Don’t let that tree outside your kitchen window fool you. Though it may not look like it’s doing much more than adding a little shade to your yard, it is, in fact, working to curb global warming. Through the process of photosynthesis, green plants use the energy from the sun to draw down carbon dioxide and combine it with water to create the carbohydrates plants need to grow and animals need to live. While a single tree does not have much effect on the atmospheric levels of carbon dioxide, en masse plants draw down millions of tons of this potent greenhouse gas and effectively help keep the Earth cool.
Given their positive effect on the climate, one would think that we’d
want to keep the world’s plants, especially those in our backyards,
intact. But each year humans destroy untold amounts of plants and fertile soil
through the process of urbanization. Every time a subdivision is built, a strip
mall is erected, or a road is laid, the local vegetation is uprooted and the
soil is turned. Though new grass and trees may sprout afterwards, this newly
grown canopy of vegetation is usually much less dense.
|Urbanization tends to the reduce vegetation density. These two images show city lights and Normalized
Difference Vegetation Index (NDVI) for the eastern United States. Bright areas of the lights image are more urbanized. Dark green regions of the
vegetation image represent dense vegetation, while brown indicates sparse vegetation. Between the two images, urban areas correspond to regions with sparse vegetation.
(Lights image data courtesy Marc Imhoff of NASA GSFC and Christopher Elvidge of NOAA NGDC. NDVI data courtesy Compton Tucker and Bob Mahoney of
Tracking the precise impact of urbanization on vegetation, however, is no simple task. Urbanization moves relatively fast and its outlines are often hard to discern. It’s also difficult to gauge what the urbanized areas were like before the cities were erected. Recently, a group of researchers at NASA’s Goddard Space Flight Center led by biologist and remote sensing specialist Marc Imhoff found a way to overcome these obstacles. Using satellite images of city lights at night, they constructed a map of the urbanized areas of the United States. They then retrieved vegetation density readings of present day American cities as well as simulated readings of the landscapes that pre-dated these cities. By combining the vegetation data with the urbanization maps, Imhoff was able to calculate the effects of urbanization on many types of ecosystems across the country.
Our Changing Landscape
“When considering global climate change, carbon is really at the heart of the issue,” says Imhoff. As the human population has grown and industrialized over the past 250 years, factories and farms have sent out a constant, ever-growing stream of methane, carbon dioxide, and other carbon-based greenhouse gases. These gases trap solar radiation and heat the atmosphere in much the same way that a windshield traps heat in a car on a sunny day. According to the Intergovernmental Panel on Climate Change (IPCC), the Earth’s average surface temperature has increased between 0.4°C (0.7°F) to 0.8°C (1.4°F) over the last hundred years largely due to these gases. Over the next hundred or so years, it is feared that temperatures could rise another 1.4°C (2.5°F) to 5.8°C (10°F), causing sea levels to rise, rainfall patterns to change, and ecosystems to shift (IPCC 2001).
One of the purely natural phenomena that actually reduces the amount of
greenhouse gases in the atmosphere is vegetation growth. Through the process of
photosynthesis, plants draw down more carbon dioxide than anything else. And
carbon dioxide is the most abundant greenhouse gas generated by humans.
“When the Northern Hemisphere greens in the spring or summer, the change
in carbon dioxide in the atmosphere is actually a measurable event,” says
Imhoff. Were plant density to increase dramatically over the next century,
global warming could be curbed dramatically. On the other hand, the wholesale
destruction of plants through clear cutting and burning could raise atmospheric
carbon dioxide levels above expectations.
||Carbon dioxide levels in the atmosphere have been rising steadily since industrialization began in the late 1700s. Data from Mauna Loa, on the island of Hawaii, show that the rate continues to increase. The sawtooth pattern in the monthly data is due to the large amount of vegetation in the Northern Hemisphere that consumes carbon dioxide each spring and releases it each fall. (Graph by Robert Simmon, based on data from the NOAA Climate Monitoring & Diagnostics Laboratory)|
So keeping track of the amount of vegetation around the Earth is crucial to estimating carbon dioxide levels and ultimately global climate change. Imhoff explains that urbanization has a significant impact on vegetation and the surrounding ecosystem. Though rural communities also alter vegetation appreciably, buildings are spread out and the local vegetation is largely left intact for long periods of time or replaced with fields of fertile crops. When people construct cities or even subdivisions, however, forests are clear cut, shrubs are removed, and much of the ground is paved. The only vegetation left standing afterwards is typically the standard urban fare of grass, loosely scattered trees, and hedgerows. The soil, which tends to be some of the most productive in a given region, is often severely degraded. Consequently, once an area has been urbanized, it is very difficult to bring the land back to its natural state.
