A global analysis of factors controlling VIIRS nighttime light levels from densely populated areas
Introduction
Artificial nighttime lights present one of humanity's unique footprints that can be seen from space (Croft, 1978). Resulting light pollution has been shown to negatively impact the community of astronomers and our ability to observe the night sky (Cinzano et al., 2001). However, the negative effects that light pollution has on ecological systems and on our health, through changes in circadian exposure to light and changes in the wavelengths we are exposed to, might have more important and far-reaching consequences (Longcore and Rich, 2004, Falchi et al., 2011, Gaston et al., 2013). Light pollution and artificial lighting has been shown to vary greatly in space and in time, as a function of population and economic activity. However, most studies examining the factors explaining global spatial variability in lit areas were conducted at national and provincial levels using the DMSP/OLS sensor (e.g., Elvidge et al., 1997, Chen and Nordhaus, 2011, Wu et al., 2013, Keola et al., 2015). While offering the only globally available time series of nighttime lights imagery from 1992 onwards (Bennie et al., 2014a), DMSP imagery has various drawbacks as it is not calibrated, its spatial resolution is coarse, it contains overglow beyond urban boundaries and it is saturated in urban areas (Small et al., 2005, Doll, 2008). Temporal changes in cities' lights and the spatial characteristics of cities' nighttime brightness have been examined in several countries using DMSP data (e.g., Lo, 2002, Ma et al., 2012, Zhang and Seto, 2013). Most of the studies which used DMSP data for urban studies have used annual datasets, whereas daily and monthly datasets were used to identify more dynamic and time varying features, such as forest fires, wars and fishing vessels (Huang et al., 2014). New studies using DMSP datasets for quantifying urban patterns are continuously being published (e.g., Ma et al., 2015, Weidmann and Schutte, 2016), however, annual products of DMSP night lights data are no longer being produced, the last one available being that of 2013.
Recently, new studies have attempted using finer spatial resolution (≤ 1 m) nighttime imagery to examine the factors explaining spatial patterns of nighttime lights within cities (Kuechly et al., 2012, Hale et al., 2013, Levin et al., 2014, Katz and Levin, 2016). Astronaut photography taken from the International Space Station presents an additional source of information about spatial patterns of cities at nighttime (de Miguel et al., 2014, de Miguel, 2015). Levin and Duke (2012) have used ISS imagery showing that not all towns and cities are equally lit, and that economic, infrastructure and demographic factors can explain differences in brightness levels of localities in Israel and the West Bank. Kyba et al. (2014) have used VIIRS DNB data to study the relationship between population size and the sum of lights from cities and communities in the USA and Germany, finding differences in light emission between cities of these two countries, and several recent studies have used VIIRS data to examine the nighttime brightness of cities in China (Ma et al., 2014a, Ma et al., 2014b, Shi et al., 2014a, Shi et al., 2014b) and in the USA (Chen et al., 2015). In addition, Elvidge et al. (2016) have used VIIRS data to detect and measure radiant emissions from gas flares globally, forming one of the major industrial sources of light pollution, which can even be detected night-time images of Landsat 8 in the visible bands (Levin and Phinn, 2016).
Urban areas are of high importance as most of the world's population resides in cities, with 78% of global carbon emissions attributed to cities (Grimm et al., 2008). In this paper our aim was to use the new monthly global cloud-free mosaics from the VIIRS sensor onboard the Suomi-NPP (launched in 2011), to examine the factors explaining spatial variability in nighttime lights at the city level, comparing densely populated areas (mostly urban areas) globally. We hypothesized that urban form and urban density (and other factors including percent urban area, NDVI, snow cover etc.) will also affect brightness levels, and not just socio-economic factors such as national GDP and population size. In addition, we aimed to examine the difference between using lit areas (i.e., areas above a certain threshold of nighttime lights brightness, as usually done in studies using DMSP data) and using calibrated brightness levels in radiance values, on the resulting factors explaining inter-city variability in nighttime lights.
Section snippets
Methods
The Visible/Infrared Imager/Radiometer Suite (VIIRS) was launched in October 28, 2011, collecting high quality nighttime images at a spatial resolution of 750 m in the Day/Night Bands (DNB), between 500 and 900 nm (Miller et al., 2012, Miller et al., 2013). Recent studies have shown the improved quality of VIIRS nighttime lights images over those acquired by the DMSP/OLS sensor (Elvidge et al., 2013, Li et al., 2013, Miller et al., 2013, Shi et al., 2014a, Shi et al., 2014b). There are now
City level
Altogether, we identified 4153 populated areas globally, mostly corresponding to cities and metropolitan areas (Fig. 1; see Supplementary KML file for the polygons of all cities). Their median area was 29.3 km2 (with a maximum of 3927 km2, for Jakarta, Indonesia), their median population being 172,000 (with a maximum of 30.4 million people for Tokyo, Japan), the median population density being 5476 people/km2 (with a maximum of 39,605 people/km2 for Hong Kong), and the median brightness of these
Discussion
Overall, our global mapping identified 4154 densely populated areas, 13.9% more than the 3646 metropolitan urban areas identified by Angel et al. (2011) who used MODIS derived urban land cover and population data. Previous global studies which analyzed differences in nighttime light brightness at the country or state level often focused on four main variables: population size, urban area, GDP and electric power consumption (e.g., Elvidge et al., 1997, Elvidge et al., 1999, Small et al., 2005,
Conclusions
Nighttime light remote sensing is still in its infancy stage and is basically qualitative, compared with daytime optical remote sensing and microwave remote sensing. There is still a lack of understanding of the mechanisms behind nighttime light remote sensing, due to the lack of studies at the ground level and the relative lack of understanding of nighttime light transfer from lighting sources through the air to the sensor. To advance nighttime light remote sensing, there is an urgent need for
Acknowledgments
The work of Qingling Zhang was supported in part by the One Hundred Talents Program of the Chinese Academy of Science ([2015], ARP: Y674141) and the Fundamental Research Program of Shenzhen S&T Innovation Committee ([2016], JCYJ20160429191303198). We thank the anonymous reviewers and Editor whose suggestions contributed to the manuscript.
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