Temperature at Roque de los Muchachos rose in 20 years twice as much as predicted by climate models

Researchers from the Department of Physics of the Universitat Autònoma de Barcelona led a long-term study of climatic conditions at one of the main astronomical observation sites in the northern hemisphere: the Roque de los Muchachos Observatory, on the island of La Palma. In an area located 200 metres below the ridge on which most of the optical telescopes such as the Gran Telescopio Canarias are located, are the MAGIC Telescopes, built to detect very high energy gamma-ray sources by a collaboration in which UAB researchers were also involved. It is also the site of the construction of the northern hemisphere’s Cherenkov Telescope Array Observatory (CTAO).

UAB researchers Markus Gaug and Lluís Font analysed a unique series of meteorological data from the area recorded over a period of 20 years by the meteorological instruments installed on the roof of the MAGIC telescope control building. The weather station has sent data on temperature, relative humidity, atmospheric pressure and wind speed and direction every two seconds during all this time.

“The station was built with the intention to have some guidance for telescope operations, not to characterise local weather professionally, let alone the effects of climate change on the measured parameters,” explains Markus Gaug. But “the fact that they were relatively low-cost devices has been an advantage, since they had to be changed and recalibrated every two years or so, which has favoured the reliability of the data and greatly limited the effects of long-term sensor drifts, which are difficult to detect,” he adds.

The Spanish meteorological service, AEMET, did not install a professional station at Roque de los Muchachos until 2022. However, thanks to data from 36 other AEMET weather stations spread across the island of Tenerife, it has been possible to carry out studies of temperature variation in the Canary Islands for many years. Some studies show an increase of 0.3 ºC per decade between 1970 and 2010. Other more recent studies had obtained an increase per decade of up to 0.25 ºC in the same area with historical data up to 2014, and the models predicted between 2015 and 2050 a temperature increase of 0.3 to 0.5 ºC per decade.

^{Temperature times-series observed with the MAGIC weather station}

Based on data recorded since 2004 by the MAGIC telescopes, and using advanced statistical analysis, researchers from the UAB Department of Physics were able to very accurately determine the climate trends in this area. The experimental data obtained show an increase of 1.1 ºC over the past 20 years, i.e. 0.55 ºC per decade. This is more than double the increase predicted by climate models for the same area, and even more than expected for the next 20 years.

The results also show an increase in the diurnal temperature range, the difference between the highest and lowest temperatures during a 24-hour period, of 0.13 ºC per decade, and also an increase in seasonal temperature oscillations of 0.29 ºC per decade.

One of the most surprising data observed is the increase in average daily relative humidity of 4% per decade. This is an increase that was also observed in other studies at the Mauna Kea observatory on the island of Hawaii, and contrary to the decrease in humidity recorded at the other continental observatories. The researchers consider that a possible explanation for this difference between the island and continental observatories is the increase in seawater evaporation as a result of global warming.

Scientists monitored the variations in the climate at these sites after the scientific community expressed concerns about the impact of climate change on the viability and quality of data collected by unique observatories in the world, such as those at Roque de los Muchachos. In particular, the atmosphere of the subtropical zone of the Canary Islands is highly valued for astronomical observation as it is very stable throughout the year. The MAGIC telescopes are located above a thermal inversion layer with low pressures and excellent observing conditions with very dry and clean air.

Fortunately for the astronomers, the UAB study found no significant changes in the frequency of rainfall or, above all, in the occurrence of strong storms. In this sense, one of the data analysed by the researchers was the rate at which temperature may change, which has remained below variations of half a degree per minute in all measurements, a condition that meets the requirements for the proper functioning of the future Cherenkov Telescope Array Observatory (CTAO).

“At the moment, while using linear models, there is no evidence that the changes detected in the meteorological conditions could affect the operation of the telescopes or increase the time they will have to remain inactive due to inclement weather in the coming years, but these results are further evidence of the worrying pace of global warming”, says Lluís Font. “In addition, we all know that after a certain temperature increase, the linearity of the climate system ends and that can lead us to the 'tipping points' where the climate changes its behaviour drastically”, adds Markus Gaug.

The research was published in the journal Monthly Notices of the Royal Astronomical Society. Led by researchers in the UAB Department of Physics Markus Gaug and Lluís Font, also attached to the Centre for Space Studies and Research (CERES-IEEC) of the UAB, the study included the collaboration of IFAE scientists and several universities in Italy and Germany. The work is also co-authored by Sofia Almirante Castillo, who participated in the research with a final degree project in 2022, when she was a student at the UAB, under the direction of Markus Gaug.