Heat waves become more frequent and deadly

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 (Image: Pixabay CC0)
(Image: Pixabay CC0)
The risk of fatal heat waves has already risen sharply over the past twenty years. In the future, such extremes will become more frequent and heat-related excess mortality will increase. Europe is particularly affected by this, as researchers show.

Heat waves like the one we are currently experiencing are particularly deadly for the elderly, the sick and the poor. The heat wave of 2003, with temperatures in Europe reaching 47.5 degrees, was one of the worst natural disasters of recent decades, with an estimated 45,000 to 70,000 deaths within a few weeks. Forests were in flames, fields withered, and emergency departments in cities filled up. The global cost was around $13 billion. Yet public awareness of the risks of such heat waves is still low compared to other climate-related extremes. This is risky, as an external site study call_made recently published in the journal Nature Communications shows. Heat waves like the one in 2003 could become the new norm in the coming years.

Epidemiology and climate modeling combined

Researchers from ETH Zurich’s Institute for Environmental Decisions collaborated with an international group of epidemiologists for the study. Since 2013, this group has systematically collected data on daily heat-related excess mortality for 748 cities and towns in 47 countries in Europe, Southeast Asia, Latin America, the United States and Canada. Using this dataset, researchers calculated the relationship between daily average temperature and mortality for all 748 localities. This results in an ideal temperature for each location at which the lowest excess mortality occurs. In Bangkok, for example, this value is 30 degrees, in São Paulo 23, in Paris 21, and in Zurich 18.

Physically plausible weather extremes modeled

Every tenth of a degree above this ideal value increases excess mortality. "Not all heat is the same," says Samuel Lüthi, lead author of the study and a doctoral student at David Bresch’s Professorship of Weather and Climate Risks. "The same temperature has completely different effects on heat-related excess mortality in the population in Athens and Zurich." This depends not only on temperature, but also on physiology (habituation), behavior (long siestas over midday), urban planning (green spaces vs. concrete), population demographics and the respective healthcare systems.

Using the ideal values, the researchers calculated how excess mortality would develop with an average global warming of 0.7 degrees (value for the year 2000), 1.2 degrees (value for the year 2020), 1.5 and 2 degrees. To do this, 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, each time with slightly different weather conditions. This results in a large number of possible weather systems that are likely for a given CO2 content in the atmosphere," Lüthi explains. The researchers then coupled these data with an epidemiological model to calculate the corresponding heat mortality.

Previous forecasts of heat mortality were usually based on calculations using a climate model for a specific time period. "With our method, we can quantify extremes in the climate system much better and reduce uncertainties due to peculiarities of certain models." Using supercomputers, Lüthi has calculated the heat-mortality effects of more than 7000 years of physically possible weather phenomena. The corresponding data set amounts to more than a terabyte.

Up to 15 percent heat-related deaths

The results show that the risk of heat waves with large excess mortality has already increased sharply over the past 20 years. "The excess mortality of a heat summer like 2003, which used to be considered an extreme event occurring every 100 years, we now expect every 10 to 20 years," says Lüthi, "and in a world two degrees warmer in many places even every two to five years." Heat mortality rates that were considered highly unlikely as recently as 2000 (once every 500 years) will occur 14 times every 100 years in a two-degree world. Assuming no adaptation to the heat, this increases the probability of mortality from such extreme heat waves by a factor of 69.

Some regions are particularly at risk of increasing heat waves, including the Gulf and Atlantic coasts in the United States, the Pacific coast of Latin America, the Middle East, Southeast Asia, and the Mediterranean region. Even in moderate climate scenarios, a hot summer in these areas can result in ten percent of total deaths in a country being due to heat. In Paris, which was particularly affected by the heat wave in 2003, the figure was five to seven percent at the time. That’s around 2700 people who died prematurely in the French metropolis alone due to the climate heating up. From dehydration, heat stroke or cardiovascular collapse. "In the future, according to our calculations, up to 15 percent of deaths in Paris could be heat-related," Lüthi says. Europe, and southern Europe in particular, are among the "hotspots." Two factors would come together there: Temperatures are rising twice as fast as the global average, and the population is older than average.

Frightening outlook

"The results scared me," says the 30-year-old climate scientist. "I kept trying during the study to see the human lives affected behind the numbers. It’s scary." At the same time, he says, the assumptions underlying the modeling are rather conservative. With current greenhouse gas emissions, the world is not on a path to a maximum warming of 1.5 to 2 degrees Celsius, as assumed in the study, but to 2.6 degrees. In addition, population growth, migration to cities, and increases in the elderly are not accounted for in the future scenarios - all factors that are likely to further increase heat-related excess mortality. Finally, the study lacked epidemiological data on Africa and India, both regions severely affected by the climate crisis and poverty.

The researchers write that the results highlight the urgency to act. To at least curb increasing heat waves, the most important step is to phase out fossil fuels as soon as possible, Lüthi says. The study shows that the risk at 1.5 degrees is already high, but still much lower than at two degrees. However, society can also adapt to higher temperatures to some extent to reduce the impact of future heat waves. "We should now prepare for the inevitable as quickly as possible and prevent situations that can no longer be controlled at all costs," advises Lüthi.
Samuel Schlaefli