C. Aubrecht, C.D. Elvidge, D. Ziskin, T. Longcore, and C. Rich
A consequence of the explosive expansion of human civilization has been the global loss of biodiversity and changes to life-sustaining geophysical processes of Earth. The footprint of human occupation is uniquely visible from space in the form of artificial night lighting – ranging from the burning of the rainforest to massive offshore fisheries to omnipresent lights of cities, towns, and villages. This article describes a novel approach to assessing global human impact using satellite observed nighttime lights. The results provide reef managers and governments a first-pass screening tool for reef conservation projects. Sites requiring restoration and precautionary actions can be identified and assessed further in more focused investigations. We hope to create a mental picture for others to see and encourage participation in maintaining and restoring the natural world.
Assessing and protecting the diversity of life on Earth has been a major focus of scientific in-vestigation and public attention in recent years. Closely related to concerns about global climate change, it is one of the ‘global issues’ affecting society.
The variety of life on Earth – its biological diversity – and the natural patterns it forms are commonly referred to as biodiversity (Shah 2008). Plants, animals and microorganisms, the enormous diversity of genes they contain, and the ecosystems that they form (e.g. rainforests, deserts, coral reefs) are all the product of billions of years of evolutionary history, shaped by natural processes and recently and increasingly by anthropogenic influence.
Why is biological diversity important? Many recognize the aesthetic, cultural, and intrinsic value of other species and see preventing extinction as a moral issue (Norton 1987). Biological resources are also the foundation upon which civilizations are built. The loss of biodiversity threatens food supplies and limits natural sources for energy and medical progress. Protecting biological diversity should therefore be our highest priority. Klaus Töpfer, Executive Director of the United Nations Environment Programme (UNEP), states that “although scientists are now able to appreciate the complexity of interacting natural processes, we are still a very long way from understanding how they all fit together. What we do know is that if any part of this web of life suffers breakdown, the future of life on the planet will be at risk.” (Secretariat of the Convention on Biological Diversity 2000).
Coral reefs are among the most biologically rich ecosystems on Earth (Bryant et al. 1998). It is estimated that between one and nine million species are associated with coral reefs. Some estimates, such as the Global Biodiversity Status compiled by the World Conservation Monitoring Centre (WCMC 1992), reach a number of 14 million species altogether, including land and aquatic environments. Others suggest that even greater diversity is possible when including microbial life. Locations of coral reef hotspots are shown in the UNEP-WCMC World Atlas of Biodiversity (Groombridge and Jenkins 2002), which maps biodiversity-related issues.
Coral reefs are important to local communities. They protect the coast, sustain local fisheries, are the source of medicines, and support economies through ecotourism. These values are at risk because corals and coral reefs are extremely sensitive. Slight changes in the reef environment may have detrimental effects on the health of entire coral colonies. The impact of natural and anthropogenic disturbances can have multiplicative effects on coral reef eco-systems (Hughes and Connell 1999). The interactions of stressors are not fully known, but evidence suggests that human-damaged reefs may be more vulnerable to some types of natural disturbances and take longer to recover after disturbance.
Satellite observed nighttime lights for environmental impact assessment
Because of the global distribution of coral reefs and their occurrence in remote locations, the most practical approach for standardized monitoring of relevant conditions is through the use of remotely sensed data. The U.S. National Oceanic and Atmospheric Administration (NOAA) has established a global sea surface temperature (SST) tracking system focused on coral reefs, known as ‘Coral Reef Watch (CRW)’, using meteorological satellite data. The system automatically detects prolonged periods of high sea surface temperatures in coral reef locations and issues alerts for coral bleaching events. Satellite coral bleaching monitoring products include SST, SST anomalies, coral bleaching hotspots, coral bleaching Degree Heating Weeks (DHW), SST/DHW time series, and the above mentioned Satellite Bleaching Alerts (SBA). These products are produced in near-real-time using NOAA/NESDIS operational composite nighttime POES (Polar Operational Environmental Satellites) AVHRR (Advanced Very High Resolution Radiometers) featuring a spatial resolution of 0.5-degree (approximately 50-km). The adverse effects of rising sea surface temperatures on reef ecosystems are also highlighted in the technical paper ‘Climate Change and Biodiversity’ of the Intergovernmental Panel on Climate Change (IPCC 2002).
Another ecological stressor, maybe not so obvious or well-known, is artificial light at night. Night lighting from cities, gas flares and light-induced fisheries can have direct and in-direct effects on marine organisms, including seabirds (Montevecchi 2006) and fish (Nightingale et al. 2006). Adverse effects are likely to occur in the biologically diverse communities of coral reefs (Aubrecht et al. 2008). Many examples of serious ecological effects of artificial night lighting are found in Rich and Longcore (2006). In addition to ecological consequences, extensive research (e.g., Kloog et al. 2008) shows that excessive exposure to light at night may pose a serious threat to human health (e.g. increased risk for breast cancer).
Mapping the presence of light on the Earth’s surface on a global scale is made possible through the use of the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) operated by NOAA’s National Geophysical Data Center (NGDC).
