Reaping What
You Sow
Mapping the Urbanization of
Farmland using Satellites and City Lights
by John Weier
November 01, 2000
Part two of a three part series.
Part 1: Bright
Lights, Big City
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.
Scenes like this (above)
—rolling farmland punctuated by isolated
farmhouses—are becoming increasingly rare as cities
and suburbs expand into rural land. (Photograph
courtesy USDA Photography Center)
The
right 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
"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.
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.
Population growth and urbanization go
hand in hand. (Photograph 'above' copyright
Photodisc)
The Aura of Urbanization
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.
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.
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)
A Bumper Crop of Brand New Homes
"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.
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)
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.
This pair of images shows the
suitability of 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)
Saving What We’ve Got Before
It’s Gone
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)
References
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
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)
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