Reaping What We Sow

  Part two of a three part series.
  Part 1: Bright Lights, Big City
  Part 3: Urbanization’s Aftermath

      Page 2

When most animals in the wild multiply to the point where they require more food than is available in their habitat, they eat what they can and then starve in droves. From dinosaurs to present-day deer populations, this basic rule of nature has held fast for nearly every animal species with one notable exception—us. Many anthropologists believe that 10,000 years ago, when the human population reached its natural limit of 10 million people (Imhoff et al., 2000), the agricultural revolution began so that the hunter-gatherers could ensure their survival. Ever since, we humans have been growing in number, precariously and diligently avoiding what seems to be a Malthusian fate by engineering new ways of reviving our soil, changing the flow of the Earth’s water, and even genetically altering our crops.

Now that the number of people on the planet has surpassed the six billion mark, it is more important than ever that we actively protect our natural resources. Yet, many researchers fear we may be doing the exact opposite. As our population continues to swell, our self-made urban and suburban habitats have begun to consume enormous tracts of once rural landscape. What is worse, some researchers believe, a majority of this landscape is prime farmland.

Tracking this phenomenon, however, has always been difficult. Urbanization moves relatively fast and its outlines are often hard to discern. Recently, a group of researchers at Goddard Space Flight Center, led by climatologist and remote sensing specialist Marc Imhoff, came across a solution. Using satellite images of city lights at night, they constructed a map of the urbanized areas of the United States and several other countries. They then integrated this map with a soil map that the United Nations prepared. These NASA researchers found that while the residents of these countries are not going to starve tomorrow, they may indeed be destroying their best soils and putting future generations at risk.

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The data used in this study are available in one or more of NASA's Earth Science Data Centers.


Scenes like this—rolling farmland punctuated by isolated farmhouses—are becoming increasingly rare as cities and suburbs expand into rural land. (Photograph courtesy USDA Photography Center)

Chicago at Night
The above photo of the lights of Chicago was taken by an astronaut aboard the space shuttle. The bright lights of the city center are bounded by the black waters of Lake Michigan on the right, and they fade into dark rural landscapes on the left. Scientists assembled a dataset of lights detected from space to measure the extent of cities and urban areas worldwide. (Image courtesy NASA, Photo ID STS081-376-1)

  Biting the Hand that Feeds Us   Page 1Page 3

"We’re living at a very special point in human history with respect to population growth," says Imhoff. "We’re adding whole country-sized populations of people in decreasing time intervals." The human race reached one billion people in 1818. Since then it has been growing geometrically, reaching two billion by 1932, four billion by 1982 and close to six billion by 2000. Though the growth rate is slowing down now, the Earth is expected to house 10 billion people by the year 2050.


Population Graph

On the whole, Imhoff explains that the human population now consumes and burns as much as 40 percent of all new plant growth on the Earth every year (Imhoff et al., 2000). Most of the best soils in the world have already been cultivated in one fashion or another to grow everything from asparagus to cotton to pine trees to wheat. As the human population expands, it is likely that we will have to keep all the farmland we have as well as cultivate much of the remaining arable land on Earth.

We may, however, be sabotaging ourselves. Along with staggering population growth over the last century has come a mass movement towards the cities. Worldwide, human flight towards large urban areas is boosting the urban population upward three times faster than the general population growth. Only a third of the planet’s population lived in urban areas ten years ago. Now it’s up to 50 percent and in ten more years it will be up to two thirds. This mass movement to the cities has caused urban areas to expand at an enormous rate. In the United States alone, 19,000 square miles of otherwise rural cropland and wilderness were developed between 1982 and 1992 (World Resources Institute, 1996).

  Global population increased by more than 3 times—from 1.65 billion in 1900 to 6.06 billion in 2000—in just a century. In contrast, the Earth’s population never reached more than one billion people before 1800. (Graph by Robert Simmon, based on data from the United Nations)

The problem with all this urban development has to do with where most major cities around the world are located. "Because we are biological entities, we follow biological resources," Imhoff says. He explains that in the past, people laid down the foundations of our modern day metropolises in areas where the land was flat, the water and soil were good and the climate was temperate. These are the same regions that make for good farmland. Though urban sprawl today normally only covers two to five percent of the total land in any given country, that very land may be our most arable. And once an area of land is urbanized, it is very difficult to bring the soil back to its former state.

