Quantifying the Escalation of Heatwave Events in Spain: A Study Based on the new XHW Index

Since the early 2000s, Spain has experienced a consistent increase in the frequency and severity of heatwaves. Studies indicate that the number of days with extreme heat summer temperatures has increased, particularly in regions such as Andalusia, Valencia, and Madrid. The 2003 heatwave, regarded as a pivotal event in Europe, resulted in over 70,000 additional deaths across European countries, with Spain contributing almost 20% to this statistic (approximately 13,000 deaths). Further studies emphasise that extreme heatwave events, like the 2003 episode, are no longer exceptional phenomena but are becoming recurrent, even in areas where such events were rare before 1980. These extreme heatwave episodes can pose a serious risk to human health, even leading to severe heat illnesses such as heatstroke, hyperthermia, and critical dehydration. Furthermore, they can also exacerbate pre-existing pathologies like cardiovascular and respiratory diseases. This results in a substantial increase in hospitalisations and even fatalities. This study aims to employ a novel Extreme Heatwave (XHW) characterization index, based on the human body's response to water loss during such events, to evaluate the temporal evolution of these occurrences across Spain, with a special focus on the 13 most populated cities (> 300,000 inhabitants). The analysis utilised ERA5 reanalysis (~25 km resolution) for the period 1950–2024. The results showed that, through the application of a k-means clustering technique, XHW occurrences in Spain could be categorised into three distinct periods: 1950–1977 (P1), 1978–2002 (P2), and 2003–2024 (P3). For this assessment, the daily XHW index values were accumulated monthly and then annually at each grid point of the reanalysis data subsequently interpolated using Bilinear Interpolation for the 13 Spanish cities. The highest percentage increases in cumulative XHW index values were observed in Andalusia, exceeding 1000% in certain grid points when comparing the sums of P3 and P1. Some regions and cities, such as Madrid and Barcelona, experienced virtually no XHW episodes during P1, but these became relatively frequent (in over 80% of the years) during P3. In certain cities and regions, particularly in the Southwest of the country, it was found that during P3, one in every four summer days, on average, presented an XHW index value greater than zero. The results revealed and quantified an alarming scenario regarding the increased intensity and frequency of XHW episodes in Spain. This trend is compounded by the broader context of climate change, which coincides with the warming of the North Atlantic Ocean near the western coast of the Iberian Peninsula and the Mediterranean Sea along the remaining coastal regions of the country. Such developments are likely to exacerbate this situation further in the coming years, potentially precipitating a severe crisis in the Spanish public healthcare system.