January 20, 2021

The temperature in Spanish cities has risen twice the world average in 50 years | Science

The temperature in Spanish cities has risen twice the world average in 50 years | Science

Temperatures in Spanish cities have not stopped rising in recent decades. In the most recent 30 years, the average temperature of provincial capitals has risen by almost one degree. And where the data allow us to go back further, such as Barcelona, ​​Alicante, Madrid and some thirty other cities, the increase is even greater, above 2 degrees Celsius. Such increases are superior to those of the rest of the country and double those suffered by the planet as a whole. And although the rise is not homogeneous (there are cities that suffer more climate change and that, predictably, will suffer more in the future, and others, such as Salamanca, where even temperatures have dropped) is undoubtedly the era of urban climate change is here.

The Observatory of Sustainability has compiled data from meteorological stations of 52 Spanish cities for its latest report on the decarbonisation of Spain. Most are provincial capitals and stations are usually in the center of the city or nearby, they can also be installed at the airport. There are records that go back to the 19th century (Madrid, 1894) or the first third of the 20th century (San Sebastián since 1918 or Barcelona since 1925). But we must wait until the eighties for there to be presence of all capital cities and autonomous cities. The information comes from the State Meteorological Agency and is expressed in annual averages.

An analysis of the data shows an average increase of the urban temperature of almost 1º, from the 15.10º of the five-year period 1988-1992 to the 16.06º of the period 2014-2018. The five-year comparisons seek to neutralize the interannual variability in which an unusually cold or warm year can distort the data. This average of + 0.96º hides values ​​that deviate, and much, from it. For example, in Barcelona, ​​thermometers have risen 1.89º. Avila and Murcia also support an increase of more than 1.8º in the last 30 years.

The phenomenon, moreover, is widespread. There is only one city where temperatures have dropped in the last three decades. In Salamanca, specifically at the meteorological station installed at the Matacán airport (17 kilometers away), the temperature has dropped 0.09º. Although it is a discrete descent, it stands out when compared with the general urban rise. But, as with global warming, in which, along with an average increase in temperatures, regional chillings occur, in the old Castile something similar could be happening: cities near the capital of Salamanca, such as Valladolid, Zamora or Palencia, are among those with the lowest relative increases.

If you look at the map of Spain you can see that urban warming is greater in the Mediterranean portion of the country. Leaving aside Ourense, with its particular topography, at the bottom of the depression of the river Miño, the ten cities where the temperature has risen the most is in the eastern and southern part of the country.

Increase in temperatures

Variation between the five years 1988-1992 and the last one (2014-2018)

The data from the Observatory of Sustainability allow us to go back much further than 30 years. Although there are no records of the 52 cities beyond the eighties, there are a good number of them for the seventies, sixty and even fifty. And that is when, even though it is a less representative sample of the whole, the data allow to qualify the urban warming as exceptional.

In the thirty cities for which there are records for 50 years (lustrum 1965-1969), the average of the urban temperature has gone up 1.6º. But there are cities in which the increase throughout the historical series is much greater. At the end of the 19th century, Madrid had an average annual temperature of 13.72º (in the 1894-1898 period). In the last five years, the average has been 16.19, almost two and a half degrees of increase. Cuenca, with 2.91 from 1956 and the anomalous case of Ciudad Real, with 3.57 of increase since 1976, are the only ones that have risen more than the capital of Spain. They are followed by Zaragoza, Alicante or Barcelona.

The temperature in Spanish cities has risen twice the world average in 50 years

To understand what is 1.6º more, you have to look outside. According to data from the AEMET, the average of the country in the last five years (2014-2018) was 15.90º, one tenths below the average of the cities. Keep in mind that to obtain this average data have been used from the 290 stations that the agency has distributed throughout the country, many of them in urban areas, which may have smoothed the difference between the global of Spain and the urban. But this increase in temperatures in Spanish cities in the last 50 years almost doubles the one experienced in the rest of the planet. US agencies such as NASA or NOAA (National Oceanic and Atmospheric Administration) agree that temperatures have risen around 0.8º since the 1960s.

"The global average temperature in 1965 was 13.82º (-0.08º below the 20th century.) In 2018, the global average temperature was 14.69º (0.79º above the 20th century)", recalls the scientist of the NOAA Ahira Sánchez-Lugo.

Going further back, the last report of the Berkeley Earth project estimates that mercury rose an average of 1.5º in the last 250 years. That is to say, the Spanish cities have experienced in a few decades higher thermal rises than those experienced by the planet in a quarter of a millennium. Such thermal amplification is rare, although not unique, nor exclusive of the capitals of Spain. But, at a regional level, only in a few areas, such as the Middle East, central Europe and, especially, the polar regions, is such an acceleration of climate change occurring.

