Global artificial light masks biologically important light cycles of animals

We document the importance of low‐light conditions in 136 animal species and then translate the new world atlas of skyglow, which reports artificial night sky brightness, into estimates of anthropogenic illuminance (that is, artificial light reaching Earth's surface). Quantifying habitat illuminance from skyglow facilitates understanding of the disruption of natural light cycles, such as new moon conditions, which are critical to animal ecology. We corroborated this transformation of sky brightness by comparing concurrent field measurements of skyglow and illuminance. We then quantified global artificial illuminance caused by skyglow, finding that skyglow artificially doubled illuminance of new moon conditions—a critical phase for biological processes, such as foraging, courtship, and mating—for 22.9% of the Earth's terrestrial surface, 51.0% of Key Biodiversity Areas, 77.1% of Global Protected Areas, and ~20% of highly diverse areas for mammals, birds, and amphibians. We provide summaries of artificial illuminance at 750‐m pixel resolution for each protected area to aid land managers and guide policy in reducing skyglow in areas that may yield the greatest benefits for conserving animal biodiversity.