Thermal Persistence İndex (TPI): A Novel Measure of Prolonged Heat Stress in the Mediterranean, 1950–2024

Abstract

The Mediterranean basin is a prominent ‘hotspot’ highly vulnerable to anthropogenic climate change, where heatwaves are increasing in frequency, intensity, and duration. Traditional heat stress metrics, often focusing on instantaneous conditions or daily averages, fail to capture the cumulative impact of uninterrupted intra-daily exposure. In this study, we introduce the Thermal Persistence Index (TPI), a novel metric quantifying the persistence of human-relevant heat stress on an hourly scale. TPI is defined as the maximum number of consecutive hours within a day during which the Universal Thermal Climate Index (UTCI) exceeds + 32 °C (‘strong heat stress’). Using ERA5-Land reanalysis (1950–2024), we analyze long-term trends, regime shifts, and spatial hotspots for three TPI-derived metrics: seasonal mean (tpi_mean), seasonal maximum (tpi_max), and days with TPI ≥ 6 h (tpi_ge6days). Results reveal widespread, statistically significant increases in all metrics, with consensus hotspots in eastern Spain, northern Italy, the Aegean, and the Levant. Pettitt change-point analysis identifies the 1990s—particularly 1997—as the dominant period of regime shift. These findings indicate that the Mediterranean has entered a new thermal regime characterized not only by hotter days but also by longer and more frequent periods of intra-daily heat stress. TPI offers a concise framework for assessing climate-driven changes in heat stress duration and supporting targeted adaptation in public health, labor, and agriculture.