f. Greenhouse gases:
1) carbon dioxide (CO2), produced by fossil fuel and wood burning
2) nitrous oxide (NO2), produced by fertilizer use and released from decomposition of animals wastes
3) methane (CH4), produced by bacteria (especially in animal guts), in sediments, and in flooded rice paddies
4) chlorofluorocarbons (CFCs), in particular Freon (a refrigerant)
III. Acid precipitation (rain)
A. Rainfall pH
1. Pure water has a pH of 7; dissociation of H2O produces equal numbers of H+ and OH- ions
2. Atmospheric CO2 combines with water to produce a weak carbonic acid and an increased number of H+ ions
3. Rain normally has a pH of 5.6 rather than 7.0
4. However, near urban areas, rainfall pH is nearer 4.0 and some fog clouds drop to as acidic as 1.7!
5. Living vegetation and building limestone rapidly deteriorate under such "acid rains"
B. Sources
1. Coal and oil routinely burned by power plants emit sulfur dioxide (SO2) into the air
2. Kuwait oil has high sulfur content; the oil well fires released much sulfur dioxide (SO2) into the atmosphere
3. Automobile exhaust contributes nitrogen oxides to the air
4. Both sulfur dioxide and nitrogen oxides are converted to acids when they combine with water vapor in the air
5. Sulfur and nitrogen oxides are emitted in one locale; deposition occurs in another location across boundaries.
6. Acid deposition is responsible for the following:
a. killing lakes and forests in North America and Europe
b. killing small invertebrates and decomposers and threatening the ecosystem
c. reducing agricultural yields
d. causing extensive structural damage by corroding marble, metal, and stonework
e. degrading water supplies by leaching heavy metals from the soil into drinking-water supplies
f. statistically is implicated in increases in lung cancer and colon cancer
IV. Photochemical smog
A. Photochemical smog is air pollution that contains nitrogen oxides (NOx) and hydrocarbons (HC), which react in the presence of sunlight to produce ozone (O3) and peroxylacetyl nitrate (PAN)
B. Both NOx and hydrocarbons come from fossil fuel combustion, but additional hydrocarbons come from various other sources as well, including paint solvents and pesticides.
C. Breathing O3 affects respiratory and nervous systems, results in respiratory distress, headache, and exhaustion.
D. Ozone is damaging to plants, resulting in leaf mottling and reduced growth.
E. Carbon monoxide
1. Carbon monoxide (CO) is a gas that comes from burning of fossil fuels in the industrial regions
2. High levels of CO increase the formation of ozone (O3)
3. CO combines preferentially with hemoglobin and prevents hemoglobin from carrying oxygen
F. Thermal inversion
1. Normally, warm air near the ground rises and dissipates into the upper atmosphere
2. Air pollutants, including smog and soot, are sometimes trapped near earth due to a thermal inversion
3. Thermal inversion occurs when a layer of dense cold air is trapped under a layer of warm air
4. Areas around hills are susceptible because air stagnates, with little mixing
V. Ozone depletion
A. The ozone shield is a layer of ozone (O3) in the stratosphere, some 50 km above the earth's surface
B. It absorbs much of the ultraviolet (UV) radiation of the sun, preventing it from reaching the earth's surface
C. In the history of life, ozone shield permitted life on land to exist; aquatic organisms are shielded by water
D. Hazards of UV radiation include increased mutation rate, which can lead to skin cancer and cataracts; depression of immune system; impaired crop and tree growth; and death of plankton
E. It became apparent in the 1980s that some depletion of worldwide ozone had occurred.
F. By the 1990s, there was an annual loss of 40-50% of ozone above Antarctica
G. U.N. Environment Program predicts a 26 percent rise in cataracts and nonmelanoma skin cancers for every 10 percent drop in ozone; this is 1.75 million cases of cataracts and 300,000 more cases of skin cancer every year.
