Rapid Brightening of Night Skies Globally: Recent Results from Citizen Science and Solutions
Teznie Pugh McDonald Observatory, UT Austin
John Barentine Dark Sky Consulting, LLC
James Lowenthal Smith College
Aparna Venkatesan University of San Francisco
Constance Walker NSF's NOIRLab / IAU CPS
A recent paper by Kyba et al. (2023) found that light pollution of the world’s night skies has increased by as much as 10% a year since 2011, based on star counts made by citizen scientists. Paraphrasing the authors, night skies would brighten by a factor of about four over the duration of human childhood, strongly reducing the visibility of stars. This has been widely covered in the media and articles. Here, we share an overview of these results, related consequences, and one ray of hope: ground-based light pollution can be addressed through mitigating solutions that have already been successfully demonstrated.
Although our newsfeeds and attention are dominated by SpaceX Starlink launches and the impact on astronomy from low-earth orbiting satellites, ground-based light pollution remains the largest threat to astronomical science and to humanity’s relationship with the skies. The recent citizen-science analysis in Kyba et al. 2023 revealed that terrestrial light pollution has not gone away and is, in fact, increasing faster than expected, by as much as 10% each year over the previous year. Although even this is likely an underestimate, there is some good news: ground-based light pollution can be mitigated successfully.
Skyglow — the most familiar symptom of light pollution — is caused by atmospheric scattering of light from ground-based sources (direct or reflected). Short wavelengths are scattered most effectively leading to the familiar light domes and distinctive glow that yield a washed-out appearance to the skies above light-polluted regions at night. Anyone who has frequented a major metropolitan area well knows that seeing more than a handful of stars is rare. And as astronomers, we’re all familiar with the forlorn husks of once-productive research facilities now stranded amongst bustling, brightly lit city streets or university campuses.
As the global population continues to grow and cities expand, the problem of light pollution grows and expands with them. This seems like a faraway concern for most astronomical sites these days, with the largest telescopes being built in remote regions of the planet with little light pollution to worry about, for now. However, with growth rates as high as 10% per year, the impact of light pollution poses an increasing problem for our science.
Light pollution isn’t just bad for our astronomical sites. How many astronomers were motivated to consider an astronomy career by childhood experiences of the night sky or an astronomical event (Comet Hale-Bopp, anyone?)? As fewer and fewer people are able to experience the night sky, we lose the inspiration that drives our science, something that makes astrophysics one of the easiest physical sciences to "sell" to the public and that brings talented engineers to our field to design and run our facilities.
Beyond these human concerns, there is increasing evidence that ground-based light pollution is responsible for disruption to human and animal circadian rhythms (Cao et al. 2023, Touzot et al. 2023), migratory patterns (Torres et al. 2020), and plant seasonal cycles (Meng et al. 2022), as well as changes in reproductive cycles of insects (Firebaugh & Haynes 2016) to name a few of the problems. It may also make urban air quality worse (Stark et al. 2011, Shith et al. 2022). Poor-quality outdoor lighting, which is the source of light pollution, wastes energy (Tatro 2020) and is in part responsible for workplace accidents (Wren & Locke 2015). So, even if your own scientific endeavors are not impacted by ground-based light pollution, it is highly likely that your life or environment is.
For more than a decade, NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab) has been leading a citizen-science program known as the Globe at Night. This program collects estimates of naked-eye limiting magnitude (NELM) and Sky Quality Meter measurements submitted by volunteers around the world. Contributions typically exceed some 10,000 to 30,000 observations per year with more than a quarter of a million data points from 180 countries over the last 17 years.
