Lighting Up the Night

ImaGeo iconImaGeoBy Tom YulsmanApr 8, 2013 5:23 AM


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Changes in nighttime lighting in Asia are depicted for three years in this map, based on satellite data. (Image: Christopher Small) I'm sure most of you have seen those beautiful images of Earth at night, including the latest ones from the Suomi-NPP Satellite. They're a great illustration of how much of the planet's surface we humans dominate. But how is that domination changing over time in fast-growing Asia? That's a question Christopher Small of the Lamont Doherty Earth Observatory has explored using satellite data. The beautiful image at the top of this post is the result of his research. I'll explain the details in a minute. But first, the overall findings... As described in a paper Small published recently with a colleague in the International Journal of Applied Earth Observation and Geoinformation, between 1992 and 2009, south and east Asia experienced an 18 percent increase in the number of spatially distinct lights, and an 80 percent rise in the lighted area. For China and India specifically, the changes were even more significant. While the number of lights in both countries increased by 20 percent, the lighted area expanded by a whopping 270 percent. (China's increase came later than in India.) "Our analysis shows development in both India and China becoming measurably more interconnected in less than 20 years," Small told me in an email message. "In other words, interconnection of the largest networks is outpacing the nucleation of smaller settlements. This is presumably in part a result of accelerating migration to urban centers and the intergrowth of those centers into larger networks." What does the image show? The colors are keyed to night lights, as detected by infrared sensors on spacecraft of the Defense Meteorological Satellite Program, in three specific years. What those colors mean is a bit more nuanced than you might think — and they tell a really interesting story. The redder a pixel is on the map, the brighter it was in 2009 relative to 2001 and 1992. A glance at China shows what looks to be a very significant recent expansion in lighting. No surprise there, really. What about the other colors? The bluer a pixel is on the map, the brighter it was in the first year, 1992. In other words, those blue areas actually grew dimmer in the ensuing years. (But with this important caveat: For technical reasons, areas that are saturated with light — meaning the cores of cities — tend to show up blue. That's particularly evident in Japan.) Meanwhile, the greener a pixel, the brighter it was in the middle year. In other words, these areas were brighter in 2001 than they were in 1992, but then dimmed in 2009. So here is where I think things get kind of interesting. There's quite a lot of green in places, particularly in India, parts of Thailand, Malaysia and Indonesia. If development is occurring at breakneck speed, why would lighting have increased, and then dimmed? I put that question to Small, and here was his answer:

There are some plausible reasons why places may go from dark to brighter to dimmer. Think of construction sites. During construction there is a lot of exposed soil (which is bright, especially in the infrared), not much shadow (flat) and often lighted at night (at least somewhat, presumably to prevent theft of all the materials too large to lock up). Once construction is finished, most of the area is un-lighted, or the light is occluded by the buildings. We consistently see highways in China go from dark to light to dark in the 2000's.

He emphasized that there may be other explanations for what happened. And he cautioned that "there is still much unknown about the relationship between night lights and what's actually happening on the ground. A small number of people are trying to figure this out." The findings are independently supported by Landsat data. But even so, this research can't provide a final, definitive answer. Small likes to think of the method as a kind of "reconnaissance tool." As far as I can tell, that reconnaissance is turning up some fascinating — and suggestive — details about life in the Anthropocene.

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