Heatwaves over the Mediterranean region under climate change: characterisation, recent evolution, and impacts

The Mediterranean region is one of the areas most affected by global warming and is considered a climate change hotspot due to its pronounced warming trend and general decline in precipitation. It suffers severe climate extremes, such as heavy rainfall, droughts, wildfires, and atmospheric and marine heatwaves, which are becoming more frequent and intense. More than 500 million people live in the region, over 70% in coastal cities. Adaptation plans based on rigorous regional knowledge are essential to protect communities from climate extremes. In this context, atmospheric heatwaves have been recognised as one of the major impactful extreme phenomena in the region. According to the World Meteorological Organisation, an atmospheric heatwave is a marked warming of the air or the intrusion of very warm air over a large area, lasting from a few days to weeks. Atmospheric heatwaves negatively affect sectors such as energy, agriculture, and especially public health, contributing to increased mortality. In the summer of 2025, heat caused 24400 excess deaths in European cities, 68% attributable to anthropogenic climate change. Future climate scenarios suggest that atmospheric heatwaves will become more intense and frequent globally in the next decades, highlighting the need to improve adaptation strategies. Mediterranean Sea surface temperatures, which play a key role in regional climate dynamics, have risen 20% faster than the global average in recent decades. This warming has led to more frequent marine heatwaves, increasing the likelihood of their co-occurrence with atmospheric heatwaves. However, the interaction and concurrence of atmospheric and marine heatwaves remain poorly understood. This thesis investigates atmospheric heatwaves in the Mediterranean region, focusing on three aspects: their characteristics and changes over recent decades, their impact on heat-related mortality, and their interaction with marine heatwaves. Results reveal a generalised but uneven increase in atmospheric heatwave frequency, intensity, duration, and spatial extent from the 1950s to the present over mainland Europe and the Mediterranean Sea. Atmospheric heatwaves show different characteristics across climates. In the Iberian Peninsula, they are 20-30% more frequent than in France and Central Europe, whereas in the latter regions they are 16-22% more intense. These differences translate into variations in thermal comfort during atmospheric heatwaves. Mainland Spain – in the western Mediterranean – exhibits one of the strongest increases in atmospheric heatwave frequency. In Spain, heat-related mortality increases during atmospheric heatwaves, although the effect strongly depends on local conditions. The largest impact is found in the Mediterranean coastal city of Valencia, where heat-related mortality risk increases by 8.0% (95% confidence intervals [3.3, 12.8]) during atmospheric heatwaves. Results indicate that a finer scale than provincial division is necessary to adequately design heat alerts. In the Mediterranean Sea, atmospheric heatwaves locally intensify concurrent marine heatwaves by up to 60% compared to non-concurrent events through changes in air-sea heat fluxes. These findings improve our understanding of atmospheric heatwave characteristics, trends, and heat-related mortality at the local scale, as well as their interactions with marine heatwaves and air-sea heat exchange, supporting the design of effective adaptation strategies and improved prediction of extreme events in the region.