Heat Waves Becoming More Frequent, Deadly, Study Finds

The risk of fatal heat waves has risen sharply over the past 20 years, and such extreme weather will become more frequent in the future, increasing heat-related excess mortality, a study shows. The study, published in the journal Nature Communications, found that Europe will be particularly affected.

Heat waves of the kind we are currently experiencing are particularly deadly for the elderly, the sick and the poor, the researchers said. The 2003 heat wave, which saw temperatures in Europe reach 47.5 degrees Celsius, was one of the worst natural disasters of recent decades, claiming an estimated 45,000 to 70,000 victims in the space of a few weeks, they said.

The researchers from ETH Zurich in Switzerland found that such heat waves could become the new norm in the coming years. Since 2013, they have been systematically collecting data on daily heat-related excess mortality for 748 cities and communities in 47 countries in Europe, Southeast Asia, Latin America, the US and Canada.

The researchers used this dataset to calculate the relationship between the average daily temperature and mortality for all 748 locations. From this, they were able to establish each location's ideal temperature, where excess mortality is at its lowest. In Bangkok, for example, this value is 30 degrees Celsius, in Sao Paulo 23 degrees, in Paris 21 degrees and in Zurich 18 degrees.

Every tenth of a degree above this ideal value increases excess mortality, according to the researchers. "Not all heat is the same," explains Samuel Luthi, lead author of the study and doctoral student under David Bresch, Professor for Weather and Climate Risks. "The same temperature has a completely different impact on heat-related excess mortality in the populations of Athens and Zurich,” Luthi said.

This depends not only on the temperature, but also on physiology, behaviour (long siestas in the middle of the day), urban planning (green spaces versus concrete), the demographic structure of the population, and the local health care system. Using this ideal value, the researchers calculated how excess mortality would develop with an average global temperature increase of 0.7 degrees Celsius (the value in 2000), 1.2 degrees Celsius (the value in 2020), 1.5 degrees Celsius and 2 degrees Celsius.

They used five particularly powerful climate models, known as SMILEs (single-model initial-condition large ensembles). "We ran the same model up to 84 times, with slightly different weather conditions each round. That gave us a multitude of possible weather systems that are likely to occur if there is a certain amount of CO2 in the atmosphere," explained Luthi. The researchers then coupled this data with an epidemiological model to calculate the corresponding heat mortality.