Our Changing Landscape

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“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.
 

   

Graph of
Carbon Dioxide Levels Since 1958
 

  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.

 

Photo of New Housing
The houses, roads, and shopping malls of urban sprawl replace native vegetation. (Photograph copyright David Hanauer)

 

Graph of Global Population
 

 

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.

next Viewing Vegetation in a New Light
back Urbanization’s Aftermath

  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)
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