It's 5 AM and it's over 20 degrees: tropical nights are multiplied by four in Spanish cities

The data of the daily minimum temperatures recorded by the State Meteorological Agency (AEMET) in the 52 stations of the provincial capitals show that in 40 of them tropical nights have increased in recent decades. Only in some very cool places in the territory is this phenomenon still not observed (none or very few times).

"The trend towards an increase in tropical nights is statistically significant", says Javier Martín-Vide, Professor of Physical Geography at the University of Barcelona. This increase "is not the result of chance," he explains, "it is a true phenomenon that has its causes."

The main one is global warming, says the professor. The AEMET meteorologist, Ricardo Torrijo, explains it: "In the last century and a half, temperatures have risen one degree, and in Spain more. If the temperature increases and you are under the influence of warmer air, both the maximum and the minimum". This occurs especially in the Mediterranean area, where the warming is "parallel to global warming but more intense, it is a hot spot with a higher rate of temperature increase," adds Martín-Vide.

This rise in temperatures is enhanced by other factors, such as heat islands. The way big cities are built makes it hard to cool down at night, so temperatures stay high 24 hours a day when it's hot.

In the following map you can see how much tropical nights have increased in each province compared to the 50s or the first decade with available data. It must be taken into account that in some cases the figures may be conditioned by changes in the environment of the weather stations: alterations in the level of humidity, type of terrain and there are even some that were outside the urban centers and are now surrounded by buildings.

In the early 1980s, the first decade with temperature data for most stations, only 5 provinces recorded more than 40 tropical nights on average per year. They were in Santa Cruz de Tenerife, Las Palmas, Melilla, Cádiz and Almería. In the last decade, however, almost twenty provinces have exceeded this threshold. In all the cities of the peninsular Levante, in addition to Madrid, there are already more than 40 tropical nights a year.

This increase has health consequences, especially when it comes to resting, since the heat "decreases the effectiveness of self-regulation mechanisms such as sweat", explains Beatriz Hervella, spokesperson for the Aemet, necessary "to enter deep sleep" . When a temperature of 20ºC is recorded in the street, inside the houses it is usually higher, which makes it "impossible to fall asleep", recalls Martín-Vide.

A study published in the journal One Earth, which monitored the hours of sleep of nearly 50,000 adults combined with temperatures in their city, showed how "increasing temperature shortens sleep mainly due to late onset, which increases the probability of sleeping less than seven hours a day" . In addition, they specify that "the effect is multiplied among low-income countries and is more acute among older people."

Along the same lines, Hervella explains that, although we are all vulnerable to heat, there are groups that are especially so. First of all, children and the elderly "because their thermoregulatory system is not mature or is too mature". But also "chronically ill, with cardiovascular or neurodegenerative diseases and also people linked to energy poverty who cannot afford to turn on air conditioning," says Hervella.

The night heat, moreover, also affects mortality: The figures from the Carlos III Institute analyzed by indicate how deaths skyrocket during heat waves in Spain. A study published in the scientific journal Environmental Epidemiology in 2021 focused on hot nights in southern Europe: they found a strong relationship between the duration and excess of hot nights with higher mortality, especially in Portugal.

The capitals with the most tropical nights continue to be those located in the Canary Islands, Melilla and Cádiz, with more than 100 days a year with this phenomenon. But some of the most notable increases are observed in Murcia, Barcelona, ​​Castellón, Valencia or Seville, where more than 50 are registered, and they are two, three and even four times more than in previous decades.

The data show a concentration of the cities where the temperature rises the most on the Mediterranean coast. "The sea, which is warming up in the Mediterranean area, helps that there is not so much relief at night," explains Hervella. "If you have a very high sea temperature, at night the temperatures are going to be very high. They can influence that there are more tropical nights," adds Torrijo.

Although these are not the seasons where the thermometer stays above 20ºC most times, the great increase in tropical nights in cities like Albacete stands out, where they have gone from an average of 1 tropical night a year in the fifties to a dozen in the present; Balearic Islands, from 6 to 40; Lleida from 5 to 23; and Ciudad Real, from none to 36.

In cities, a key explanation for the increase in night temperatures is the heat island effect. "In summer, if you're inside a city, the ground heats up a lot with the sun," says Torrijo. This happens, according to the meteorologist, because "buildings impede air circulation and even the air conditioners themselves or cars radiate a large amount of heat". And this phenomenon is not just a summer thing. "In winter the same thing happens, the heaters and the cars also influence the temperatures," he adds.

Although without the effect of the heat island, global warming is also noticeable in rural places. In mountain stations, isolated from the urban environment, the minimum temperatures have also risen. In the Izaña station, located in the Teide National Park, no tropical night was recorded in the 1950s and 1960s and in the last twenty years there have been 8. It is also observed in the Puerto de Navacerrada station (Madrid ), with only one tropical night from 1950 to 1970 and with seven in the last two decades.

Traditionally, in our country, nights with temperatures above 20ºC were concentrated in the months of July and August. However, in recent years there has been a considerable increase in these hot days during the month of June, and even in September and May. "Summer is extending into the month of June, where high temperatures are already noticeable," explains Ricardo Torrijos, AEMET meteorologist.

In the 70s and 80s, only Jaén and Cádiz recorded more than 1 out of every 5 nights in July with minimum temperatures above 20 degrees. Since the turn of the century there are already 14 provinces that exceed this threshold.

Santa Cruz de Tenerife is the province with the most tropical nights recorded in June in the last twenty years: the temperature does not drop below 20 degrees for 16 days of the month, on average. This means that they spend more than half of June (54% of the days) with tropical nights, while from 1950 to 1970 they were only observed during 14% of the month. They are followed by Cádiz (45%), Melilla (44%), Las Palmas (43%) and Almería (41%).

So far this summer, almost all the provinces have recorded more tropical nights than those observed, on average, in the previous ten years.

In the case of Barcelona, ​​more than 90% of the nights the temperature has not dropped below 20ºC. In total, twenty provinces have had tropical nights for more than half of the summer. Only in Almería and Las Palmas the records for this summer are lower than those of the last decade, leaving aside the six provinces that have not had any tropical night.

Torrijo recalls what the specialists say: the increase in temperatures "will increase." To deal with tropical nights, Martín-Vide advocates "changing cities and making their soils more permeable." "The so-called hard squares were an icon but they are useless because in the summers they are a real desert. To alleviate the heat islands it is necessary to fluff up the city, more green elements, put green roofs, use white and light colors, reduce the traffic levels... In short, to achieve "a city that gives off less heat and pollutes less", concludes the professor.

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