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Population growth and urbanization go hand in hand. (Photograph copyright Photodisc)

  The Aura of Urbanization   Page 2Page 4

Since researchers first suspected that this trend was taking place, the single biggest problem in tracking it has been in finding a way to measure the full extent of urbanization across very large regions, such as whole continents. Several years ago, Imhoff came across a solution. He discovered satellite images displaying the illumination cities and towns generate at night. The images were taken by the Defense Meteorological Satellite Program’s Operational Linescan System (OLS). The satellite network was originally designed to aid in aircraft navigation by detecting the lunar illumination off of nighttime clouds. What the Air Force realized is that on evenings when there was a new moon, the satellite was sensitive enough to record the illumination from city lights. Over a period of several new moons, the data the satellite retrieved could be pieced together to produce a global image of city lights.

  North American Lights

Using computer algorithms, Imhoff figured out a way to create maps of the approximate population density across an entire country or continent from the images (see Bright Lights, Big City). "We essentially scaled back on the brightness levels of the imaging data," says Imhoff. The first full map of population density he constructed was of the United States. With help from U.S. Census Bureau statistics, the Goddard team was able to classify all land area in the United States into three categories—urban, peri-urban, and non-urban areas.

An urban region, Imhoff elaborates, is defined as an area with 1000 people or more per square mile. These are regions where humans have developed and completely transformed the natural ecosystem. Any scientist looking at a region classified as urban on the map can be fairly certain that there are parking lots, office buildings, some strip malls, and maybe a fast-food restaurant or two. Peri-urban areas, on the other hand, have only been lightly populated. They usually consist of farmland, light suburban development or small towns and are classified as having an average of 100 people per square mile. In most instances, this is the type of land development that occurs as cities expand. Finally, non-urban areas are regions such as central Montana and western Maine, where only ten people or less live per square mile.

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Based upon satellite measurements of city lights, this image is a map of the urban population density of North America. Red yellow and green are urban areas, and blue is peri-urban. The city light data is laid over elevation data (black is sea level, light grey is over 10,000 feet). Most major cities are in level areas along an ocean bay, large lake, or navigable river. (Image courtesy Marc Imhoff, NASA GSFC, and Flashback Imaging Corporation, Ontario, Canada)

  A Bumper Crop of Brand New Homes   Page 3Page 5

"So we had this map of where the urbanized places are in the U.S. We then wanted to merge it with the soils map of the U.S. created by the United Nations Food and Agricultural Organization (UNFAO)," says Imhoff. The UNFAO regularly samples and tests soils all across America from the tip of Maine to Southern California in an effort to determine the amount and location of productive farmlands throughout this country. They then lay out their results on a map of the U.S., classifying each soil in terms of limiting factors.

"The more limiting factors you have, the more expensive it is to produce agricultural products because those limiting factors have to be overcome," explains Imhoff. One limiting factor may be that the soil is too damp and needs to be drained before it can be farmed. Another limiting factor may be that the soil is too acidic and would have to be limed. Generally, a soil with three or fewer of the less severe limiting factors is considered to be prime soil because very little has to be done with it in order to plant a crop.


United States Soils

Imhoff and his group used Geographic Information System (GIS) software tools to merge his map and the UNFAO map point-for-point in terms of latitude and longitude so that the Goddard team could compare the two. When they tallied up the amount and type of soils covered by urbanization, they found that the soils with just one or two limiting factors were being urbanized the most. For the entire United States, nearly five percent of these high-quality soils had been dug up for development. However, soils with no limiting factors came in at fewer than three percent. "What we think is going on across the country is that there might be some preservation of the very best soils, but it’s at the expense of the next best."

