Imagine walking along Ipanema beach on a summer afternoon. The sand is golden, there’s a cooling sea breeze, the shade of a parasol and a cold drink in hand. Now look up.

Clinging to the hillside just a few hundred metres away is Vidigal, one of Rio’s favelas in the Brazilian city. Here, thousands of people live in a heat trap with metal roofs, no parks and no formal public transport networks.

In nearby sprawling suburbs, families face the same suffocating nights and concrete pavements radiate heat long after sunset. If there are no cool public spaces to retreat to, no water fountains or drinking water sources to guarantee relief, extreme heat is inescapable.

Rio is far from alone. Last summer, Europe sweltered. Spain recorded highs of 46°C. Portugal hit 46.6°C. France experienced its second-hottest June since 1900. In the US, more than 150 million people faced extreme heat warnings. In south Asia, west Africa and Latin America, extreme heat is not just seasonal.

But the consequences of heat are not evenly distributed. They vary between countries, regions and neighbourhoods. Differences in demographics, infrastructure and capacity to adapt all shape how badly people are affected.

Our new study shows that this “systemic cooling poverty” is widespread yet unequal across 28 – predominantly developing – countries.

Across the 3 billion people represented by our sample, nearly 600 million are experiencing severe levels of systemic cooling poverty. People in south Asia and sub-Saharan Africa bear the heaviest burden.

Yet countries facing similar extreme heat can highlight different outcomes. Indonesia and Bangladesh both face exposure to........

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