Aim: Exposure to artificial light at night (ALAN) is a risk factor for organisms. Considering the spread and increasing intensity of night brightness across the globe, and the key role of light at all biological levels, alterations of ecosystems are expected. Yet, we cannot predict the severity of the effects of ALAN in several biomes because little information is available outside the temperate zone. We reviewed current knowledge and identified traits that could be targeted to fill this knowledge gap in order to contribute to the elaboration of a biogeographical framework for the study of ALAN at the global scale.

Location: global.

Time period: current and next decades.

Method: We analysed the latitudinal variation of ALAN and focused on environmental factors that vary with latitude but that have been overlooked. We reviewed biological traits that exhibit latitudinal variation and depend on light and photoperiod and compiled information about the predicted changes of human demography and road networks across different world regions.

Results:  Cloud cover amplifies ALAN far away from urbanized areas. Because of the higher frequency of overcast sky nights, exposure effects may be stronger both at high latitudes and across a large fraction of the intertropical zone, though at different times of the year. Intertropical biomes host the largest fraction of global biodiversity. Although currently they are not the most exposed to ALAN, their human populations are growing, and urbanized areas and road networks are expanding. Hence, ALAN could have strong ecological consequences, with cloud cover as an aggravating factor.

Perspectives:  Knowledge gaps currently limit our ability to predict the effects of ALAN in different biomes. Therefore, it will be important to start investigating the consequences of this novel environmental factor across the globe, in order to develop a relevant theoretical framework.

This is mainly a review and perspective paper but we used already published data on the intensity of artificial light at night, cloud cover, time of twilight, and human demography. Light and cloud cover data were provided with permission from authors. The others were free of use.

• Artificial light at night - from Falchi, F., Cinzano, P., Duriscoe, D., Kyba, C. C. M., Elvidge, C. D., Baugh, K., … Furgoni, R. (2016). The new world atlas of artificial night sky brightness. Science Advances, 2, e1600377. https://doi.org/10.1126/sciadv.1600377

• Cloud cover fraction - from the moderate resolution imaging spectroradiometer (MODIS) mission and provided by the Giovanni online data system, developed and maintained by the NASA Goddard Earth Sciences Data and Information Services Center. https://giovanni.gsfc.nasa.gov/giovanni/

• Twilight - from https://www.esrl.noaa.gov/gmd/grad/solcalc/ and https://earthsky.org/?p=235797

> Time of dusk and dawn for each month of the year were tabulated for three towns at three latitudes and graphs created using R

• Human demography - from United Nations Department of Economic and Social Affairs Population Division. (2017). World population. Retrieved from https://population.un.org/wpp/Publications/Files/WPP20 17_Wallchart.pdf

> Data were mapped using QGIS with no transformation.

File: Data Dryad-latitudinal variation in ALAN cloud cover

Latitudinal distribution of artificial light at night (ALAN) and cloud cover fraction and their standard deviations were measured from satellite images and data were binned in 5° categories using QGIS. Only terrestrial areas have been included in the analysis. Seas and large lakes have been excluded.

median_latitude: median position of the 5°latitudinal band

alan_count and cloud_count: number of pixels with a value

alan_sum and cloud_sum: sum of values for all counted pixels of the latitudinal band 

alan_mean and cloud_mean: mean value for each latitudinal band computed as sum / count

alan_sd and cloud_sd: standard deviation for each latitudinal band

File: Data Dryad-Daylength and ALAN

The table provides the geographical coordinates of three towns (Reykavik, Vladivostok and Lagos) as well as the time of sunset and sunrise and the duration of twilight at these three locations. Values are reported for three definition of twilight: civil twilight (sun declination between 0 and 6° below the horizon), nautical twilight (declination 6–12°), astronomical twilight (declination 12–18°).