Physical Geography, Environment, and Global Change
Physical geography is concerned with the natural world around us—in short, with the human environment. Because natural processes are constantly active, the Earth's environments are constantly changing. Sometimes the changes are slow and subtle, as when crustal plates move over geologic time to create continents and ocean basins. At other times, the changes are rapid, as when hurricane winds flatten vast areas of forests or even tracts of houses and homes.
Environmental change is now produced not only by the natural processes that have acted on our planet for millions of years but also by human activity. The human race has populated our planet so thoroughly that few places remain free of some form of human impact. Global change, then, involves not only natural processes, but also human processes that interact with them. Physical geography is the key to understanding this interaction.
Environment and global change are sufficiently important that we have set off these topics by placing them in special sections identified with Eye on Global Change that open each chapter. What are some of the important topics in global change that lie within physical geography? Let's examine a few.
GLOBAL CLIMATE CHANGE
Are human activities changing global climate? It seems that almost every year we hear that it has been the hottest year, or one of the hottest years, on record. But climate is notoriously variable. Could such a string of hot years be part of the normal variation? This is the key question facing scientists studying global climate change. Over the past decade, nearly all scientists have come to the opinion that human activity has, indeed, begun to change our climate. How has this happened?
The answer lies in the greenhouse effect. As human activities continue to release gases that block heat radiation from leaving the Earth, the greenhouse effect intensifies. The most prominent of these gases is CO2, which is released by fossil fuel burning. Others include methane (CH4), nitrous oxide (NO), and the chlorofluorocarbons that until recently served as coolants in refrigeration and air conditioning systems and as aerosol spray propellants. Taken with other gases, they act to raise the Earth's surface temperature, with consequences including dislocation of agricultural areas, rise in sea level, and increased frequency of extreme weather events, such as severe storms or record droughts.
Climate change is a recurring theme throughout this book, ranging from the urban heat island effect that tends to raise city temperatures to the El Nino phenomenon that alters global atmospheric and ocean circulation, to the effect of clouds on global warming, and to rising sea level due to the expansion of sea water with increasing temperature.
THE CARBON CYCLE
One way to reduce human impact on the greenhouse effect is to slow the release of CO2 from fossil fuel burning. But since modern civilization depends on the energy of fossil fuels to carry out almost every task, reducing fossil fuel consumption to stabilize the increasing concentration of CO2 in the atmosphere is not easy. However, some natural processes reduce atmospheric CO2. Plants withdraw CO2 from the atmosphere by taking it up in photosynthesis to construct plant tissues, such as cell walls and wood. In addition, CO2 is soluble in sea water. These two important pathways, by which carbon flows from the atmosphere to lands and oceans, are part of the carbon cycle. Biogeographers and ecologists are now focusing in detail on the global carbon cycle in order to better understand the pathways and magnitudes of carbon flow. They hope that this understanding will suggest alternative actions that can reduce the rate of CO2 buildup without penalizing economic growth. The processes of the carbon cycle are described in Chapter 8.
Among scientists, environmentalists, and the public, there is a growing awareness that the diversity in the plant and animal forms harbored by our planet—the Earth's biodiversity—is an immensely valuable resource that will be cherished by future generations. One important reason for preserving as many natural species as possible is that, over time, species have evolved natural biochemical defense mechanisms against diseases and predators. These defense mechanisms involve bioactive compounds that can sometimes be very useful, ranging from natural pesticides that increase crop yields to medicines that fight human cancer.
Another important reason for maintaining biodiversity is that complex ecosystems with many species tend to be more stable and to respond better to environmental change. If human activities inadvertently reduce biodiversity significantly, there is a greater risk of unexpected and unintended human effects on natural environments. Biogeographers focus on both the existing biodiversity of the Earth's many natural habitats and the processes that create and maintain biodiversity. These topics are treated in Chapters 8 and 9.
Human activity is reducing the biodiversity of many of the Earth's natural habitats. Environmental pollution degrades habitat quality for humans as well as other species. Extreme weather events, which will become more frequent with human-induced climate change, as well as other rare natural events, are increasingly destructive to our expanding human population.
As we all know, unchecked human activity can degrade environmental quality. In addition to releasing CO2, fuel burning can yield gases that are hazardous to health, especially when they react to form such toxic compounds as ozone and nitric acid in photochemical smog. Water pollution from fertilizer runoff, toxic wastes of industrial production, and acid mine drainage can severely degrade water quality. Such degradation impacts not only the ecosystems of streams and rivers, but also the human populations that depend on rivers and streams as sources of water supply. Ground water reservoirs can also be polluted or turn salty in coastal zones when drawn down excessively.
Environmental pollution, its causes, its effects, and the technologies used to reduce pollution, form a subject that is broad in its own right. As a text in physical geography that emphasizes the natural processes of the Earth's land surface, we touch on air and water pollution in several chapters—Chapter 4 for air pollution and Chapter 14 for surface water pollution, irrigation effects, and ground water contamination.
Catastrophic events—floods, fires, hurricanes, earthquakes, and the like—can have great and long-lasting impacts on both human and natural systems. Are human activities increasing the frequency of these extreme events? As our planet warms in response to changes in the greenhouse effect, global climate modelers predict that weather extremes will become more severe and more frequent. Droughts and consequent wildfires and crop failures will happen more often, as will spells of rain and flood runoff. In the last decade, we have seen numerous examples of extreme weather events, from Hurricane Katrina in 2005—the most costly storm in U.S. history—to the Southeast drought of 2007, which devastated crops in large parts of the southeastern United States. Is human activity responsible for the increased occurrence of these extreme events? Significant evidence now points in that direction.
Other extreme events, such as earthquakes, volcanic eruptions, and seismic sea waves (wrongly called tidal waves), are produced by forces deep within the Earth that are not affected by human activity. But as the human population continues to expand and comes to rely increasingly on a technological infrastructure ranging from skyscrapers to the Internet, we are becoming more sensitive to damage and disruption of these systems by extreme events.
This text describes many types of extreme events and their causes. In Chapters 4 and 6, we discuss thunderstorms, tornadoes, cyclonic storms, and hurricanes. Droughts in the African Sahel are presented in Chapter 7. Earthquakes, volcanic eruptions, and seismic sea waves are covered in Chapter 12. Floods are described in Chapter 14.