H. The cause of ozone depletion can be traced to the release of chlorine (Cl-) atoms in the stratosphere
1. Chlorine atoms combine with ozone and strip away oxygen atoms one by one
2. One atom of chlorine destroys up to 100,000 molecules of ozone before settling to earth's surface
3. Chlorine atoms come from breakdown of CFCs, which were in heavy human use from 1950 to 1990
4. Freon is a common CFC used in refrigerators and air conditioner
5. Other CFCs occurred in cleaning agents, production of plastics, and as spray can propellants
6. Most countries have agreed to stop use of CFCs by 2000; CFCs in atmosphere will remain for a century
VI. Water
A. Pollution of Surface Waters
1. Pollution contributes to the shortage of fresh water
2. Solid wastes include household trash, sewage sludge, agricultural residue, mining refuse, and industrial waste
B. Sewage
1. Sewage treatment plants degrade organic wastes, which would otherwise cause oxygen depletion in lakes
2. Human feces contains pathogens that cause cholera, typhoid fever, and dysentery; these kill many children
C. Pollution of oceans
1. The oceans are the final recipients of wastes deposited in rivers and along the coasts
2. Waste dumping occurs at sea, and ocean currents sometimes transport both trash and pollutants back to shore
3. Solid pollutants cause death of birds, fish, and marine mammals that mistake them for food and get entangled
4. Offshore mining and shipping add pollutants to the oceans
a. Five million metric tons of oil a year, over one gram per 100 square meters of ocean surface, ends up in oceans
b. Large oil spills kill plankton, fish larvae, and shellfishes, as well as birds and marine mammals
c. The Exxon Valdez oil spill in Alaska's Prince William Sound leaked 44 million liters of crude oil
d. In Gulf War, 120 million liters were released from damaged onshore storage tanks in the Persian Gulf
e. Petroleum is biodegradable; takes a long time because low-nutrient content does not support bacteria
5. Some species of fish are in dramatic decline from combined effects of pollution and overfishing
VII. Soil erosion, desertification, and deforestation
A. City populations increase
1. In 1950, 20 percent of the world population lived in cities; by 2000 it is predicted that 60 percent will live in cities
2. As new housing is built around new cities, it takes over agricultural land
B. Soil erosion and desertification
1. In agricultural areas, wind and rain carry away about 25 billion tons of top soil yearly, worldwide
2. At such a rate, it is estimated that practically all top soil will be lost by the middle of the next century
3. Soil erosion causes a loss of productivity; it is compensated for by fertilizers, pesticides, and fossil fuel energy
4. Desertification is transformation of marginal lands to desert conditions due to overgrazing and overfarming
a. This is most evident along the southern edge of the Sahara Desert in Africa
b. Over 240,000 square miles of once-productive grazing land has become desert in the last fifty years
c. A similar process can occur if U.S. rangeland is overgrazed
C. Deforestation
1. In Canada, vast stands of trees are scheduled to be felled for paper, wood products and particleboard
2. Tropical rain forests are more biologically diverse than temperate forests
3. One million species are in danger of disappearing in 20 years due to deforestation
4. Lost species that have never been studied may have been sources of food or medicine
5. Logging in tropical forests meets the demand for furniture and also the desire of local people to farm the land
6. Slash-and-burn agriculture also contributes to the destruction of tropical rain forests
a. The ashes provide temporary nutrients to raise crops
b. After a few years, the fertility of the land is insufficient to raise crops and farmers move on
c. Cattle ranching usually takes over from farming
VIII. Conclusion
A. As previously noted, human population growth has largely followed the J-shaped growth curve. Our intelligence, tool making abilities and capacity for harnessing energy sources has so far enabled us to move into new habitats and increase the carrying capacity of the earth.
B. However, we can only postpone the inevitable for so long. Sometime in the future population growth will exceed the carrying capacity of the earth, as predicted by Malthus over 200 years ago.
C. Eventually our J-shaped growth curve will begin to change into the S-shaped curve. This will be done either voluntarily or nature will do it for us.
D. Recall that in the process of succession some organisms modify their environment so much that they can no longer live in it. If humans exceed the "self cleansing" capabilities of the earth, it is possible that we may become just another transitional community, replaced by other organisms.