|Part one of a three part series.
Part 2: Reaping What We Sow
Part 3: Urbanization’s Aftermath
Watching the familiar, rural landscapes of our youth give way to suburban sameness has become as much a part of modern American life as portable electronics, instant food, and wasted time in front of the television. Nearly all of us have had the disappointing experience of returning to what used to be the woods near our childhood homes and finding a new subdivision. Or we have been shocked to see that some corporate entity has erected aluminum-sided duplexes and an outlet mall in the middle of our favorite vacation spot.
Like it or not, throughout this century, the United States has undergone a steady process of urbanization as a larger and larger percentage of the population has moved towards the cities. While increasing urbanization may have some positive impacts on our environment, such as the lower birth rates that come with a city lifestyle, scientists are becoming more concerned about the negative long-term effects. Unlike rural communities, urban sprawl completely transforms the landscape and the soil and alters the surrounding ecosystem and the climate.
Marc Imhoff, a biologist at NASAs Goddard Space Flight Center, is one of these concerned scientists. For the past six years, he and a team of researchers have been looking for ways to measure the effects of urbanization on the biological productivity in the U. S. and other countries around the world. They created a method of mapping urbanization on a countrywide scale by using satellite images of the light cities generate at night. With the resulting city lights maps, they are now zeroing in on the impacts urban sprawl has on the food we eat, the air we breathe, and the ecosystem within which we live.
Global city lights. The Eastern U.S., Europe, and Japan are brightly lit by their cities, while the interiors of Africa, Asia, Australia, and South America remain (for now) dark and lightly populated. (Data courtesy Marc Imhoff of NASA GSFC and Christopher Elvidge of NOAA NGDC. Image by Craig Mayhew and Robert Simmon, NASA GSFC.)
High-resolution versions of this image are available on the Visible Earth
|Seeing the Light|
Urbanization in any country generally begins when large-scale commerce takes root and most new jobs are to be found in the factories and financial centers in cities. In the United States, urbanization began to occur roughly around the turn of the last century. Since then, the percentage of people living in the United States in urban areas has risen from 39 percent to more than 73 percent (U.S. Census Bureau, 1995). From decade to decade the amount of rural land that has been consumed by urbanization is enormous. Between 1982 and 1992, for instance, 19,000 square miles of otherwise rural cropland and wilderness were developed in the U.S. This would be the equivalent of covering half of Ohio into one big subdivision in a ten-year period (World Resources Institute, 1996).
Urbanization is not just an issue in the United States. Right now researchers estimate that worldwide movement towards cities is growing at three times the rate of population expansion worldwide. Only a third of the planets population lived in urban areas ten years ago. Now that number is up to 50 percent and in ten more years roughly two thirds of humanity will live in the cities (World Resources Institute, 1996).
Imhoff says that while working at Stanford University as a
post-doctorate, he initially became intrigued by the biological
implications of urbanization after he saw how urban sprawl appeared on
the Earths surface when viewed from orbit. "I spent a number
of years looking at the Earth from space, and I found it remarkable how
human development of land looked a lot like biological growth. Like mold
on an orange," he says.
Cities and suburban areas are growing rapidly, as shown by these two images of Plano, near Dallas, Texas. (Images courtesy USGS Earthshots)
He wanted to know how urban sprawl was changing the landscape on a global scale, and whether this increased development was affecting food supplies, local ecosystems, and even the global climate. Early on, he realized that the best way to do such a study would be to construct a map of urbanization using remote-sensing data from satellites. "Satellite data would give us a synoptic view of the globe from which we could get an explicit idea of where the human-dominated surface features are especially with respect to cities. We could also merge that information with soil maps and biosphere data from other satellites to assess the impact of urbanization on ecosystems," he says.
To construct such a map Imhoff needed a satellite instrument that
would give him a snapshot of the urbanization on an entire continent all
at once. When he began this research, he had access to data from a
number of remote-sensing satellites, such as Landsat 5 and NOAAs
operational satellites, that record the reflected sunlight and heat
emissions from the surface of the Earth. But demarcating urban sprawl
with the instruments on these satellites would require the researchers
to retrieve close-up images of cities and separate each individual,
urbanized area from the surrounding farms, parks, and wilderness. Doing
so for an entire continent would have been labor intensive and tedious.
St. Louis, Missouri appears as a grey smudge against green woods and brown, freshly plowed fields in this early autumn satellite image. (Image courtesy Reto Stöckli and Robert Simmon, NASA GSFC)
Imhoff found a solution in an unlikely place. He recounts a weekly astronomy club meeting he attended in 1996. "The people at the meeting were talking about how light pollution was a problem. They pulled out a satellite-generated city lights map. They were looking at it and saying, Look at all that awful light pollution. And Im there thinking, This is exactly what I need. Theres my global map of where human beings are," he recalls.
The images were taken by a Defense Meteorological Satellite Programs (DMSP) Operational Linescan System (OLS). This network of satellites was originally designed to pick up on lunar illumination reflecting off of clouds at night in order to aid nighttime aircraft navigation. What the Air Force discovered is that on evenings when there was a new moon, the satellites were sensitive enough to record the illumination from city lights. Over a period of several new moons, the data the satellites retrieved could be pieced together to produce a global image of city lights.
Imhoff explains that in the past census researchers have tried to use the city lights images to estimate the number of people in urban areas. In general they could only get a rough count simply because there was no way to tell, by looking at the lights alone, whether a very densely populated city such as Seattle had more people per square mile than a sprawling city such as St. Louis. However, Imhoff and his team were not interested in population counts. They wanted to measure the spatial extent of urbanization. For this purpose, he reasoned, the maps would be perfect.
Wherever there are cities, there are city lights. Some satellite sensors, designed to view clouds by moonlight, also detect lights on the ground. Scientists use satellite derived city light data to map urbanization. (Photo courtesy Steven Dahlman)