Mitigating artificial light at night: effectiveness of policies and implications for plant phenology.

Light pollution, an increasingly recognized environmental hazard, disrupts urban ecosystems, including plant phenology, such as leaf-out and senescence. In recent years, municipalities have enacted legislation to reduce artificial light at night (ALAN) at city levels. However, the effectiveness of these mitigation strategies on reducing ALAN and their impacts on urban ecosystems remain underexplored. Here, we present a proof of concept analysis to explore the potential of ALAN-mitigation policies on changes in intensity and spectrum composition of ALAN and consequent changes in plant phenology using Sustainable Development Science Satellite 1 data for 2022-2024 in paired US cities-one with mitigation policies and one without. Results suggest that ALAN declined in cities with ALAN-mitigation policies (New York City and Washington DC), with the largest decline in blue band. In contrast, ALAN increased in cities without policies (Newark and Philadelphia). The reduction in shifts of spring and autumn phenology observed in cities with mitigation was consistent with declines in ALAN exposure. These findings highlight the potential of multispectral nighttime light data in quantitatively assessing the efficacy of ALAN-mitigation policies in reducing light pollution and its ecological impacts.