Over the next century, urbanization is predicted to move at a breakneck pace. In fact, it’s estimated that worldwide the migration towards the cities has been moving at three times the rate of population growth. Only a third of the planet’s population lived in urban areas 10 years ago. Now it’s up to 50 percent and researchers believe that in 10 more years it will be up to two thirds. When you consider that the human population will grow from six billion to nearly 10 billion over the next 50 years, an enormous amount of land is likely to be urbanized in a relatively short time.
Most of the current flight towards urban areas is occurring in third world countries such as India and China. In developed countries such as the United States, the flight towards the cities occurred during the first half of the last century. But the increase in population and the unerring belief in a two-story, aluminum-sided American dream has led to further expansion. Between 1982 and 1992, 19,000 square miles of otherwise rural cropland and wilderness were urbanized in the United States (World Resources Institute 1996). This is the equivalent of covering half of Ohio with one big subdivision. All of this takes its toll on vegetation. “We are simply converting more and more of the most fertile land areas to a non-productive state by covering them with parking lots and buildings that spread out over a larger and larger area,” says Imhoff.
|Although the majority of the U. S. population lived in cities by 1950, the country continues to urbanize [yellow line (left axis)]. Worldwide, the percentage of people living in cities is expected to increase 15 to 20 percent from 2000 to 2030 (blue line (left axis)]. At the same time, global population levels are steadily climbing [black line (right axis)]. As a result, cities are continually growing and consuming green space. (Graph by Robert Simmon, based on data from the UN Population Information Network)|
Viewing Vegetation in a New Light
Several years ago, Imhoff and a group of researchers at NASA’s Goddard Space Flight Center set out to examine just how all this urbanization affects vegetation and, consequently, carbon dioxide levels. Their first obstacle lay in simply measuring urbanization’s extent. Cities and suburbs sprout up fast and their boundaries are uneven, often spreading over the land in a seemingly organic fashion like mold on fruit. Getting an accurate up-to-date map of just one urban area can be exceedingly difficult even with the help of planes or conventional remote sensing satellites such as Landsat.
Imhoff says he came across a solution when he discovered satellite images displaying the illumination that cities and towns generate at night. The images were taken by 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.
Employing computer algorithms and additional data, Imhoff figured out a way
to create maps of population density across an entire country or continent from
the images. “We
essentially scaled back on the brightness levels of the imaging data,”
says Imhoff. The first full map of urban areas he constructed was of the
United States. Using statistics taken from the U.S. Census Bureau, the Goddard
team was able to place all land area in the United States into three classes Ð
urban, peri-urban, and non-urban—and assign population densities to those
Urban regions, Imhoff explains, constitute any area that has 1, 000 people or more per square mile. These are regions where the ecosystem has been significantly transformed into a human-devised habitat filled with office buildings, housing developments, and strip malls. 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. Finally, non-urban regions, such as central Montana and western Maine, harbor only ten people or less per square mile.
Now that Imhoff and his team had the outlines of the urban and peri-urban areas, the next step was to calculate how urbanization has transformed the local vegetation. In short, he would have to compare the present day vegetation in urban areas to the vegetation that existed before the cities were built.
Measuring vegetation density for the present day cities was fairly
straightforward. For years scientists have gathered global readings of
vegetation using satellites. Remote sensing satellite instruments such as the
Advanced Very High Resolution Radiometer (AVHRR) aboard NOAA’s polar
satellites have been observing the Earth’s surface on a regular basis
since the 1970s. The instrument records light of different bands (colors)
reflected and emitted from the Earth, including red, green, and near infrared.
To extract the information about the amount and extent of vegetation from the
satellite imagery, scientists typically use what is known as the normalized
difference vegetation index (NDVI). NDVI is produced by calculating the
difference in reflectance between the visible and near-infrared light in a
satellite image. It is a relative measure of the vegetation density and an
indicator of how much photosynthesis is taking place on a given plot of ground.
To obtain current readings of urban vegetation density, Imhoff simply acquired
the most recent NDVI readings of the United States from NOAA. He then laid the
vegetation density data on top of his city lights map and retrieved the values
of vegetation density for regions designated as urban and peri-urban.
|The map at left shows the urban areas of Sacramento, California derived from city lights data and superimposed on census tracts. Each census tract has a similar total population, so smaller tracts correspond to denser populations. Light blue represents urban areas, dark blue peri-urban areas, and light grey rural areas. (Image courtesy Marc Imhoff, NASA GSFC)|
Obtaining vegetation density readings for these areas from before the cities were constructed presented a more complex problem. After all, many major cities in the United States were founded well before people ever took flight or even knew what photosynthesis meant. “We looked at what the NDVI is inside urban areas and compared it to what the NDVI is immediately outside urbanized areas in the same land cover class,” says Imhoff. The team consulted a map created by the United States Geological Survey (USGS) displaying natural ecosystems such as savanna, mixed forest, and shrub land across the United States. Since these maps were general in nature and ignored the presence of urban areas, the scientists could pinpoint the natural ecosystems that predated present day cities and suburbs. They then retrieved satellite-derived vegetation density readings in untouched areas around the city with the same ecosystem. In Miami, for instance, the USGS maps indicated that most urban areas lay on savanna. The team located relatively untouched savanna nearby and obtained NDVI values. They swapped these readings for the vegetation density values in their present day urban area maps to get a picture of what vegetation was like before the city was built.
