Once-​in-a-century heatwave may soon happen every other year

As the world experiences increasingly frequent and deadly heatwaves, researchers are shedding light on the alarming reality of this pressing issue. The 2003 European heatwave, which led to an unprecedented 45,000 to 70,000 fatalities, may be a foreshadowing of the new normal for the world’s climate. 

A team from the Institute for Environmental Decisions at ETH Zurich, partnering with international epidemiologists, has collected heat-related mortality data from 748 cities (spanning 47 countries, from Europe to Southeast Asia) since 2013.

Ideal temperatures

Samuel Lüthi, the study’s lead author, elaborated on the team’s innovative approach. The researchers combined daily temperature data with mortality rates to identify each location’s “ideal temperature” – a point at which excess mortality is minimized. 

For example, while Bangkok’s ideal temperature stands at 30 degrees Celsius, Paris is most comfortable at 21 degrees. Every tenth of a degree above this ideal value increases excess mortality. 

“Not all heat is the same,” said Lüthi. “The same temperature has a completely different impact on heat-​related excess mortality in the populations of Athens and Zurich.”

Factors such as acclimatization, urban planning, population demographics, and local healthcare play pivotal roles in ideal temperature and heat-related mortality. 

How the team studied heatwaves

To analyze future heatwave scenarios, the researchers turned to five powerful climate models known as SMILEs. 

“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 Lüthi. 

The researchers combined this data with an epidemiological model to calculate the corresponding heat mortality. 

By comparison, previous projections of heat-​related mortality were typically based on calculations that used one climate model over a specific period of time. 

“Our method allows us to quantify extremes in the climate system much more effectively and reduce uncertainties that arise from the idiosyncrasies of certain models,” said Lüthi.

What the researchers discovered 

The results were startling. The researchers found that the once-in-a-century excess mortality rate from the hot summer of 2003 is now anticipated to recur every 10 to 20 years. 

Should global temperatures rise by 2 degrees, such deadly heatwaves could occur every other year or even more frequently. 

Worst-case scenarios suggest that regions like the Gulf and Atlantic coasts of the U.S., Southeast Asia, and southern Europe might see up to 10% of their deaths linked to heat. 

“The excess mortality of a hot summer like 2003 used to be considered an extreme, once-​in-a-century event. We now expect it to occur once every 10 to 20 years, or, in a world that is 2 degrees warmer, every two to five years in many places,” said Lüthi.

Europe, particularly its southern belt, is doubly vulnerable. Not only are temperatures here ascending at double the global rate, but an aging population compounds the risk.

Frightening results

“The results frightened me,” said Lüthi. “While I was working on the study, I always tried to look behind the figures and see the real lives of people who are affected by the changes. It’s worrying.” 

Particularly, as he points out, because the assumptions underlying the modeling are actually on the conservative side. 

The results are based on the assumption that the global average temperature is on track to increase by a maximum of 1.5 to 2 degrees Celsius. However, with greenhouse gas emissions at their current levels, the more likely figure is 2.6 degrees.

Urgent need for action

According to the researchers, the results highlight the urgent need for action. In order to at least curb increasing heat waves, the most important step is to phase out fossil fuels as quickly as possible, said Lüthi. 

The experts say that society can partly adapt to higher temperatures to reduce the impact of future heat waves. “We should now prepare and manage the unavoidable, while avoiding the unmanageable at all costs,” said Lüthi.

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