Kyba et al. (2023) studied the NELM estimates and based on these data inferred a global average increase in the light pollution of 9.6% (10.4% in North America) per year between 2011 and 2022. This is a much larger increase than has been reported by studies using only satellite remote sensing observations of light emissions (Kyba et al. 2017, Sanchez de Miguel 2017), which had found a roughly 2% per year increase on a global average basis. Satellite instrumentation is focused on wavelengths of 500–900 nm, which misses short-wavelength optical emissions characteristic of modern white light-emitting diodes (LEDs) that increasingly dominate the light budgets of cities. In addition, blue light (i.e., shorter wavelengths) is more effectively scattered in the atmosphere than other colors. These two effects give a possible reason for the lower estimate from orbital-based light pollution measures versus the ground-based estimates studied by Kyba et al. The team also points out that direct glare from poorly shielded LED street lights could blind observers near them to faint stars, biasing the NELM estimates toward brighter values without increasing skyglow locally (Bará, Bao-Varela & Kocifaj 2023).
It is probable that the global average change of +9.6% per year is in fact an underestimate of the true rate of increase in light pollution. Since artificial nighttime lighting is strongly correlated with economic performance metrics (Rybnikova 2022) and regions with high rates of economic growth are under-sampled in the Globe At Night data, it is likely that the true rate of increase exceeds 10% per year.
For our work as astronomers, this means that skies over existing observatories are getting noticeably degraded over timescales far less than one astronomer's lifetime, and the options available for sufficiently dark locations for new observatories are dwindling rapidly. This finding also demonstrates (as the authors note) that existing lighting policies are not adequate for the protection of the night sky.
There is, however, some good news — the sky glow from artificial light at night can be reduced. The strategies for cutting light pollution are straightforward: use outdoor lighting only when, where, and how it is needed (timing, area, and brightness), minimize blue light content, and use fully shielded fixtures (see Outdoor Lighting Basics for more information). Those approaches may be simple, but as the Kyba et al. study shows, more effort is needed to put these recommendations into ordinances, bylaws, and other regulations to reverse the degradation of our shared night sky, which is a millennia-old resource and inspiration for us all.
Please join us at AAS 242 in Albuquerque to hear about recent successes in mitigating light pollution, the creation of protected dark-sky places, recent documentaries on the many ways we connect to dark skies, and how good lighting practices have been implemented in lighting ordinances in different regions of the US.
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Cao, M., Xu, T., & Yin, D. 2023. “Understanding light pollution: Recent advances on its health threat and regulations” Science Direct, vol 127, 589-602. https://doi.org/10.1016/j.jes.2022.06.020
Firebaugh, A. & Haynes, K. J. 2016. “Experimental tests of light-pollution impacts on nocturnal insect courtship and dispersal” Oecologia, 182, 4, 1203-1211. https://doi.org/10.1007/s00442-016-3723-1
Kyba, C. C. M., Kuester, T., Sanchez de Miguel, A., et al. 2017. “Artificially lit surface of Earth at night increasing in irradiance and extent” Science Advances, vol 3, 11. https://doi.org/10.1126/sciadv.1701528
Kyba, C. C. M., Altintas, Y. O., Walker, C.E., et al. 2023. “Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022” Science, vol. 379, 6629, 265-268. DOI: 10.1126/science.abq778
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Shith, S., Ramli, N. A., Awang, N. R., Ismail, M. R., Latif, M. T., & Zainordin, N. S. 2022. “Does Light Pollution Affect Nighttime Ground-Level Ozone Concentrations?” Atmosphere, vol. 13, 11, 1844. https://doi.org/10.3390/atmos13111844
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Tatro, K., 2020. “Light Energy: Our Wasted Resource” Consilience, vol. 22, 65-72. https://doi.org/10.7916/consilience.vi22.6731
Torres, D., Tidau, S., Jenkins, S., et al. 2020. “Artificial skyglow disrupts celestial migration at night” Current Biology, vol 30, 12, R696-R697. https://doi.org/10.1016/j.cub.2020.05.002
Touzot, M., Dumet, A., Secondi, J., et al. 2023. “Artificial light at night triggers slight transcriptomic effects on melatonin signaling but not synthesis in tadpoles of two anuran species” Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 111386. https://doi.org/10.1016/j.cbpa.2023.111386
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