Heat waves have killed millions. Here’s how scientists tally lives lost

Heatwaves claim thousands of lives each year, with their death toll rising over time. An estimated 210,000 additional individuals succumb to heat-related causes annually from 2012 to 2021 compared to the average from 1990 to 1999. Heatwaves in Asia and Europe have been linked to significant mortality, with 75,000 deaths per hot season in Asia from 1990 to 2019 and nearly 14,000 deaths in Europe during a record-breaking heatwave in June 2023.

Accurately counting heat-related fatalities is challenging, as heat elevates the risk of heart attacks and worsens numerous serious health conditions, but death certificates rarely attribute heat as the cause of death. To bypass reliance on death certificates, researchers have devised methods to differentiate heat-related deaths from total fatalities.

The 'excess deaths' approach compares the total deaths during a heatwave to a baseline period without extreme heat, calculating the difference as the number of heat-related fatalities. This method is rapid and simple but cannot pinpoint the exact death count and only provides estimates after a heatwave has concluded, limiting its use in predicting future heatwave fatalities.

An alternative method employs exposure-response functions, which reveal the relationship between temperature and mortality rates at specific temperatures. These functions can be incorporated into models that estimate deaths for a given temperature increase, enabling real-time application during heatwaves or predictions for future scenarios.

However, precise temperature forecasts are crucial for these models to yield accurate predictions, and non-heat-related shifts in mortality can diminish their accuracy. The UK Health Security Agency recently reported 2,877 heat-related deaths in England during May and June, using the excess-deaths method. Additionally, a study by Gasparrini and colleagues applied the exposure-response method, yielding a similar estimate of 2,736 deaths in England and Wales, with nearly half attributed to human-induced climate change.