The light of urban development seen from space can tell us much about the shape of the world's economy.
Economists have previously measured what they call the “economic center of gravity” of the world. It’s that theoretical point on the globe – which has been migrating particularly rapidly over the past decade – that sits at the geographic center of all the world’s countries when they’re weighted by GDP.
This map, from a report last year by the McKinsey Global Institute, traces the movement of that center of economic gravity over the past two millennia (and projected into the next 15 years). Industrialization and urbanization in Europe and America tugged the center westward starting in the 17th century. Then around 1950, this trajectory abruptly reversed, reflecting the rise of whole economies – and cities – in the East.
Nicola Pestalozzi, then a master’s student at ETH University in Zurich, was intrigued by this concept in looking at an entirely different set of data that has sometimes been used by researchers as a proxy for GDP: satellite measurements of the nighttime light produced by human development. Light has a kind of clean appeal that GDP lacks.
“Of course, GDP for most countries is an estimate, every country measures GDP in a different way, and GDP data is difficult to gather,” Pestalozzi says. Light, on the other hand, can be measured more scientifically, evading some of the politics of economic data. “That’s basically the whole point about light: It’s an observable quantity, and it’s objective. You cannot fake it because you look at it from outside the earth.”
For his master’s thesis, Pestalozzi was looking at a massively comprehensive database from the Defense Metrological Satellite Program, run by the U.S. Air Force. Since 1992, its satellite measurements of the world’s light output have been digitized and publicly available. Other researchers have used this data to measure economic development across the world. Pestalozzi wanted to use it to try to calculate the center of the world’s light, in much the same way that other researchers have tried to calculate the center of the world’s economy.
These satellites passing overhead collect data on much more than light (they were originally deployed to measure cloud cover). But in a given year, they pass over most spots on the globe – a pixel on one of these maps represents a square kilometer – between 50 and 100 times. The average of the measured light output from all of those observations can then be quantified (Pestalozzi’s metric pegs light on a scale of zero to 64), enabling him to similarly calculate the center of all of our night light.
This is the map he produced, in a paper co-authored with Peter Cauwels and Didier Sornette, illustrating that "center" in a time lapse since 1992:
Our center of light has been migrating eastward, too, at a rate of about 60 kilometers a year (the McKinsey report put that number instead at about 140 kilometers a year).
So what does all of this tell us? For one thing, as Chinese cities like Guanzhou and Shenzhen have grown in population and economic might, they’ve gotten brighter, too. This chart shows how the sum of light in China has increased in two decades, when all of the satellites measuring that data are calibrated with each other:
These two images from the research show the agglomeration of light around Shenzhen-Guangzhou in 1992, at left, and 2009, at right:
At the same time, the lights have actually been dimming in some parts of Russia and Eastern Europe as populations have dwindled there or migrated elsewhere. In some parts of the West, particularly in Canadian cities, light output has also decreased not because economies or populations there are stagnating, but because initiatives to reduce light pollution have actually been successful. In the last 20 years, Pestalozzi has measured a 50 percent decrease in light out of Canada.
In that same time, Egypt has doubled its measurable light. As Pestalozzi points out, we seldom think of Egypt alongside rapidly developing countries like China, India and Brazil as rising economies. These two images of the growing urbanization along the Nile (again from 1992 and 2009) suggest that perhaps we should:
In many ways, these satellite images can illuminate narratives about global development that even economic statistics can’t describe. The border between India and Pakistan is visible from space, Pestalozzi says, capturing the tension between the two countries in the way they keep their contentious border floodlit for security.
He has also tried to use all of this data to calculate a Gini coefficient for light, borrowing from the metric commonly used to capture income inequality. He expected that less developed countries would have a stark inequality of light – that it would be centralized in a few mega-cities while people living outside of them sat in the dark. Surprisingly, though, this turns out not to be true. Light appears to be spatially dispersed in countries across the globe with an almost universal ratio. This means that as lights brighten in Karachi, Pakistan, smaller villages around the country aren’t necessarily left behind; they brighten, too.
Some of these patterns mirror what traditional economics can already tell us. "But the best way to measure economics is to combine several factors: GDP could be one factor, light could be another factor," Pestalozzi says. "If you try to combine several independent observations, then you probably have a better image the world."
The Middle East
All images, including the above map of Central Africa, courtesy of Nicola Pestalozzi.