Heat stress in the Mediterranean, 1940–2023: trends, drivers and vulnerability

We used the ERA5-HEAT reanalysis to analyze changes in the Universal Thermal Climate Index (UTCI) over Europe from 1940 to 2023, separate the contributions of its meteorological drivers, and map heat vulnerability at NUTS-2 level. The Mediterranean is the region where the signal is clearest: it has warmed quickly, and the bioclimatic and health consequences are now measurable over decades. Across the continent, cold-stress hours decrese fastest in the north (more than 4 h decade^-1), while heat-stress hours increase across southern Europe (more than 3 h decade^-1). In the Mediterranean, the annual heat-stress total already exceeds 670 h, roughly four weeks per year, and is increasing by  4–6 h yr^-1. A one-at-a-time perturbation between 1940–1960 and 2003–2023 attributes most of the rise to 2 m air temperature and mean radiant temperature. At the city scale we analyze 118 cities from 42 European countries. Athens, Seville and Antalya already see 750–1,000 heat-stress hours per year, with trends of 3–4 h yr^-1. A Heat Vulnerability Index was developed that combines heat stress exposure with the population aged 65 and over, population share at risk of poverty, the share of outdoor workers, and GDP per capita as a proxy for adaptive capacity. The index showed values above 0.5 across southern Spain, southern Italy, Greece, Bulgaria and Romania, and above 0.6 in several NUTS-2 regions. The regions where heat exposure is rising fastest are also the regions with the low adaptive capacity, which makes Mediterranean heat a socio-economic problem as well as a meteorological one. That points to two complementary policy directions: urban cooling and green infrastructure on the climate side, and occupational heat-safety rules and heat-health warnings on the public-health side, with older and lower-income residents as the priority group.