Science Advances
Continued improvement in the luminous efficacy of light sources and increases in gross domestic product (GDP) have resulted in tremendous growth in artificial light use over several centuries. Historically, lighting has been subject to a strong rebound effect, in which increases in luminous efficacy result in correspondingly greater light use rather than energy savings. Regardless of historical or geographical context, humans tend to use as much artificial light as they can buy for ~0.7% of GDP. Outdoor lighting became commonplace with the introduction of electric light and grew at an estimated rate of 3 to 6% per year during the second half of the 20th century. As a result, the world has experienced widespread “loss of the night,” with half of Europe and a quarter of North America experiencing substantially modified light-dark cycles.
A critical question for sustainable development is whether the use of outdoor light will continue to grow exponentially or whether developed countries are nearing saturation in demand. In addition to the possibility that the existing light levels are already sufficient for any desired visual task, factors that reduce demand include greater public recognition of the unintended ecological and astronomical impacts of outdoor light pollution, official warnings that overexposure to artificial light may be affecting human sleep and health, efforts to transition to a sustainable society with decreased electricity demand, the desire of local governments to reduce the costs of lighting, and the establishment of protected “dark sky” areas. If demand saturation has not been reached, then the increasing luminous efficacy made possible by the solid-state lighting revolution will increase light emissions instead of saving energy.
Changes in outdoor lighting can be measured on the global scale only via Earth-observing satellites, but no calibrated satellite sensor has made global observations of night lights until recently. The well-known older images of Earth at night were based on an uncalibrated sensor from a defense satellite [Defense Meteorological Satellite Program (DMSP)], which had frequent and unrecorded changes in sensor gain. Despite this drawback, there have been attempts to use statistical methods to try to intercalibrate the time series. These methods sometimes rely on questionable assumptions, such as the assumption that Sicily experienced no changes in lighting over a 15-year period. In addition to the lack of an on-board radiance calibration, DMSP experienced saturation in cities and had low (8 bit) radiometric resolution and an intrinsic spatial resolution of 5 km. Nevertheless, the inherent connection between artificial light and human activity means that DMSP data display strong correlations with many socioeconomic factors.