The impact of heatwave characteristics on the mortality in Spanish cities

The Iberian Peninsula is known as a hotspot of climate change where heatwaves (HW) have become more frequent and severe over the last decades. These extreme temperature events are associated with devastating socioeconomic consequences.

 

Here we address the relationship between HW characteristics and impact on health at 12 Spanish cities over the last 45 years. The findings will contribute to developing accurate proxies to monitor mortality and to develop mitigation strategies at local scale. 

 

This study aims to explore how different HW characteristics, namely duration, intensity and frequency, influence mortality in 12 different cities of Spain. The ensemble of cities reflects a diversity of climates in Spain, with for example coastal and inland, Mediterranean and Atlantic and lower and higher population density cities. Besides, the performance of two health indices, the recovery factor (RF) and the excess heat factor (EHF), as proxies of HW health impact is investigated to assess which type HWs are potentially more harmful for society and which is its dependence with the city characteristics. The RF measures the capacity to recover at night from daytime heat exposure, while the EHF measures the impact of a sudden heat income. 

 

We used the high resolution (5km) observations of daily maximum and minimum temperatures (AEMET, in English Spanish MetService) to identify HW events, defined as at least 3 consecutive days with daily maximum temperatures above percentile 95th of the reference period, and daily time series of mortality information (INE, in English National Institute of Statistics) for the 12 different Spanish cities. In this study HWs are identified at each gridpoint and then the HWs affecting each city are analysed.

 

Here we study how HWs and its characteristics have evolved during the extended summers (May to September) of the 1975-2019 period across cities. Afterwards, we analyse the relationship between HWs and mortality using a quasi-Poisson regression model and distributed lag non-linear models. Models were adjusted by daily mean temperature to obtain the independent effect of HWs from the risk associated with temperature effect. We additionally tested for potential interactive effects by fitting interaction models between temperature and HW typologies (i.e., using different indices). 

 

Results show that despite a general HW magnification affecting the whole study area, there are significant differences in the characteristics of the HWs at local scale, which could affect the relationship between HW and mortality. Preliminary results for the 3 most populated cities show a significant added risk of mortality during a HW compared to a day without a HW in Mediterranean cities (up to 1.5% in Barcelona and up to 1.2% in Valencia) while non significant results are obtained in Madrid, in central Spain (less than 1.1%). No substantial differences in mortality risk were found in the other cities when comparing a day with a HW and without a HW. 

 

Our findings support the need for local analysis of the synergies between HW and mortality.