Which cities are warming fastest, and why is heat exposure rising?

Cities around the world are getting hotter. But how quickly they are warming, when that warming occurs, and why more people are being exposed to extreme heat vary from one place to another.

In our new study, published in Communications Earth & Environment, we analysed two decades of satellite observations covering more than 1,400 cities worldwide. We found the fastest warming in cities with cold climates and during winter, particularly across Eastern Europe and Western Asia.

By Dr Marzie Naserikia, Postdoctoral Research Associate, UNSW.

Hot nights are becoming more frequent

Cities are experiencing more hot extremes and fewer cold extremes, with the clearest changes occurring at night. In arid cities, the frequency of hot nights increased by around 47% over two decades.

For this research, an extreme temperature was one falling within the hottest or coldest 10% of observations for that particular city. This allowed us to compare changes across cities with very different climates.

Hot nights are especially concerning because cooler overnight conditions normally give people an opportunity to recover from daytime heat. When temperatures remain high, that recovery is limited, potentially increasing heat stress and associated health risks.

More people are being exposed to extreme heat

Warming is only part of the picture. A city can experience a relatively modest temperature increase but still see a large rise in heat exposure if its population is growing rapidly. Conversely, in cities with slower population growth, exposure may increase mainly because extremely hot conditions are becoming more frequent.

To capture both influences, we combined satellite temperature observations with population data. We estimated exposure in person-days, which combines the number of people potentially exposed to extreme heat with the number of extreme heat days they experience. For example, if one million people experience three days of extreme heat in a year, that equals three million person-days of exposure.

Globally, estimated daytime exposure increased by 51%, from 5.6 billion person-days in 2003 to 8.4 billion in 2020. Climate warming increased the annual rate of growth in daytime heat exposure by 82% beyond what population growth alone would have caused.

A global picture, with local differences

Climate warming and urban population growth are occurring together, but their relative importance varies across climate zones, regions and individual cities. In rapidly growing regions, population growth can be the dominant factor. In many cities in colder climates, however, warming plays a much larger role.

This distinction matters for adaptation. Two cities experiencing a similar increase in heat exposure may require very different responses depending on what is driving that change. Understanding the local balance between warming and population growth can help cities direct their resources towards the most effective measures.

The study, “Two decades of urban heat intensification and exposure across 1400 cities,” is published in Communications Earth & Environment.