Artificial light at night (ALAN) can cause plants to open their buds and sprout leaves earlier in the spring. ALAN can also cause trees to delay changing the color of their leaves in the autumn. These changes can ripple through ecosystems, affecting pollinators, herbivores, and insects that lay eggs on plants. ALAN also disorients many migrating birds and other animals. Some cities have attempted to reduce overall ALAN using techniques such as shielding fixtures, implementing adaptive dimming, reducing light output, and choosing warmer color temperatures, but little is known about whether such efforts have been effective and what their downstream ecological effects might be.
Lin Meng and colleagues used data from the Sustainable Development Goals Science Satellite 1 (SDGSAT-1)—in orbit since late 2021—to measure ALAN between 2022 and 2024 in two pairs of cities: New York City, NY, and Newark, NJ; and Washington, DC, and Philadelphia, PA. In each pair, the first city has implemented ALAN-mitigation measures, whereas the second is a nearby city that has not. The authors found that New York City and Washington, DC, showed a reduction in ALAN after the policies were implemented, especially in the blue band, while Newark and Philadelphia did not.
Plant growth cycles also differed between the cities. ALAN typically advances the start of the growing season and delays the end of the season, but cities with mitigation policies show a start of season that is several days less advanced than it would otherwise be, as measured by the emergence of new green growth visible in satellite images. And cities with mitigation policies also show a less delayed turn to fall color than cities without such policies. According to the authors, the findings demonstrate the feasibility of using high-resolution satellite images to identify relative differences in ALAN levels and point toward ways in which this tool could support urban ecological research.
