bio

The human population is becoming more demanding especially with the continuous increase in the human population size. As humans use energy and resources from ecosystems, they deposit large amounts of wastes into the environment; thus, they are threatening the stability of ecosystems by causing pollution on land, in water, and in the atmosphere. Pollution can be defined as the addition of harmful materials or pollutants to the environment (e.g., air, water, land). Even some materials that are not usually harmful can cause pollution, if they build up in the environment. In this first section, we will discuss the causes and effects of air pollution.

Air pollution results from the addition of pollutants to the air. The burning of fossil fuels is a major source of air pollution, as it results in the release of toxic emissions into the atmosphere. Most motor vehicles, factories, and power plants burn fossil fuels for energy (Figure 14.1). Common air pollutants that result from burning fossil fuels

include sulfur dioxide, oxides of nitrogen, oxides of carbon, unburned hydrocarbons, and suspended particles. Other common air pollutants include chlorofluorocarbons (CFCs) that are often used in refrigerators and aerosol cans.

Sulfur dioxide gas (SO,) is mainly released into the atmosphere when fossil fuels are burned. This gas gives rise to sulfuric acid when it mixes with rainwater in the atmosphere. The burning of fossil fuels in factories, power stations, and vehicles also releases nitrogen oxides (NO,) into the atmosphere.

These compounds can also dissolve in rainwater to form nitric and nitrous acids. Sulfuric acid, nitric acids,

and nitrous acids are key components of acid rain-rain whose pH levels have been lowered by the presence of dissolved acids (e.g., sulfuric acid). This form of acidic precipitation can damage buildings and harm plants by reducing their rate of growth and affecting their leaves; acid rain washes out important nutrients from the soil

rendering plants in short supply of them. Acid rain also renders the water of lakes and rivers acidic, which threatens the survival of aquatic organisms and the stability of aquatic ecosystems. In addition, certain substances that are normally found in the mud of lakes, such as aluminum salts, have a higher solubility in acidic solutions than in basic or neutral solutions. Thus, these substances can get dissolved in the water rendered acidic by acid rain and harm aquatic living organisms (e.g., some fish can be killed by elevated aluminum levels in the water).

The burning of fossil fuels can also produce microscopic liquid or solid particles (such as dust) that can become suspended in the atmosphere.

These suspended pollutant particles can cause a number of respiratory diseases in humans.

Carbon monoxide is a pollutant that is generally released

when

carbon-containing

compounds

burn in the presence of low amounts of oxygen.

When

humans

inhale

carbon

monoxide,

the

carbon monoxide molecules bind to hemoglobin molecules in red blood cells. The carbon monoxide molecules take the place of the oxygen molecules that normally bind to hemoglobin. As a result, the red blood cells become unable to transport enough oxygen to the body's cells. Thus, in elevated concentrations, carbon monoxide poisoning can be fatal; it is particularly dangerous to people with heart disease or anemia (a condition characterized by low numbers of red blood cells or low amounts of hemoglobin).

Smog is a form of air pollution made of a combination of fog and smoke. Photochemical smog is a thick,

brownish mist formed when

some gases,

such as unburned hydrocarbons

(i.e.,

hydrocarbons

emitted by the incomplete

combustion of petroleum in engines) and nitrogen oxides,

react in

the presence of sunlight in a

photochemical reaction. Photochemical smog is mostly observed in heavily populated big cities with large numbers of motor vehicles. When the level of photochemical smog is high, it may cause eye and throat irritation, breathing difficulties, and numerous other side effects, such as increased risk for birth defects and premature death?

The Greenhouse Effect and Global Warming

Several gases present in Earth's atmosphere help keep the planet warm enough for life processes to occur normally. These gases allow most of the shortwave radiation from the sun to pass freely through the Earth's atmosphere, with only a small portion being reflected back into space.

Some of the radiation that reaches from the sun to the Earth's surface is also reflected back into space, and the remaining radiation is absorbed by Earth's surface. The absorbed radiation is then reemitted into the atmosphere as longwave radiation (i.e., infrared radiation).

The blanket of gases found in the atmosphere does not allow radiation of long wavelengths to pass as freely as the shortwave radiation. Thus, only a small portion of the reemitted longwave radiation passes into space; the rest is absorbed by gases in the atmosphere and reflected back into Earth's surface to warm Earth some more (Figure 14.4).

