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February 11, 2004
Cities Built on Fertile Lands Affect
Climate
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.
Although cities account for just 3 percent of the continental U.S. land area, the loss of plant growth due to urban use roughly offsets the gain made by agricultural expansion. 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.
For more information and images on the
Internet, visit:
http://www.gsfc.nasa.gov/topstory/2004/
0202cityland.html
For more information about NASA on the
Internet, visit:
www.nasa.gov
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Contacts:
Elvia H. Thompson
Headquarters, Washington
(Phone: 202/358-1696)
Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 607/273-2561)
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Comparing Post-Urban U.S. to Pre-Urban
U.S., Difference in Total Annual Net Primary
Production
This graphic compares modern U.S. annual Net
Primary Production (NPP) to a computer-derived
estimate of what the annual NPP would be in the
absence of urbanization. The graphic shows areas
with reductions or gains in NPP as a result of
urban development. NPP measures plant growth by
describing the rate at which plants use carbon
from the atmosphere to build new organic matter
through photosynthesis. Units are in grams of
carbon per meter squared. Credit: Marc
Imhoff/NASA
U.S. Urbanization and Net Primary
Production
a) The top map (shown in thumbnail) depicts
urbanized areas across the continental U.S. The
map was generated from nighttime satellite images
from the Defense Meteorological Satellite’s
Operational Linescan System (DMSP/OLS) collected
from October 1994 to March 1995. Red indicates
urban areas; yellow marks those smaller towns,
and suburbs on the peripheries of cities, or
peri-urban areas; and black represents non-urban
or rural areas. b) The lower map (shown in
full-size image) shows simulated total annual NPP
for the U.S. at 1x 1km horizontal resolution.
Units for NPP are in grams of carbon per square
meter. Credit: Marc Imhoff/NASA
Seasonal dynamics of the impact of
urbanization on NPP for the Mid-Atlantic Region
of the U.S.
These two graphs show how urbanization
influences the seasonal dynamics of NPP in the
Mid-Atlantic Region of the US. The Mid-Atlantic
region defined here includes (New Jersey,
Delaware, Maryland, Virginia, and North
Carolina). NPP in urban areas is higher than
non-urban areas in the cold months thereby
extending the growing season. However, NPP in
urban areas is greatly reduced during the warm
months when compared to non-urbanized areas. The
warm season reduction is more than enough to
offset the cold season gains. a) The top graph
(shown in thumbnail) shows monthly mean NPP rates
for urban areas (diamonds, red), peri-urban
(squares, yellow), and non-urban (triangles,
green) areas. b) The lower graph (shown in
full-size image) shows the differences between
modern annual urbanized NPP rates and pre-urban
NPP month by month. The graph depicts the losses
(negative, red) or gains (positive, green) in NPP
rates resulting from urbanization (urban -
non-urban). Units for NPP are in grams of carbon
per meter squared.
Bars represent ± 1 standard
deviation. Credit: Marc Imhoff/NASA
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