The definition of a heatwave depends on where you live. In Sweden, it means exceeding 25C for five consecutive days, while in India it varies between 40C if your home is on the hot plains and 30C if you live in the mountains. Even on a small island such as Britain, the definition of a heatwave varies: in London it is three days above 28C and on the Scottish borders 25C.
It matters because heatwaves are killing people in increasing numbers and governments have a duty to warn their citizens of the risks. Research in India shows that as soon as temperatures stay elevated above what the local population is used to the number of excess deaths begins to climb.
Rather obviously, the hotter the temperature, and for longer, the greater the number of excess deaths, as much as 33%. What was perhaps surprising is that even in a minor, short-lived heatwave 10% more people died than was expected. Because these “minor” heatwaves were far more frequent than the extreme ones overall they claimed more lives as a result. In a heating world where such episodes are becoming more frequent in almost every country, the researchers wonder whether governments need to do more to give their populations earlier warnings of the dangers they face.
