PARIS, Jan. 27 -- Nearly 3.8 billion people worldwide could be exposed to extreme heat by 2050, and countries across all climate zones remain dangerously unprepared for the scale of adaptation required, scientists warned on Monday.

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Researchers from the University of Oxford said demand for cooling would rise sharply in large tropical countries such as Brazil, Indonesia and Nigeria, where hundreds of millions of people currently lack access to air conditioning or other effective cooling measures. They cautioned that even regions with traditionally cooler climates would face serious challenges as temperatures climb.

The findings, published in the journal Nature Sustai...