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Extreme heat has been a far greater challenge for urban populations around the world than rural ones over the past 40 years.

Cities are vulnerable to heat because of both climate change and something called the 鈥� a phenomenon where urban areas are much hotter than surrounding rural regions due to their higher concentration of built infrastructure and human activities.

at the University of California, Santa Barbara, and his colleagues used worldwide urban population data combined with global fine-resolution maximum air temperature and relative humidity estimates to figure out just how much urban regions have been affected by daily temperatures greater than 30 掳C between 1983 and 2016.

鈥淚 really think we鈥檝e been flying blind in terms of understanding the health and heat impacts, and I think this study provides a map, so to speak, for us to start thinking about where to target interventions and policies,鈥� says Tuholske.

The team combined data for 13,115 urban settlements 鈥� including some in rapidly urbanising regions, such as sub-Saharan Africa, the Middle East and southern Asia, that are frequently underrepresented in urban studies.

Tuholske and his colleagues estimated that global exposure to extreme heat 鈥� assessed using a metric that considered the number of individuals affected and the number of days each person was affected 鈥� increased by almost 200 per cent from 1983 to 2016. In 1983, there were 40 billion person days of exposure per year, while that figure was 119 billion in 2016.

鈥淭his work points to the need for further research on the drivers of heat-health outcomes in urban areas,鈥� from the diversity of urban fabric to mobility, social inequality and temperature-related mortality, says at University College London.

The data gathered will be made public to allow others to assess how their communities are being affected by extreme heat, says Tuholske.

鈥淭he majority of major cities around the world are very much aware of these issues, and a lot of cities are actually taking a lot of initiative to address these issues,鈥� says at the City University of New York.