Initially designed to detect moonlit clouds, it includes a pair of visible and thermal spectral bands. With 14 orbits collected per day and a 3.000 km swath width, each OLS is capable of collecting a complete set of images of the Earth every 24 hours. The sensor is equipped with a photomultiplier tube (PMT), which intensifies the visible band signal at night. It thus provides up-to-date information on the location and impact zone of oil and gas producing and processing facilities, heavily lit fishing boats, plus the artificial night sky brightness that can extend many kilometers out from major urban areas (see figure 1). Algorithms for processing the long-term DMSP data archive (e.g., identifying image features, such as lights and clouds, and creating annual composites) have been described by Elvidge et al. (1997, 2001).
The ‘Lights Proximity Index’ and related applications
We developed the Lights Proximity Index (LPI) to assess potential areas at risk from lights both as direct ecological threats (e.g. disrupted spawning and forage cycles, disorientation of birds and sea turtle hatchlings) and as proxy measures for indirect impacts (e.g. human-associated chronic water pollution, tourist overuse, bomb fishing; Aubrecht et al. 2008).
The approach has already been used to assess global anthropogenic stresses to coral reefs (see figure 2) based on an annual composite of DMSP-OLS nighttime lights (satellite F15, year 2003) having 30 arc second resolution cells (approximately 1 km² at the equator). The incoming data have a ground sampling distance (GSD) of 2.7 km. The produced global grids integrate lighting from a wider area surrounding the nominal 30 arc second grid cell (foot-prints on the ground are in the 5-7 km² range). A complete list of analyzed coral reef locations and associated LPI values is available for download on the web page of NOAA-NGDC’s Earth Observation Group.
Our current work involves adapting the algorithm for ecological risk assessment of sea turtle nesting sites (Ziskin et al. 2008). The relation between artificial night lighting and sea turtles has already been extensively analyzed (e.g., Salmon 2003).
Initial results of the analysis related to human-induced coral reef stress measured by the LPI (see figure 3) indicate that reefs in Puerto Rico, the Red Sea and the Persian Gulf are at high risk from direct and indirect impacts of human settlements. The Red Sea and Persian Gulf are also greatly affected by gas flaring while fishing activities pose the greatest threat in the Gulf of Thailand.
According to the time series analysis (Aubrecht and Elvidge 2008), human development (i.e. cities and towns) as a reef stressor has been increasing since 1992. Regions like the Red Sea, Malaysia and Singapore especially stand out, which is confirmed by local monitoring (Wilkinson 2004). Nonetheless some areas actually show a decreasing LPI. The island of Oahu in the Hawaiian archipelago serves as an example for such an improvement. Presumably this is because of law-enforced management activities against light pollution.
With nightly fishing boat activity remaining approximately stable, a slight improvement can be observed in the Gulf of Thailand in contrast with a slight degradation in the Philippines. Looking at the temporal trend of the gas flaring LPI in the Persian Gulf, the region most affected by this particular stressor, the potential threat to coral reefs has been increasing steadily since 1992 and is likely to increase even more in the future.
Nighttime lights, in addition to serving as a proxy measure for other human activities, pose a serious threat to biologically diverse ecosystems and even to human health. The ability to easily visualize regions particularly subjected to artificial light at night can be valuable to identify degraded sites requiring immediate restoration actions and pristine areas with high priority for protection.
Solving problems caused by light pollution can be very different from mitigating other pollutants, although lights can be indicators of a range of well-known stressors that deserve attention themselves. But in theory, harmful light could be eliminated immediately by “flipping a switch at the source” (Witherington and Martin 2000) if it can be recognized first. Obscured human risk perception may be the key problem because we are all used to seeing lights at night. Our collective challenge is to recognize the diverse direct and indirect threats of light pollution and try to use this knowledge to help protect global biodiversity.
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Christoph Aubrecht is a research associate at the systems research division of the Austrian Research Centers (Vienna, Austria). He has been teaching at the Institute of Geography and Regional Research at University of Vienna and is a cooperative doctoral researcher at the Institute of Photogrammetry and Remote Sensing at Vienna University of Technology. He spent several months at the U.S. National Oceanic and Atmospheric’s (NOAA) National Geophysical Data Center (NGDC) carrying out research on satellite observed nighttime lights.
Chris Elvidge is leading the Earth Observation Group at NOAA/NGDC (Boulder, CO, USA). After his Ph.D. in Applied Earth Science from Stanford University he was a National Research Council research associate at NASA’s Jet Propulsion Laboratory. He moved on to faculty in Biological Sciences in the Desert Research Institute at University of Nevada. Prior to coming to NGDC he was a visiting scientist in the EPA’s global change research program office in Washington, DC.
Daniel Ziskin is a research associate at NOAA/NGDC (Boulder, CO, USA). Previously he worked at NASA Goddard Space Flight Center and at the National Center for Atmospheric Research.
Travis Longcore is research associate professor of geography at the Center for Sustainable Cities at the University of Southern California (Los Angeles, CA, USA) and science director of The Urban Wildlands Group.
Catherine Rich holds degrees in law and geography and is co-founder and executive officer of The Urban Wildlands Group (Los Angeles, CA, USA). She is lead editor of ‘Ecological Consequences of Artificial Night Lighting’ published by Island Press (2006).