Looking at Earth at night from space turns the planet’s extremely varied and complex surface into a much simpler view of light and dark. The ghostly outlines of continents, laced with webs of light, are beautiful and eerily peaceful. But these images can also reveal clues about what the humans on Earth are up to, and how that is changing.
Earlier this month, when NASA released a new global mosaic of Earth’s night-lights based on images collected throughout 2016, cartographer John Nelson was intrigued by the comparison with the 2012 mosaic. NASA’s image sliders, which show a spot of Earth from both of those time periods, revealed changes in the quantity or brightness of lights. Nelson decided to map those changes.
“I was swiping back and forth ... and was fascinated by where things had changed,” he says. “So I thought a change-detection map would let me see that really easily, in one go.”
Nelson, a cartographer at the mapping-software company Esri, compared the equivalent pixels on two different maps and calculated the difference by subtracting the brightness value of one from the other. He then mapped those differences, showing places where the 2016 mosaic was brighter in blue, and places that had dimmed in pink.
The resulting firefly-style map shows just how much Earth’s night-lights changed in four years, revealing some big differences that are easily explained, such as the darkening of war-torn Syria in the image of the Middle East above.
That India has brightened dramatically, as shown by all the blue in the image below, is also not a surprise. The country is home to more than its fair share of people living without electricity, and its government has been working to change that by establishing a rural electrification program and investing heavily in renewable energy. There is still a lot of work to be done, but the results are already easy to see on Nelson’s map.
Other parts of the map highlight changes that aren’t as easily explained, such as a marked dimming of lights in parts of the developed world. “I was surprised to see an overall darkening of American and European night-lights,” Nelson says. “I suppose it might have to do with more efficient lighting technology.”
Though there’s a fair amount of new light in the United Kingdom, shown in blue on the map below, the majority of Europe is heading in the opposite direction. Perhaps increased awareness of light pollution, and campaigns to reduce it, have started to have an impact.
NASA earth scientist Miguel Román, who leads the team that analyzes the nighttime light data from the Suomi-NPP satellite, says caution should be used when trying to ascribe the changes shown on Nelson’s map to specific events on the ground. If a pixel shows an increase in light, it could be because that spot has become electrified, he says. “But it could also mean a change in the type of streetlights being used.”
Maps like these are good for highlighting qualitative change. But to really study the change, Román says the best thing to do is look at the magnitude of the change of each pixel, measured in radiances, which takes into account things like viewing angle.
“If you are trying to attribute this change to a human-driven pattern or process, then you really need to get down to the pixel level,” he says. “This is a continuing area of nighttime satellite research — state-of-the-art, never-before-done stuff — which is exciting!”
The change maps are perfect for finding and exploring curious developments. There are lots of changes to wonder about in the image of the eastern United States above. Why are the Carolinas brighter, while Georgia is dimmer? What’s causing the brightness along the Gulf Coast to be turned down? You can zoom in on any spot on Earth with the interactive version of Nelson’s map at the bottom of this story map.
To get a better sense what might be causing the change, Román says it’s also useful to track changes in a given area over time. NASA is moving toward making daily nighttime views of Earth available to scientists, which the agency says could be useful for all sorts of things, including spotting illegal fishing, responding to disasters, targeting sources of light pollution, and protecting vulnerable ecosystems.
“The NASA data is just terrific, and I love the work that they are doing,” Nelson says. The latest “Black Marble” nighttime data inspired him the moment he read about it, and he started creating maps immediately. “I think the best maps tend to be the fun, rushed ones that result from me just wondering something about a data set.”
Nelson’s other projects include working on ways to visualize a big data set on ocean chemistry. “I can't believe I get to do this for a living,” he says. “I have so much fun sliding around in amazing data and trying to figure out a good distilled way of presenting it. I'm never for want of a cool project to chime in on.”