Extreme heat: The rising ‘silent killer’ set to strike cities the hardest

Tuesday, 14 October 2025

Extreme heat could occur more frequently in the future, research suggests. (File photo)

Experts are warning of the increasing threat extreme heat is set to have on cities, with one referring to the risk as a “silent killer”.

Extreme heat has been around for decades, but research has pointed to more events estimated to happen more frequently.

The 2024 State of the Global Climate report showed that global air temperatures in 2024 were the highest on record for the tenth consecutive year.

People walk at Trocadero plaza near the Eiffel Tower during a heat wave July 2, 2025, in Paris.

Research released by the University of Otago in August this year showed that New Zealand is on track for a major spike in extreme heat.

“We know that extreme heat is becoming more frequent and intense,” said urban climate professor at Singapore Management University's College of Integrative Studies, Winston Chow.

“I believe that extreme heat is one of, if not the most, biggest driver of deaths for climate change impacts.”

‘A silent killer’

According to World Health Organisation (WHO) studies between 2000 and 2019, about 489,000 heat-related deaths occur each year.

In 2003, the Northern Hemisphere experienced a summer heatwave which caused temperatures to soar 10 to 12 degrees above normal - 35,000 people died as a result - most of whom were elderly and living alone, said Chow.

In June this year major heatwaves struck across Southern Europe causing temperatures to soar above 40C, and tens of millions of people across the Midwest and East of the US endured dangerously hot temperatures during a June heatwave.

Dr Luke Harrington says very hot days have become much more likely with the effects of climate change.

Senior climate change lecturer at the University of Waikato, Dr Luke Harrington, estimated very hot days and multi-day hot spells have become about three times more likely to occur today because of climate change.

“The dice are being loaded towards extreme heat events happening more often,” he said.

Chow said people tend to see the visual effects of severe storms, hurricanes and floods that “capture attention”. He said heatwaves were a “silent killer”.

“It will cause issues to any city in any part of the world, that’s the scary, insidious nature of heatwaves.”

Urban heat hits the vulnerable hardest

Cities face a compounding problem - they experience additional warming on top of climate change itself, said climate change expert David Dodman, from the Institute for Housing and Urban Development Studies in the Netherlands.

Built-up areas absorb heat in buildings and roads, which in turn release that heat, meaning cities can generate their own climate.

David Dodman, climate change expert, focuses his work on how cities in low-income countries are affected by climate hazards like heat waves, flooding and sea level rise - and how they can adapt to keep people safe.

Surfaces like concrete and asphalt store heat during the day and release it at night, keeping temperatures elevated.

“You get more warming in cities than you would just get as a result of climate change, [creating] a concentration of the effect,” he said.

The deadly impact comes from sustained high temperatures that prevent people cooling down at night.

According to World Health Organisation studies between 2000 and 2019, about 489,000 heat-related deaths occur each year.

“People physiologically can often deal with the peak, but it's the consistent peak, and they're not cooling down at night, that seems to have the physiological effect,” he said.

Chow added inner-city activities like vehicles and air conditioning, which transfer heat from indoors to outdoors, make the problem worse.

While the elderly and people with pre-existing health conditions are particularly at risk, Dodman said living and working conditions can be just as deadly.

Outdoor workers on low wages doing strenuous work - shown in cities like Barcelona - are especially vulnerable, as are people living in poor quality housing that is poorly ventilated or in small, densely packed rooms.

“When you have a physical and human driver of vulnerability, and then these social drivers around living conditions or working conditions come together, that's where you get to a crisis of risk,” he said.

Chow said cities could take three main actions - creating more green spaces to reduce temperatures, blue spaces such as lakes and ponds, and physical infrastructure such as air conditioning and smart urban design.

Early warning systems and community networks were vital to ensuring people stayed safe. But Chow said governments needed to have strong policy, law and regulation to prepare for the risk ahead.

“This is not just a simple ‘people dying of heatwaves’. This has ramifications for the food that we eat or the way we live … from having a stable power grid or a stable 5G network,” he said.

Dodman and Chow will be at the Adaptation Futures conference at Te Pae in Christchurch this week, the flagship event of the United Nations World Adaptation Science Programme, bringing together about 1500 climate researchers and policymakers.

Both experts will be facilitating discussions on climate issues such as “climate overshoot” - the reality that the world is set to exceed the Paris Agreement target of 1.5C of warming.