"The global average does not reflect what is happening at the regional level," says Sánchez-Lugo. "We are observing that the Arctic is heating up much faster that the rest of the planet. "In this polar region, temperatures are increasing twice the global average at least since the nineties, here the explanation is simple, the melting caused by warming reduces the albedo effect, the amount In parallel, the dark waters of the Arctic Ocean trap more heat, feeding back the process.

In the Spanish cities, as in the rest of the cities of the world, the role of the dark waters is interpreted by cement and asphalt. In general, the materials with which cities are made have a higher specific heat than rural land or vegetation cover. But there are other causes that explain why cities get hotter. The same paving decreases evaporation and pipes and drainage reduce the capacity of urban transpiration even more. In addition, the urban geometry captures more solar radiation and brakes the winds that could cool the environment. Nor does pollution help dissipate accumulated heat, as does the smallest portion of visible night sky. But it is also that the city generates its own heat. Large amounts of energy arrive in this form in the form of electricity or fuels, an energy that releases heat when used. All this explains why cities have an extra temperature and the phenomenon has a name: urban heat island effect.

"Urban centers, cities that have grown significantly in the last 40 or 50 years suffer from a thermal island effect," says AEMET researcher Yolanda Luna. It is a change due to the growth of the city. "The land is modified, filled with buildings, asphalt and then filled with vehicles, heating and air conditioning," he adds. But it clarifies that the cities are heated as the environment that surrounds them. "The city of Madrid is heated to the same level as the rest of the Community, but to that we must add the heat island effect," he adds.

The degree and evolution of this effect can be observed if two nearby meteorological stations are compared but one inside the city and the other outside. The AEMET has in Madrid, for example, three weather stations. The fourth closest, either, is that of the Madrid-Barajas Adolfo Suarez airport. The oldest one in Madrid with continuous data until today is the one inside the Retiro park, which dates back to 1920. The one in Barajas started recording data since 1945 but, except for that year and that of 1951, the data was lost. There are only complete series since 1960.

Well, if you compare the temperatures of Barajas and Retiro since the sixties, you can see how the island of urban heat is emerging. In the lustrum of 1961-1965, the temperatures in the park were slightly lower, 0.14º. Keep in mind that being inside a large park, the relative temperature tends to be lower than in the rest of the city. It could be assumed that then it made the same heat / cold both in Madrid and in its vicinity. But things change in the following decades. At the beginning of the seventies, in Retiro there is an annual average of 0.44º above the airport. And, in the nineties, the difference rises to 0.69 °. Since then the temperatures have been equal, perhaps because the expansion of the city has engulfed the airport area. Even so, it is still getting hotter in the capital of Spain than in Barajas.

Spanish cities have experienced higher rises in a few decades than those experienced by the planet in a quarter of a millennium

"In 1965 and until the eighties, Madrid and Barcelona, ​​or better, both metropolitan areas grew, thus increasing the intensity of their heat islands, which would explain that difference, now, for some time, they no longer grow", recalls the professor of Physical Geography at the University of Barcelona, ​​Javier Martín Vide, who has been studying this phenomenon for decades. "The topography of the city, its layout, the building density … give shape to this island of heat, but for its intensity, the contrast between the temperature of the center of the city and the periphery, the most decisive factor is the number The higher the volume of population, the greater the intensity of the urban thermal island, "adds the coordinator of the Group of Experts on Climate Change of Catalonia.

For Martín Vide and the rest of climatologists, 30 years ago the heat islands of Madrid or Barcelona were anecdotal. "Today I rate them as climate risk," he warns. The plus of urban temperature has consequences in the most diverse aspects. "From the meteorological point of view, it has reduced frost and snowy days in the cities, economically, heat islands save energy in winter, but waste it in summer, due to cooling. can live in urban parks and gardens exotic tropical species, both animals and vegetables ", summarizes Martín Vide.

"But the greatest impact is suffered by humans, the weakest humans." In the context of climate change, when it arrives a heat wave, this thermal plus increases mortality and morbidity between people with chronic diseases and, especially, the elderly. For Fernando Prieto, of the Observatory of Sustainability, "the cities, in general, are not taking the necessary measures to adapt to these strong temperature rises, such as green rings, sheets of water, green facades …"

All reports and studies indicate that, if CO emissions are not drastically reducedtwo, global warming will continue to increase. There are estimates that place the global average rise up to 5º. And the cities? Martín Vide recalls that the key is the population: "If the city does not grow, and the Spanish cities are not doing it, the intensity of the heat island will not increase, but it is evident that, by increasing the temperatures in the countryside equally and the city, those who live in urban centers will experience more extreme temperatures. "


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