Does summer last more days than before?

Is global warming changing the length of the seasons, climatically speaking?

From a purely climatic point of view, summer days have gone from 90 to 145, in the last 50 years, in the main Spanish cities, according to a study by the Polytechnic University of Catalonia – BarcelonaTech (UPC). In other parts of the world, surely there has also been a lengthening of summer.

In the main cities of mainland Spain, the average increase in temperatures has been 3.54°C, between 1971 and 2022, and they are among those that register the most pronounced climatic anomalies in the world. In the last 50 years, summer days have gone from 90 to 145, which represents another two months of warm days. And tropical nights have increased by 18 and have gone from an average of 45 to 63. This is clear from a study prepared by the Center for Land Policy and Valuations (CPSV) of the UPC.

2022 was the second warmest year on record in Europe since the start of systematic measurements, with 0.9°C warmer on average. In many southwestern European countries, it was the warmest. The highest temperature anomalies occurred in northeastern Scandinavia and in countries bordering the northwestern Mediterranean Sea.

Precisely, the Mediterranean is considered one of the most vulnerable points to climate change in the 21st century; the average temperature over the Mediterranean has a higher increase than the global average and is a key factor to explain the increase in temperatures in Spain. In fact, the Spanish coast, which has experienced increases of more than 2°C in recent years, is one of the hot spots for climate change within the Mediterranean area.

The Center for Land Policy and Valuations (CPSV) of the Polytechnic University of Catalonia – BarcelonaTech (UPC), linked to the Barcelona School of Architecture (ETSAB), has analyzed the warming process in the main Spanish urban areas since that unified records were made, at the beginning of the 70s. To do this, it has developed a methodology with which it has analyzed the evolution experienced by temperatures between 1971 and 2022 in 21 representative meteorological stations of all the autonomous communities. Barcelona (with data collected at the meteorological stations of the Fabra Observatory and the Prat airport), Madrid (Retiro park and airport), Valencia, Zaragoza, Seville, Malaga, Bilbao, Valladolid, Ciudad Real, Badajoz, Asturias, have been studied. La Coruña, Ourense, Murcia, Logroño, Palma de Mallorca, Las Palmas de Gran Canaria and Santa Cruz de Tenerife.

According to the results of the study, the increase in temperatures, especially in the case of extreme events such as heat waves, in addition to the discomfort they generate, are a marked risk factor for health. According to the information offered by the MoMo database, during the summer of 2022 there were 22,249 additional deaths compared to the expected mortality, of which a minimum of 4,732 were due to high temperatures.

Average temperature increase and heat waves

The results show that the average increase in temperatures in the main cities of mainland Spain has risen 3.54°C, in terms of the maximum, as well as 2.73 in the case of the minimum. And 2022 has been, as a rule, the warmest year on record. The research carried out shows that the continental influence is manifested mainly in the increase in maximum temperatures, while in the area of ​​Mediterranean influence the increase in minimum temperatures is more pronounced (with a greater effect on mortality). On the other hand, the Cantabrian and Atlantic coasts, as well as, above all, the Canary Islands, present less pronounced increases, below 2°C.

The increase in temperatures between 1971 and 2022 in these cities, both during the day and at night, is higher than the Mediterranean average.

Palma de Mallorca and Barcelona (day and night), Murcia (day), as well as Ciudad Real, Zaragoza and Madrid (night) have registered the highest temperatures.

The study also presents the daytime and nighttime heat waves experienced by the cities studied. Given the lack of consensus on how to define heat waves, two complementary methodologies are used. On the one hand, the general criteria used by the State Meteorological Agency (AEMET), corrected to differentiate between daytime and nighttime heat waves: defines a heat wave as when 3 or more days occur with temperatures above the 95% percentile of the months of July and August of the reference period (1971-2000). And, on the other, the method developed by the CPSV of the UPC (Serra et al, DOI: 10.21203/rs.3.rs-2095725/v1), which makes it possible to differentiate between heat (and cold) waves, day and night, a long the whole year.

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

As for the nocturnal heat waves, they have gone from 2.7 in the decade 1971-1980, to 6.8 (1981-1990); at 8.8 (1991-2000); at 20.7 (2001-2010); to 25.7 (2011-2020), and to 30 in the decade 2013-2022. The study carried out shows that the increase in night heat waves is clearly more pronounced than that of daytime heat waves.

Additionally, the UPC CPSV method confirms the increase in heat waves over the last 52 years. In total, 2,491 daytime heat waves at the 21 weather stations, with a total of 10,348 days of proportionally high heat associated with them. This increase indicates that daytime heat waves have multiplied by 6.98 between the decade 1971-1980 and the decade 2013-2022 (and by 9.48 on hot days). As for nocturnal heat waves, 2,732 have been detected in the set of stations analyzed, amounting to 11,469 hot nights, and nocturnal heat waves have multiplied by 10.83 between the decade 1971-2000 and 2013-2022 (and for 12.94 warm nights).

For the set of 21 meteorological stations studied, summer days (TX >= 25°C) have gone from 90 in 1971 to 145 in 2022, which means that there are now two more months of summer days in the area. For its part, tropical nights (TN >= 20°C) have increased by 18, from an average of 45 in all of Spain in 1971 to 63 in 2022.

Map showing the increase in hot days in the last decades. (Image: UPC)

In relation to the period 1971-2022, climatic anomalies are much more pronounced (1.49°C) in Spanish cities than on a global scale (0.71°C). Experts therefore warn that global warming is taking its toll on the main urban systems in Spain.

The study ‘Global Warming in Spanish Cities’ of the CPSV was presented at the General Assembly 2023 of the EGU (European Geoscience Union), which took place recently in Vienna. The work has been carried out from the CPSV by Blanca Arellano, Josep Roca Cladera and Zhang Xu, professors at the Barcelona School of Architecture (ETSAB) of the UPC, with the collaboration of Dolors Martínez, from this same school, and Carina Serra and Xavier Lana, professors at the Higher Technical School of Industrial Engineering of Barcelona (ETSEIB). (Source: UPC)