While cities provide vital habitat for human beings to
thrive, it appears U.S. cities have been built on the most
fertile soils, lessening contributions of these lands to
Earth’s food web and human agriculture, according to a study by
NASA researchers and others.

Though cities account for just 3 percent of continental U.S.
land area, the food and fiber that could be grown there rivals
current production on all U.S. agricultural lands, which cover
29 percent of the country. Marc Imhoff, NASA researcher and
lead author of a current paper, and co-author Lahouari Bounoua,
of NASA and University of Maryland, College Park, added that
throughout history humans have settled in areas with the best
lands for growing food.

“Urbanization follows agriculture — it’s a natural and
important human process,” said Imhoff.Throughout history,
highly productive agricultural land brought food, wealth and
trade to an area, all of which fostered settlements.

“Urbanization is not a bad thing. It’s a very useful way for
societies to get together and share resources,” said Bounoua.
“But it would be better if it were planned in conjunction with
other environmental factors.” Studies like this one, which
appears in the current issue of Remote Sensing of Environment,
may lead to smarter urban-growth strategies in the future.

The researchers used two satellites offering a combination of
daytime and nighttime Earth observation data and a biophysical
computer model to derive estimates of annual Net Primary
Productivity (NPP). NPP measures plant growth by describing the
rate at which plants use carbon from the atmosphere to build
new organic matter through photosynthesis. NPP fuels Earth’s
complex food web and quantifies amounts of carbon dioxide, a
greenhouse gas, which plants remove from the atmosphere.

Nighttime-lights data from the Defense Meteorological Satellite
Program and a vegetation-classification map created at NASA’s
Goddard Institute of Space Studies, New York, were used to
portray urban, peripheral and non-urban areas across the United
States. In this way, the researchers calculated the extent and
locations of U.S. urban and agricultural land.

In addition, observations from the Advanced Very High
Resolution Radiometer instrument, aboard the National Oceanic
and Atmospheric Administration’s polar orbiting satellites,
were used to calculate the Normalized Difference Vegetation
Index. This index is a measure of plant health, based on the
principle that plants absorb solar radiation in the red part of
the spectrum of sunlight used for photosynthesis during plant
growth. These data were then entered into a Stanford University
computer model to derive NPP.

The computer model created a potential pre-urban American
landscape, which was used to compare and estimate the reduction
of NPP due to current urban-land transformation.

For the continental United States, when compared to the pre-
urban landscape, modern cities account for a 1.6 percent annual
decline in NPP. This loss offsets the gain in NPP of 1.8
percent annually from increased farmlands. The result is
striking, given the small area that cities cover, relative to
agricultural areas.

A reduction of this magnitude has vastly unknown consequences
for biological diversity, but it translates to less available
energy for the species that make up Earth’s complex food
web. The loss of highly fertile lands for farming also puts
pressure on other means to meet the food and fiber needs of an
increasing population. On the local scale, urbanization can
increase NPP, but only where natural resources are limited. It
brings water to arid areas, and “urban heat islands” extend the
growing season around the urban fringe in cold regions. These
benefits, however, do not offset the overall negative impact of
urbanization on NPP.

NASA scientists developed the city lights map, and the U.S.
Geological Survey used a technique to create the Normalized
Difference Vegetation Index data. Research partners include the
University of Maryland’s Earth System Science Interdisciplinary
Center, the World Wildlife Fund, and the Center for
Conservation Biology at Stanford University.

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