The trapping of heat near Earth's surface is referred to as the greenhouse effect, because the process is similar to what occurs in greenhouses used to trap heat for growing plants; the glass of greenhouses allows shortwave radiation to pass freely into the greenhouse and traps longwave radiation inside the greenhouse, warming its environment. The greenhouse effect is a phenomenon in which

certain gases in the atmosphere of the planet, such as carbon dioxide, methane, and chlorofluorocarbons (CFCs), trap some radiation on Earth. Gases that trap some radiation in this way are called greenhouse gases. These gases allow visible light energy to pass through the atmosphere but do not allow all of the longwave radiation to escape. Because long-wavelength radiation (infrared radiation) cannot escape into space, it remains in the atmosphere and keeps the planet warm.

The greenhouse effect is needed to keep Earth warm.

However,

if too much heat energy is

trapped on Earth, the planet becomes warmer, an effect known as the enhanced greenhouse effect.

The enhanced

greenhouse effect results from

an increase in the levels of greenhouse gases in the atmosphere. A consequence of the enhanced greenhouse effect is global warming, the gradual rise in Earth's average temperature.

In recent years, the levels of carbon dioxide gas in the atmosphere have increased, primarily due to the increase in the burning of fossil fuels in motor vehicles and in industry. Cutting down trees, which take in carbon dioxide for photosynthesis, can also be considered as a cause of the increase in atmospheric levels of carbon dioxide.

The levels of methane gas in the atmosphere have also been on the rise. Methane is naturally released during digestion in cattle and insects; thus, the increase in atmospheric methane could be due to the increase in cattle raising to meet the food demands of the growing human population.

Methane can also be released into the atmosphere as a result of accidental natural gas leaks. Methane is also produced as a result of the decay of organic wastes by microorganisms, which commonly

occurs in landfills; hence, an increase in the amount of dumped organic wastes can also lead to a rise in atmospheric methane.

CFCs also contribute to the enhanced greenhouse effect.

CFCs are used as refrigerants in air conditioners and refrigerators and as propellants in aerosol cans; CFCs are also used to expand plastic foam (Figure 14.5). CFCs are very stable and persist in the air for a long time. In addition to their roles as greenhouse gases, CFCs can damage the ozone layer—a layer of the upper atmosphere that is rich in ozone gas; this layer protects Earth from ultraviolet radiation emitted from the sun, which can cause skin cancer, among other side effects. Certain alternatives to CFCs, called ozone-friendly chemicals, may help reduce the damaging effects of these gases.

Global warming may result in huge climate changes in different parts of the world. It is difficult for scientists to predict the exact changes that will take place in each region of the world as a result of global warming. However, it is highly likely that changes in global temperature may affect the levels of precipitation in many regions. Some regions may experience an increase in rainfall, while other regions may become dry deserts. This may affect the stability of ecosystems since numerous organisms may become less adapted to their environments and, thus, become threatened with death and, in some cases, extinction. These factors, in turn, may cause food shortages because major components of food chains will disappear.

A rise in global temperature, up to a certain point, is likely to cause the melting of polar ice caps, leading to a rise in sea levels (Figure 14.6); higher sea levels will flood cities and other areas that lie near sea level.

Limiting global warming and climate change can be achieved by controlling the emissions of greenhouse gases into the atmosphere. The ways by which these emissions can be controlled will be discussed in detail later in this chapter.

Nuclear Pollution

lonizing radiation (gamma rays, X-rays, beta particles, alpha particles) is a high-energy radiation that can cause a number of harmful side effects, such as an increase in the rate of DNA mutations, which may in turn lead to cancer and birth defects.

The exposure to a high dose of radiation can also lead to immediate symptoms such as radiation sickness (including nausea, vomiting, and hair loss) and burns.

Nuclear power plants use ionizing radiation

emitted by radioactive materials to generate heat and electricity (Figure 14.7). If they are not properly designed and maintained, nuclear power plants can release radioactive materials into the air, water, and other parts of the environment. Organisms living in affected ecosystems may ingest food contaminated with radioactive materials or drink contaminated water and, thus, become affected by ionizing radiation emitted by the radioactive materials. Affected organisms can pass radioactivity along food chains and food webs when they are eaten by other organisms in different trophic levels, harming the whole ecosystem at different levels.

Nuclear power plants must follow strict safety regulations to limit the release of radiation or other harmful substances into the environment.

However, nuclear power plant accidents can cause the release of radioactive material into the environment. In 1986, such an accident occurred at the Chernobyl nuclear power station. The Chernobyl accident released radioactive particles into the air and produced radioactive smoke as a result of a fire that burned for nearly two weeks.

Winds carried these radioactive materials across Europe. Over time, the particles fell to the ground (fallout); in some areas, the particles fell in the form of rain and turned the rain black. Areas closest to Chernobyl were the most seriously affected.

Table 14.1 summarizes the major classes of air pollutants along with their main sources and some of their effects on ecosystems.