The number of summer days in Spain has increased from 90 to 145 in the last 50 years, according to a study by the UPC

•	A map showing the increase in hot days per decade for the period 1951-2018

• A map showing the increase in hot days per decade for the period 1951-2018

Increase in hot days and tropical nights between the periods 1951-1954 and 2015-2018

Increase in hot days and tropical nights between the periods 1951-1954 and 2015-2018

The average temperature in the main Spanish cities has risen by 3.54 °C between 1971 and 2022. The country is among those with the most pronounced climatic anomalies in the world. Over the last 50 years, summer days have increased from 90 to 145, which accounts for a two-month increase in hot days. Tropical nights have jumped by 18, from an average of 45 to 63. These are the conclusions of a study by the Centre for Land Valuation Policy (CPSV) of the Universitat Politècnica de Catalunya - BarcelonaTech (UPC).

May 04, 2023

2022 was the second-warmest year on record for Europe, at 0.9 °C warmer than the average. For many countries in southwestern Europe, it was the warmest. The highest anomalies in temperatures occurred in northeastern Scandinavia and those countries bordering the northwestern Mediterranean Sea.

The Mediterranean is considered one of the most vulnerable regions to climate change in the 21st century. The mean temperature over the Mediterranean has been increasing above the global average and is an important factor in explaining increased temperatures in Spain. In fact, Spanish coastal areas are one of the climate change hotspots in the Mediterranean basin, having experienced increases of more than 2 °C in recent years.

Linked to the Barcelona School of Architecture (ETSAB), the Centre for Land Valuation Policy (CPSV) of the Universitat Politècnica de Catalunya - BarcelonaTech (UPC) has analysed the warming process in the main Spanish urban areas since unified records started in the early 1970s. The study is based on the evolution experienced by temperatures between 1971 and 2022 in 21 meteorological stations representative of all the Spanish autonomous communities: Barcelona (data collected at the Fabra Observatory and El Prat Airport weather stations), Madrid (Retiro park and airport), Valencia, Zaragoza, Seville, Malaga, Bilbao, Valladolid, Ciudad Real, Badajoz, Asturias, Santander, A Coruña, Ourense, Murcia, Logroño, Palma de Mallorca, Las Palmas de Gran Canaria and Santa Cruz de Tenerife.

The study concludes that increased temperatures—especially in extreme events such as heat waves—not only cause discomfort but also pose a significant risk to health. According to the information provided by the MoMo database, in the summer of 2022 there were an additional 22,249 deaths compared to the expected mortality, of which at least 4,732 were due to high temperatures.

Average increase in temperature and heat waves
The results show that the average maximum temperature in the main cities of mainland Spain has risen by 3.54 °C. The minimum increased by 2.73 °C. In general terms, 2022 was the warmest year on record. The research carried out shows that the continental influence is mainly manifested in increased maximum temperatures, while the Mediterranean influence translates into a larger increase in minimum temperatures—with a greater impact on mortality. The Cantabrian and Atlantic coasts—especially the Canary Islands—show less significant increases, below 2 °C.

The increase in daytime and nighttime temperatures between 1971 and 2022 in the cities studied is higher than the Mediterranean average.

Palma de Mallorca and Barcelona (daytime and nighttime), Murcia (daytime), Ciudad Real, Zaragoza and Madrid (nighttime) have recorded the highest temperatures.

The study also presents daytime heat waves (DHWs) and nighttime heat waves (NHWs) recorded in the cities studied. Given the lack of consensus on how to define heat waves, two complementary methodologies were used. First, the general criterion used by the Spanish Meteorological Agency (AEMET)—corrected to differentiate between DHWs and NHWs—defines a heat wave as an episode of at least three consecutive days with temperatures above the 95% percentile for the months of July and August from the reference period (1971-2000). Second, the method developed by the UPC’s CPSV (Serra et al.,, which allows differentiating heat (and cold) waves for daytime and nighttime throughout the year.

The first method makes it possible to verify that the increase in daytime and nighttime heat waves (and the associated hot days) has been constant throughout the period studied. DHWs have gone from 3—the annual average in all the stations studied for the decade 1971-1980—to 9.4 for 1981-1990, to 8.8 for 1991-2000, to 13.7 for 2001-2010, to 15.8 for 2011-2020 and to 21.9 for 2013-2022.

NHWs have gone from 2.7 for 1971-1980, to 6.8 for 1981-1990, to 8.8 for 1991-2000, to 20.7 for 2001-2010, to 25.7 for 2011-2020 and to 30 for 2013-2022. The study shows that the increase in nighttime heat waves is clearly larger than that in daytime heat waves.

In addition, the CPSV’s method confirms the increase in heat waves over the last 52 years. In total, 2,491 DHWs at 21 weather stations, with a total of 10,348 proportionally high heat days associated with them. This represents a 6.98-fold increase in DHWs between the decades 1971-1980 and 2013-2022, and a 9.48-fold increase in hot days. 2,732 NHWs have been detected at the analysed stations, with 11,469 hot nights, which is a 10.83-fold increase between the decades 1971-2000 and 2013-2022, and a 12.94-fold increase in hot nights.

Considering the 21 stations, summer days (TX ≥ 25 °C) have risen from 90 in 1971 to 145 in 2022, which accounts for a two-month increase in summer days. Meanwhile, tropical nights (TN ≥ 20 °C) have increased by 18, from an average of 45 across Spain in 1971 to 63 in 2022.

Regarding the period 1971-2022, climate anomalies are much more pronounced (1.49 °C) in Spanish cities than they are on a global scale (0.71 °C). Experts warn that global warming is wreaking havoc on Spain’s main urban systems.

The study Global Warming in Spanish Cities by the CPSV was presented at the General Assembly 2023 of the European Geosciences Union (EGU), which was held in Vienna until 28 April. The work was carried out at the CPSV by professors Blanca Arellano, Josep Roca Cladera and Zhang Xu, from the UPC’s Barcelona School of Architecture (ETSAB), with the collaboration of professor Dolors Martínez, also from the ETSAB, and professors Carina Serra and Xavier Lana, from the Barcelona School of Industrial Engineering (ETSEIB).