households.

But Santiago's Huidobro said air-conditioning is generally a last resort for adaptation because of its climate impacts.

Santiago wants to plant 33 "pocket forests" that would be used as climate shelters, especially near schools and health facilities. These are an alternative to the air-conditioned cooling centers being developed in the US and Europe.

"During a heat wave people can go inside these nature-based cooling centers and get their shade, and rest and drink water," said Huidobro. 

Edited by: Jennifer Collins

This article was priginally published in May 2023, and was updated on 17 July, 2023 with the latest on the heatwaves gripping the Northern Hemisphere.