Question asked by Claire on 12/22/2021.
Do the urban illuminations of the end-of-year celebrations significantly modify the luminosity of our cities, seen from the sky? To answer this question, one would be tempted to take a look at the data collected by the satellites. Sadly, “there is no satellite solely dedicated to measuring light pollution”, explains Alejandro Sánchez de Miguel, astrophysicist from the University of Madrid, expert in light pollution issues. However, a VIIRS infrared radiation measuring device, on board the Suomi NPP satellite, continuously performs measurements from which we can, theoretically, deduce the answer to this question. Data are also available for North America. But it is of little relevance to use VIIRS data if one seeks to know the situation in Europe. Indeed, a portion of a given territory is only flown over and recorded for a few hours every sixteen days. “However, VIIRS takes its images of Western Europe very late, when the illuminations [liées aux fêtes de fin d’année] are generally extinct”, continues Alejandro Sánchez de Miguel. Christopher Kyba, a German researcher specializing in the quantification of artificial light in the nocturnal environment, confirms to us that the satellite flies over the Paris region “around 1:30 a.m.”. And at this hour, “the month of December does not stand out in an obvious way compared to the others”, he observes. “But that doesn’t mean Christmas lights have no effect: any added light increases the light visible from space, and the brightness of the sky! It simply means that this effect is relatively modest compared to the light intensity of the entire city at the time. [tardive] where the satellite passes overhead.”
So let’s focus, for the moment, on the data collected for North America. In the mid-2010s, two NASA researchers developed an algorithm that estimates, over the months, the variations in intensity “light of electrical origin” on satellite images, by applying specific processing to VIIRS data in order to take into account other phenomena that affect luminosity (reflection of light returned by the moon, clouds, air pollution, etc.).
Their work identifies, between the rest of the year and the six weeks preceding the New Year, “a uniform increase in all [les villes étudiées], at national scale”. About 22% of the cities sampled in the United States, and a third in northern Mexico, recorded during this period an increase of between 5% and 15% in the light of electrical origin emitted. At the district level, the researchers also identify many areas for which the luminosity increases by 20%, 30%, or even 50%. However, these variations are not necessarily attributable to the illuminations put in place by the municipalities. “Variations in street lighting patterns during the Christmas and New Year holidays differ across residential contexts”, explain the researchers. Taking the example of large cities in the southern United States, they find “that holiday activity is greatest in suburban and peri-urban areas, where Christmas lights are predominant. In contrast, most central urban neighborhoods, with compact housing types offering less space for illuminations, experience a slight decrease or no change in demand for energy services.” One of the authors of this work insisted in 2014 on “the very strong increase in activity [constatée] during holidays, especially around suburban areas, where there are many single-family homes with plenty of yard space to install lights”. The data analyzed thus reveal phenomena of population displacement during these holidays, with urban areas that decrease in brightness when suburban or hinterland pavilions light up during these holidays, particularly conducive to large family reunions. .
“There is undoubtedly an increase in the brightness of the lights observed by satellite [en Amérique du Nord] during the Christmas period”, confirm to CheckNews Christopher Elvidge, director of the Earth Observation Group at the Colorado School of Mines, and VIIRS data specialist.
For Europe, “few articles have yet been published on the subject”, confirms Alejandro Sánchez de Miguel. “In Madrid, we made an estimate based on ground sensors and we estimated that the Christmas lights do indeed make a contribution to the brightness of the Madrid sky”, of the order of “five percent” of the total luminosity. Apart from this work, it therefore seems – to date – quite difficult to provide a precise answer to this question.
And what about the impact of these increases in nighttime light on wildlife? Alejandro Sánchez de Miguel judges that “it probably depends on the species”. The disruptions already commonly experienced by those living in urban areas “are probably accentuated”, he judges. However, as the surplus of illumination “takes place in winter in our hemisphere, it is unlikely to have an impact on migratory birds, unlike, for example, the 9/11 memorial in New York, which is located on the trajectory of many species”. In fact, according to a study published in 2017 in the PNAS, the columns of light which symbolize the fallen towers divert about 160,000 birds from their route per year (in particular due to the presence of insects attracted by the beams). Christopher Kyba also judges that “December is a time when the impact on the environment is much less than during the summer, when the peak periods of migration in the fall and spring.”
Researcher Fabio Falchi, co-author of the World Atlas of Light Pollution, notes, however, that these considerations “about December lights having less of an effect on wildlife is only true for the Northern Hemisphere”, especially at higher latitudes. “The effect is potentially more marked at the latitudes of Italy, Spain, Greece, and in all areas below”, as well as in the countries of the southern hemisphere.