Diverging evolution of light pollution indicators: Can the globe at night and VIIRS-DNB measurements be reconciled?

The radiance of nighttime artificial lights measured by the VIIRS-DNB instrument on board the satellite Suomi-NPP increases at an average rate ∼2.2 %/yr worldwide, whereas the artificial radiance of the night sky deduced from the Globe at Night (GAN) unaided-eye observations of the number of visible stars is reported to increase at an average rate ∼9.6 %/yr. The difference between these two estimates is remarkable. This raises the question of whether the diverging temporal evolution of these indicators could be due to changes in the spectral composition of outdoor artificial light, consequence of the current process of replacement of lighting technologies. This paper presents a model for evaluating the temporal rate of change of different light pollution indicators and applies it to the VIIRS-DNB vs GAN issue, based on available data. The results show that the reported difference could be explained by spectral changes alone, if the visual GAN observations are made with scotopic or mesopic adaptation at definite times under some particular transition conditions. In case of photopic adapted observers, however, reconciling these two measurement sets requires the existence of GAN-specific light sources that affect the Globe at Night observations but do not show up in the VIIRS-DNB data. The lumen emissions of these GAN-specific sources for photopic observers should increase at a rate larger than 9 %/yr worldwide.