Human Impact on the Biosphere

Human activity damages the biosphere. Exponential population growth, destruction of habitats for agriculture and mining, pollution from industry and transportation, and many other activities all contribute to the damage of the environment. Some of the destructive consequences of human activity are summarized as follows:

1. Greenhouse effect. The burning of fossil fuels and forests increases CO2 in the atmosphere. Increases in CO2 cause more heat to be trapped in the earth’s atmosphere. As a result, global temperatures are rising. Warmer temperatures could raise sea levels (by melting more ice) and decrease agriculture output (by affecting weather patterns).

2. Ozone depletion. The ozone layer forms in the upper atmosphere when UV radiation reacts with oxygen (O2) to form ozone (O3). The ozone absorbs UV radiation and thus prevents it from reaching the surface of the earth where it would damage the DNA of plants and animals. Various air pollutants, such as chlorofluorocarbons (CFCs), enter the upper atmosphere and break down ozone molecules. CFCs have been used as refrigerants, as propellants in aerosol sprays, and in the manufacture of plastic foams. When ozone breaks down, the ozone layer thins, allowing UV radiation to penetrate and reach the surface of the earth. Areas of major ozone thinning, called ozone holes, appear regularly over Antarctica, the Arctic, and northern Eurasia.

3. Acid rain. The burning of fossil fuels (such as coal) and other industrial processes release into the air pollutants that contain sulfur dioxide and nitrogen dioxide. When these substances react with water vapor, they produce sulfuric acid and nitric acid. When these acids return to the surface of the earth (with rain or snow), they kill plants and animals in lakes and rivers and on land.

4. Desertification. Overgrazing of grasslands that border deserts transform the grasslands into deserts. As a result, agricultural output decreases, or habitats available to native species are lost.

5. Deforestation. Clear-cutting of forests causes erosion, flooding, and changes in weather patterns. The slash-and-burn method of clearing tropical rain forests for agriculture increases atmospheric CO2, which contributes to the greenhouse effect. Because most of the nutrients in a tropical rain forest are stored in the vegetation, burning the forest destroys the nutrients. As a result, the soil of some rain forests can support agriculture for only one or two years.

6. Pollution. Air pollution, water pollution, and land pollution contaminate the materials essential to life. Many pollutants do not readily degrade and remain in the environment for decades. Some toxins, such as the pesticide DDT, concentrate in plants and animals. As one organism eats another, the toxin becomes more and more concentrated, a process called biological magnification. Other pollution occurs in subtle ways. A lake, for example, can be polluted with runoff fertilizer or sewage. Abundant nutrients, especially phosphates, stimulate algal blooms, or massive growths of algae and other phytoplankton. The phytoplankton reduce oxygen supplies at night when they respire. In addition, when the algae eventually die, their bodies are consumed by detrivorous bacteria, whose growth further depletes the oxygen. The result is massive oxygen starvation for many animals, including fish and invertebrates. In the end, the lake fills with carcasses of dead animals and plants. The process of nutrient enrichment in lakes and the subsequent increase in biomass is called eutrophication. When the process occurs naturally, growth rates are slow and balanced. But with the influence of humans, the accelerated process often leads to the death of fish and the growth of anaerobic bacteria that produce foul-smelling gases.

7. Reduction in species diversity. As a result of human activities, especially the destruction of tropical rain forests and other habitats, plants and animals are apparently becoming extinct at a faster rate than the planet has ever previously experienced. If they were to survive, many of the disappearing plants could become useful to humans as medicines, foods, or industrial products.