As is well known, defining the characteristics of the climate of a place or region requires a broad set of data on temperature, precipitation and other variables over a long period of time: these are the so-called climatic series, made up of a good number of values, each of which represents weather in a given unit of time.
In this way, with a simile, the weather is like a sequence of frames, each of these being a meteorological time. The World Meteorological Organization (WMO) establishes that a minimum of 30 years of continuous meteorological records is necessary to obtain averages, the so-called climatological normals, and other statistical parameters that define the climate of a place.
It should be noted in this regard that for an element such as precipitation in the Mediterranean world, it is advisable to have more than 30 years, given its high variability, while in the case of temperature or atmospheric pressure, half the years are obtained acceptable averages in terms of their climatic significance.
In order to compare the climate of some places and others, some thirty-something determined, the so-called international climatological periods, were established on which to calculate averages and other values. Thus, there have been international periods 1901-1030, 1931-1960 and 1961-1990, and the last one was completed on December 31, 2020: 1991-2020.
In addition to allowing the comparison of the series of many places over the same period of time, necessary, for example, to build a climate atlas, the international periods have served to establish the anomalies of a given year or month with respect to the normals of the international period, thus converted into a reference period.
This today, in the context of climate change, is an important task that allows the statistical monitoring of the evolution of temperature and other variables.
Precisely because of the changing climate that we are experiencing, among other reasons, the WMO was making the definition of the reference periods more flexible and, thus, simply considered consecutive periods of 30 years starting on January 1 of a year ending in 1 and ending on 31 December of a year ending in 0, such as 1981-2010.
These are now overlapping periods, although the importance of considering the last one that fulfills the above condition is emphasized, and that coincides with the recently completed 1991-2020 (for long-term climate change studies, the period 1961-1990, which combined relatively cold years, in its first half, with the sign of global warming, in its second, also remains a reference).
The Fabra observatory, belonging to the Royal Academy of Sciences and Arts Barcelona, distinguished by the WMO, in 2018, as a centennial long-term observation station, has climatic series of 107 years in length, from 1914 to the present.
The quality of Fabra’s climate series is excellent, thanks to the dedicated work of its meteorological observers and the endorsement of the academics of the scientific institution to which the observatory belongs.
A statistical analysis of the annual mean temperature allows to establish that its trend in the entire period 1914-2020 has been +2.1 ºC. This increase is clearly higher than the planetary one, explained by the greater sign of warming that the Mediterranean basin shows, classified as hot spot, and, to a lesser extent, by the growth of the metropolitan area at the foot of the observatory.
The annual averages in the past decade were almost without exception above 16 ºC, when they were just over 14 ºC in the first half of the 20th century
The comparison of the annual average temperatures of the periods 1961-1990 and 1991-2020 shows the recent warming experienced in Fabra, which has gone from 14.6 ºC to 15.8 ºC, a statistically significant difference at 99.9%. The recently completed 2020 has also been the warmest year in the entire series, with 16.8 ºC.
It is very significant that the annual averages in the past decade were almost without exception higher than 16ºC, when they were just over 14ºC in the first half of the 20th century.
Now Fabra registers average annual values similar to those that existed in the center of Barcelona some 40 years ago.
* Javier Martín-Vide He is a professor of Physical Geography at the University of Barcelona (specialist in climatology) and an academic at the Royal Academy of Sciences and Arts of Barcelona.