|Imhoff and his colleagues used USGS land cover classification data to determine the type of vegetation that existed before a city was built. The change in productivity (which is proportional to the amount of carbon consumed by vegetation) due to urbanization differs depending on the original type of land cover. (Image by Robert Simmon, based on data from the USGS.)|
Urban Sprawl’s Impact on Vegetation
With these “before” and “after” data in hand, Imhoff
could then establish to what degree urbanization altered the natural vegetation
in the United States. “Three primary patterns kept coming up again and
again,” says Imhoff. He explains the most common pattern is best
represented in and around Chicago, which was built on a mixed forest type of
ecosystem. “Here urbanization elongates the growing season through urban
heating, but you have lower overall annual productivity,” says Imhoff. As
can be seen on any nightly newscast, temperatures are usually a degree or two
higher within urban areas. Consequently, urbanization causes vegetation to
sprout earlier in the spring and turn brown later in the fall. During the summer
months, however, urbanization lowers the total amount of plant growth
substantially. Imhoff points out that overall, urbanization has resulted in a
loss of a minimum of five full days of peak season plant growth a year in
Chicago. In other words, when a mixed forest ecosystem is completely urbanized,
plant growth is reduced by at least the equivalent of five full days a year.
But this was not the only pattern Imhoff and his team uncovered. In cities such as Miami, the native foliage is savanna, which unlike the mixed forest of Chicago is active and therefore green pretty much all year around. Here, the addition of urban heating was of no value and urbanized areas lowered the amount of plant growth all year long, resulting in the loss of 22 full days of peak season plant growth a year. And in cities such as Denver, you see the opposite effect. Denver is built on open shrub land that is sparsely vegetated. People have planted lawns and trees throughout the city and its surrounding suburbs, adding a substantial amount of greenery. For most of the year, the plant growth in the urbanized area is greater than it is in the rural dusty, tumbleweed-laden plains surrounding the city. So urbanization actually adds 11 days of productivity to Denver.
“These days many of the open spaces right outside the urban areas are farmland,” says Imhoff. “In many cases, they are right in the path of development.” So in addition to measuring the difference in vegetation between non-urban and urban areas, Imhoff also recorded the difference between urban and peri-urban areas, where most farmland can be found. The results were in some ways more disturbing. On average, when cities encroach on farmland, the area undergoes a loss of 10 days of peak season plant growth. So in areas such as Chicago, the loss in primary productivity is greater when urban areas replace farmland than when urban areas replace a mixed forest ecosystem.
|The change in plant growth due to urbanization differs based on the ecosystem surrounding a city. In Chicago the growing season is extended, but the maximimum vegetation density is lower than the original forest would be. In balmy Miami vegetation is less vigorous throughout the year. Denver, on the other hand, has slightly increased vegetation density. The graphs above show a year of NDVI values for urban vegetation as compared to the vegetation that was present before urbanization. To derive these charts, NDVI values were multiplied by the number of days in each month for each month of the year. Note the differing vertical scale. [Graphs adapted from Imhoff, Tucker, Lawrence, and Stutzer. Photographs copyright Philip Greenspun (Chicago), City of Miami (Miami), and Eric Anderson (Denver)]|
“The impacts on production are variable, but they’re generally negative. And the impacts may be disproportional to the area converted,” says Imhoff. “So while only three percent of the of the land area in the United States is urbanized, that three percent used to be the most productive soils we had.” Simply put, most of the cities in the United States are located on or near some of the most fertile soils where plant productivity is the highest. As urban areas expand onto peri-urban and non-urban areas, more of this productive soil will give way to development.
In the future, the loss of vegetation due to urban growth will likely accelerate and may affect carbon dioxide levels globally. Right now Imhoff and his colleagues are working on a way to calculate exactly how the reduced plant growth, caused by urbanization, affects carbon dioxide. “Our next step with carbon modeling is that we want to look at the impact of urbanization on photosynthesis in terms of grams of carbon fixed as plant organic matter per square meter of land over a period of months and years,” says Imhoff. With such data, those who construct climate models to forecast global warming and climate change could obtain more precise measurements on how much less carbon dioxide the land can absorb as our population and our cities expand.
|Chicago, Miami, and Denver are three of America’s largest metropolitan areas. These three scenes from the Enhanced Thematic Mapper plus aboard the Landsat 7 satellite give some sense of the scale of the cities—each images is 32.4 by 16.2 km (20.1 by 10.1 miles). (Images by Robert Simmon, based on data from the Landsat 7 science team, the Institute for Marine Remote Sensing, University of South Florida, and the Global Land Cover Facility, University of Maryland)|