Heat kills more at night than during the day in Barcelona

BarcelonaBarcelona suffers from a serious problem of nocturnal heating, which particularly affects the elderly and causes more deaths during the night than during the day. This is one of the main conclusions reached by the authors of two studies pending publication from the Polytechnic University of Catalonia (UPC). The authors attribute this reality to, among other factors, the lack of sufficient public alert measures and the fact that the energy performance certification system for housing in Spain has become outdated.

The studies promoted by the Centre for Land Policy and Valuations (CPSV)show that between 2007 and 2018, high and persistent nighttime temperatures were the main cause of heat-related mortality in Barcelona. During this period, high nighttime minimum temperatures caused 721 deaths, while daytime maximums caused 571. One of the co-authors of the research, Josep Roca Cladera, explains that this is due to the body's "low recovery capacity" after heat accumulated during the day. For this reason, he describes nighttime heat as a "silent killer," because the body "is unable to recover." "There is no escape from the heat," he insisted.

The elderly are the most vulnerable to high temperatures. According to data from the Barcelona Public Health Agency, in 2025 alone, mortality attributable to extreme temperatures reached 370 deaths, of which about 80% corresponded to people aged 75 or over. The CPSV research also concludes that during the August 2025 heatwave, high minimum temperatures led to a 73.9% increase in mortality risk among the elderly. In contrast, maximum temperatures, which might seem more dangerous at first glance, increased the mortality risk to 63.6%.

Adjust alert systems

Roca Cladera argues that many of these deaths could be avoided with public alert systems that are more aligned with climate reality and that do not "underestimate minimum temperatures and only talk about maximums." As an example, he cites the warnings from the Meteorological Service of Catalonia (SMC), which consider 27 degrees to be intense nocturnal temperature and 29 degrees to be very intense, thresholds that researchers consider "excessively high" and far from the 24 degrees established by their methodology. For this reason, they call for the incorporation of minimum temperatures into alert protocols and the establishment of differentiated warnings for age groups.

The expert explains that the CPSV sets, with its own methodology, 30.2 degrees during the day and 24 degrees at night as the thresholds beyond which there is a significant risk to health, especially for the elderly. "Throughout the summer, we have exceeded this threshold of 24 degrees average nocturnal temperature," he warns.

From Mediterranean climate to hot steppe

Due to the urban heat island effect, meaning the phenomenon by which an urbanized area –like Barcelona– registers higher temperatures than rural or peripheral areas, the city has experienced an increase in average temperature of 3.22 degrees between 1971 and 2025 (from 15.82 to 19.04 °C). This is a much more pronounced increase than that recorded in the State as a whole (1.96 °C, between 1971 and 2024), in the Mediterranean region (1.83 °C) or globally (1.29 °C).

Studies reveal that the warming caused by the urban heat island effect and the lack of cooling breeze has been particularly accentuated during nighttime hours. In the last 54 years, the minimum nocturnal temperature has increased by 3.87 °C (from 12.15 to 16.02), while the maximum diurnal temperature has risen by 2.57 degrees (from 19.49 to 22.06). According to researchers, this phenomenon is due to high urban density, high soil impermeability, low albedo –meaning the low capacity to reflect solar radiation, of which asphalt is a good example– and a deficit of green areas. Added to this urban context is climate change, which, according to Roca Cladera, is causing Barcelona to move "from a Mediterranean climate to a warm steppe climate", typical of the peripheries of large deserts.

The challenge, internally

The expert admits that both studies have a significant limitation: "The lack of knowledge and information about what happens inside homes." Beyond actions in public spaces – such as creating shade or climate shelters – he considers it essential to deploy a citizen science network to monitor climatic conditions within homes. Projects like VIP-Clima, promoted by the UPC, the EAPC, the AMB, and the FAVB, among other institutions, should allow for a much more precise picture of indoor temperatures in homes.

The studies also warn that there is a generalized overestimation of heating needs and an underestimation of cooling needs. "They are based on a past climate," summarizes Roca Cladera, who calls for urgent reforms to both the Technical Building Code (CTE) and the energy certification system. One of the reasons is that, although the lack of heating is still a greater concern, data shows that for almost 20 years, three times more energy has been spent on cooling than on heating.

Although Barcelona is already taking action to adapt to the climate emergency, Roca Cladera considers the climate shelter policy to be insufficient because "the vast majority are not open at night nor do people leave their homes to go to them." Therefore, he advocates for prioritizing the climate rehabilitation of homes rather than exclusively energy rehabilitation.

In this regard, he criticizes that this dimension still has little weight in programs such as the Barcelona City Council's Neighborhood Plan, which, he says, continues to focus mainly on maximum temperatures. As an emergency measure, he proposes fiscal incentives for these types of interventions: "There should be an income tax deduction or some mechanism that rewards climate rehabilitation."