"There is no universal definition of heatwave," Simone Russo of the European Commission Joint Research Centre and the Institute for Environmental Protection and Research, both in Italy, told environmentalresearchweb. "The Heat Wave Magnitude Index daily (HWMId), introduced in this study, merges the duration and intensity of heat events into a single number that enables researchers to compare heatwaves that have occurred in different regions and in different years."

The HWMId’s predecessor index, the HWMI, measures the maximum magnitude of heatwaves in a year, with a heatwave defined as a period of three or more days in a row where the maximum temperature is more than the 90th percentile of the daily maxima for a matching 31-day window over the period 1981–2010. The HWMId extends this concept by summing up the daily magnitude of the heatwave by including a factor relative to the 25th and 75th percentile of the annual maximum temperatures for 1981–2010. The HWMI was prone to saturation if the daily maximum was higher than the highest recorded temperature for 1981–2010.

"The previous HWMI index had some limitation in assigning magnitude to very high temperatures," said Russo. "This limitation results in an underestimation of heatwave magnitude, in particular in a changing climate. The HWMId is able to overcome this limitation and could really become a benchmark for evaluating the impacts of future climate change."

Russo believes the index could become the reference for the universal definition of a heatwave. "As an example, a heatwave could be defined as at least three consecutive hot days with a HWMId score equal to or greater than 3," he said.

The team used the HWMId to rank European heatwaves since 1950. This confirmed that the 2010 heatwave in Russia, which killed tens of thousands of people, was the worst heat event in Europe since at least 1950. Using the HWMId revealed that the heatwave in Finland in 1972 was comparable in its geographical spread and magnitude to the central European heatwave of 2003, which is considered the second strongest heatwave of the observational era. The Finland heatwave took place over a larger area but had a smaller peak magnitude and duration.

"An excess mortality of 840 deaths – 2% of all annual deaths – in the summer of 1972 in Finland was directly attributable to the heatwave," said Russo. "On the contrary, the European heatwave of this summer, 2015, had lower magnitude than that in the summer of 2003. Its spatial extent at different HWMId levels was comparable with the heatwaves that occurred in Greece in 2007, in central Europe in 1994 and in Scandinavia in 2014."

As climate warms, the frequency and magnitude of heatwaves in Europe from 2021 to 2040 is likely to increase, the team’s modelling showed. Regional climate projections suggest that Europe will see an enhanced probability of heatwaves with a magnitude, extent and duration comparable to or greater than the Russian heatwave in 2010.

"The probability of experiencing a major European heatwave in the coming decades is higher in RCP8.5 than RCP4.5, even though global mean temperature projections do not differ substantially," said Russo. "This calls for a proactive vulnerability assessment in Europe in support of formulating heatwave adaptation strategies to reduce the adverse impacts of heatwaves."

Now the researchers plan to use the index for assessing risk from heatwaves to different sectors in the future. "In particular we will investigate financial and economic resiliences to climatic shocks," Russo said.

The team reported their results in Environmental Research Letters (ERL).

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