As Imhoff suspected, the soils that weren’t being urbanized at all were those with six or more limiting factors—soils with hardly any agricultural value. "These are areas in the desert and in the high mountains. They are generally places where there are severe seasonal limits on biological activity or where there is simply less activity overall. So humans aren’t there in large numbers yet," he says.

To get a clearer picture of the way in which future urbanization was moving, the Goddard team zeroed in on and analyzed the four states that, according to the 1989 census, have the highest market value of agricultural goods–California, Wisconsin, Illinois, and Florida. For these states the team matched up both the urban and peri-urban areas to soil types. They found that the distribution of urban area over soil types was close to the national average. However, in some instances the peri-urban areas—those areas where development was likely to expand—covered more than twenty percent of the higher-grade soils in the state.

  This map is color-coded to show where the best and worst soils are located in the United States. On a scale from zero (best) to 8 (worst), the colors are an indication of a region’s soil limiting factors.” Typical limiting factors are aridity, high salinity, lack of nutrients, and steep terrain. The more limiting factors a region has, the more expensive it is to farm. (Image courtesy Marc Imhoff, NASA GSFC)

California Soils and Lights

The most extreme case is California, where there is apparently almost no effort to protect agricultural lands. "Over 16 percent of the best soils in California are already in urban use here. If you look at the peri-urban areas, those are the corridors of development. The trend is pretty clear. Development is following soil resources," Imhoff says. More specifically, if these peri-urban areas are developed in California, more than 50 percent of their best soils could be lost and replaced by the houses and businesses of people who need to eat.

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This pair of images shows the suitability of California soils for farming on the left, and urban areas on the right. The Great Central Valley, where the state’s best soils are and most of America’s fresh vegetables are grown, is becoming increasingly urbanized. (Image courtesy Marc Imhoff, NASA GSFC, and Flashback Imaging Corporation, Ontario, Canada)

  Saving What We’ve Got Before It’s Gone   Page 4

What does all this mean for the U. S.? For now, Imhoff believes that the United States still has plenty of good soil and that we are not going to run out of food anytime soon if we work to curb our urban sprawl. However, in other countries there is possibly more to worry about in the near future. "We did a similar study in China," says Imhoff. "In China, they’re also having the same trends. The best soils are being developed now with their increased economic development. But they have fewer soils to rely on." Probably the worst-case scenario is in Egypt. They have very little arable land and it is all along the Nile Delta where everybody lives. As the population in Egypt expands, people either have the choice of building on prime farmland or of moving out onto very inhospitable areas of the Sahara.



In order for the world to save its farmlands for future use as the population expands, Imhoff feels that city planners need to start building and developing city infrastructure on rocky, non-level, and arid soils. "I think that land use planning has to have some teeth. We need to leave the land that is productive in agricultural use," he says. Until then the Goddard team will continue to monitor the U.S. as well as other countries and alert people to this problem before we get to the point where we have plenty of big lawns and convenience stores, but very little food.


A photograph of Cairo, Egypt, taken from the space shuttle. Located at the southern end of the Nile Delta, Cairo is one of the world’s fastest growing cities. It is also located on what was once prime farmland. Grey urbanized areas are continually eating into the green cropland along the Nile River—Egypt’s only arable land. (Photograph courtesy NASA Johnson Space Center Astronaut Photography)

  Egypt Soils and Lights

Imhoff, M. L., W. Lawrence, and C. J. Tucker, 2000: The Impact of Urban Sprawl on Photosynthetic Production in the United States, presented at 2000 American Association for the Advancement of Science (AAAS) Annual Meeting, pp. 1-3.

World Resources Institute, 1996: World Resources 1996-97, Washington, DC.

Related Links
Assessing the Impact of Urban Sprawl on Soil Resources in the United States Using Nighttime "City Lights" Satellite Images and Digital Soils Maps

Defense Meteorological Satellite Program

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Among the soil types shown, only “fluvisols” are suitable for farming. Fluvisols are soils deposited by flowing water, specifically the rich silt laid down by the Nile River’s annual flooding. Other soils in Egypt are too dry, too salty, or too rocky for farming. (Image courtesy Marc Imhoff, NASA GSFC)