Night heat causes more deaths in Barcelona than high daytime temperatures in the current context of climate change, according to two studies, which advocate for the incorporation of minimum values into public heat alert systems and the prioritization of housing cooling needs to preserve rest.
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Compared to 571 deaths in the Catalan capital linked to maximum temperatures, 721 can be attributed to high minimum temperatures in the 2007-2018 period, according to two investigations developed by an interdisciplinary team from the Universitat Politècnica de Catalunya (UPC) formed by architecture doctors Blanca Arellano and Josep Roca and physics doctors Dolors Martínez and Carina Serra.
The cooling needs of the Catalan capital already triple those of heating
Barcelona’s climate has experienced continuous warming in recent decades. Between 1971 and 2025, the city’s average temperatures have increased by 3.22ºC, a much more pronounced increase than in Spain as a whole (1.96ºC according to Aemet, between 1971 and 2024), the Mediterranean region (1.83ºC) and the world (1.29ºC). “An absolute outrage,” said Josep Roca, professor at the Escola Tècnica Superior d’Arquitectura of the UPC.
According to the research, the growth is much more pronounced in minimum temperatures (3.87ºC) than in maximum temperatures (2.57ºC) because the sea breeze softens daytime temperatures and its absence does not allow the city to cool down at night. This phenomenon has caused “a serious problem of nocturnal warming” in Barcelona, according to the researchers.
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A problem – to which high density, high soil impermeabilization, low albedo, and a marked deficit of green areas contribute – that affects health. “The inability for nocturnal cooling prevents people from resting, which translates into higher mortality. Night heat is a ‘silent killer’ that should not be ignored,” state the authors.
They emphasize, in this regard, that the National Plan of preventive actions for the effects of excess temperatures on health 2026 of the Ministry of Health only takes into account maximum temperatures for extreme temperature warnings. “Therefore, it implicitly maintains that nocturnal temperatures do not have an appreciable effect on health (or their risk is subsumed in the maximums). Implicitly, therefore, they consider that nocturnal rest is an escape from health risk.”
Once the greater lethality of rising nocturnal temperatures has been proven, researchers advocate that public heat alert systems must also incorporate minimums: “Mediterranean cities, like Barcelona, experience high nocturnal temperatures that have devastating effects on health. There is no escape from the heat. This is what we have called No heat escape paradox.”
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The situation tends to worsen. On the one hand, due to the intensification of heatwaves. On the other, as a consequence of the progressive aging of the population, as the elderly are especially vulnerable to rising temperatures. The authors calculate that the population aged 65 or over will exceed 700,000 inhabitants in 2050, when in 2025 it was estimated at 361,914 people.
30% of cooling consumption in the city occurs at night
This age group represents 88% of the 342 deaths attributed to heat in 2025 in the Barcelona demarcation by the Carlos III Health Institute. Given the high vulnerability of older people, the research proposes that the heat warning system be differentiated by age groups.
On the other hand, the authors propose adapting the energy certification of homes to the current climate. Today, cooling needs in Barcelona (2.5 million kWh/year, according to 2025 data) practically triple those of heating (7.45 million kWh/year). And the nocturnal cooling energy load represents 30% of the total.
In light of these data, the studies consider it urgent to modify the calculation of energy certification in Spain so that the results align with the reality of the climate. “Where heating and cooling thresholds adapt to the present climate, not the past. Where diurnal and nocturnal energy loads are differentiated. And where the problem of latent heat is considered.”
These conclusions are derived from the works Vulnerability of Older Adults to Heatwaves: the Barcelona case (Vulnerability of older adults to heatwaves: the case of Barcelona) and Climate classification and energy efficiency of buildings. A new climate classification for the energy efficiency of buildings in Spain (Climate classification and energy efficiency of buildings. A new classification for the energy efficiency of buildings in Spain). The works are in the process of publication.
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