Stable Nighttime Light Decreased in Protected Area of California: Findings from DMSP-OLS Observations and the Importance for Protected Area Policy Making

Stable nighttime light, an indicator of persisting human activity and light pollution is a well-recognized disturbance to the wilderness of protected areas (PAs). Mostly supported by in situ observations, very limited studies of light pollution for PAs focused at a regional level and on a continuous time span to support policy making effectively. DMSP-OLS stable nighttime series provide continuous observation of nightlight and have been widely applied in studies focusing on human activities. In this study, we employed inter-calibrated DMSP-OLS nightlight series from 1992 to 2012 to evaluate the change of intensity and extension of stable nighttime light inside California PAs. We observed a decrease of stable nighttime light and a shrinkage in spatial extent in PAs located in all ecoregions from 1992 to 2012, especially before 2004. Such decrease and shrinkage occurred mostly in southern California and the Bay Area where mega metropolitan clusters locate. The successful application of protecting strategies in PAs and the improved technologies of lighting may contribute to the relieving of light pollution in PAs. However, the stable nighttime light slightly increased since 2004, when there was limited room for stricter protective regulations and the pressure from population growth persisted. Population density explained most spatial distribution of nightlight in years with census tract level demographic data available, except PAs with the highest wilderness such as Sierra Nevada Mts. We anticipate to improve the models with the newest remote sensing nighttime product from NASA Suomi-NPP and annually updated demographic data from American Community Survey at census tract level in the future to provide a cost-effective evaluation on protecting strategies. Such evaluation will support land managers of PAs and local policy-makers for modification and proposal of policies.