6. Explain the cause and
effects of ozone depletion, global warming and acid rain. State the role of
individual in overcoming these problems.
Ozone is a highly reactive gas comprising triatomic
oxygen. It is formed by the combination of oxygen in the presence of
ultraviolet radiation in the atmosphere. It exists as a natural component of
the atmosphere and is kept in a constant concentration of approximately 0.01
parts per million. The ozone layer is an important protective layer in the upper
atmosphere. It provides a thermal blanket for the Earth, and protects life by
filtering out harmful ultraviolet radiation from the sun.
Ozone depletion refers to a lowered concentration of ozone in the upper
atmosphere. Evidence has been found that the ozone layer can be damaged by air
pollutants, leading to the formation of ‘holes’. Today, scientists have
discovered holes in the ozone layer over the
Recent studies show that the ozone hole has been formed due to chemical effects. Since ozone is formed and broken down continuously in natural processes, the rate of breakdown can be speeded up by the presence of ozone depleting pollutants such as hair mousses and household cleaning products; and also as solvents for cleaning electronic circuit boards and computer components.
The destruction of the ozone layer
results in a significant increase in ultraviolet radiation reaching the Earth’s
surface. It may induce various adverse effects on human:
1. Ultraviolet radiation has direct fects
on DNA. It can modify the genetic information in body cells.
2. An increase in the incidence of skin cancer, cataract
and lung disease, as well as a reduction in the function of the immune system
is probably due to the increase in ultraviolet radiation reaching the Earth.
3. An increase in exposure to ultraviolet radiation can
greatly reduce the yields of crops.
4. Aquatic life, especially phytoplankton, is very
sensitive to ultraviolet light. A reduction of its productivity in the ocean
could disrupt the ecological balance of food webs. It is because phytoplankton
id the producers in aquatic habitats.
5. An increase in ultraviolet radiation leads to an
increase in the incidence of photochemical smog.
Global warming refers to the gradual
increase in the average temperature in the atmosphere as a result if the accumulation
of greenhouse gases. The increase in concentration of greenhouse gases in the
atmosphere slows down the hest loss from the Earth to the Space. As a result,
the atmosphere is warming up. This leads to changes in climate and other
subsequent effects on the Earth.
The greenhouse gases includes carbon dioxide, chlorofluoropcarbons, methane, nitrous oxides, ozone, water vapour, etc. some of them are natural components of the atmosphere, while some are released into the atmosphere as a result of human activities. They play a key role in determining the climate. Different greenhouse gases are present in different concentration in the atmosphere. They have different warm potential, i.e. the efficiency in retaining heat. Hence different greenhouse gases have different contributions to the greenhouse effect.
Although the warm potential of carbon
dioxide is quite low compared with the other greenhouse gases, it is regarded
as the most significant greenhouse gas responsible for global warming. The
reasons are that it exists in a relatively high concentration in the atmosphere
and that it can stay for relatively long period of time in the atmosphere.
Greenhouse gases are generated by
various human activities. Some of them are listed below:
1. Tremendous amounts of carbon dioxide are releases into
the atmosphere through burning of fossil fuels on motor vehicles, power
stations and factories. The consumption of fossil fuels has been increasing
since the industrial revolution. Today, the growth in human population,
improvement of living standards and rapid industrial development are further
increasing the demands for energy. A subsequent increase in release of carbon
dioxide to the atmosphere is resulted.
2. Deforestation also accounts for the increase on
atmospheric carbon dioxide content. The decrease in the quantity of trees
reduces the rate of removal of carbon dioxide in the air by photosynthesis.
Timber may be used as fuel wood and burnt, or may be used to produce paper,
furniture, etc. which are eventually incinerated or decomposed naturally. As a
result, the large quantity of carbon stored in the trees will combine with
oxygen and enter the atmosphere as carbon dioxide.
3. The extensive use of cooling agents, blowing agents,
insulating and packing materials, aerosol sprays and solvents leads to the
release of a large amount of chlorofluorocarbon compounds into the atmosphere.
4. Extensive rearing of livestock is also a cause of
global warming. A significant quantity of methane is released into the
atmosphere from the manure of livestock in some countries.
A warmer global climate could have a number of possible effects:
1. Thermal expansion of the oceans, melting of glacial
ice, and melting of ice caps at Artic and Antarctic would elevate sea levels.
Flooding would probably occur in coastal regions where about one-third of the
human population and their economic infrastructure are concentrated. Low-lying
cities, industrial areas and agricultural lands would be submerged.
2. Most of the major rainforests in the world are
low-lying and at the margin of water level. The global climate would be
affected if they are removed due to submergence.
3. The distribution of the climatic regions and ocean
currents would be changed, leading to alterations of agricultural regions.
4. Weather extremes are also expected to increase in
frequency and severity. Some places would have increments in rainfall and some
would have decrements. These would result in flooding or drought in various
areas. Damaging hurricanes, typhoons and violent storms would also increase in
intensity and occur more frequently.
5. There would be increase in the average temperature of
marine and freshwater bodies. These would affect cold-water fish and marine
mammals, and would lead to undesirable competition for habitats and food
sources.
6. Global warming would also affect biodiversity. Some
species would probably become extinct, particular those that only survive in
narrow temperature ranges and those confined to small habitats. Ecosystems such
as polar seas, coral reefs and mountains are considered to be more vulnerable
to changes in climate.
7. A warmer climate could alter disease pattern in
unpredictable ways and favour the spread of
pathogenic microorganisms. These would increase the spread of epidemics that
affect humans and other organisms.
8. Pests could become more active and grow faster.
Consequently, damages of crops and spread of plant diseases would be faster,
resulting in food shortage and famines.
Acid rain refer to precipitation in the form of dilute acid solution. It is not a simple or a single phenomenon; its formation involves a sequence of chemical reactions and it occurs in many different places globally. Acid rain is formed by the reaction of various industrial pollutants, such as sulphur dioxide and nitrogen oxides with naturally occurring oxygen and water vapour, forming acid solution. These acid solutions precipitate as rain or snow. Normally, rain water is slightly acidic owing to the presence of carbon dioxide in the air. Acid rain typically has a pH from 4.0 to 5.4. In some parts of the world, rain water with a pH as low as 2.5 has been recorded, i.e. the acidity is about 1,600 times that of normal precipitation.
The major processes of the formation
of acid rain are as follows:
1. Sulphur dioxide is the primary cause of acid rain. It
is mainly released from the burning of fossil fuels in power stations, heavy
industries and motor vehicles. In the atmosphere, SO2 reacts with
oxygen in the air. Within about 43 days, it will be converted into sulphur
trioxide. During this period, SO3 can be transported for a
considerable distance by wind and then dissolved in rain water to from a dilute
solution of sulphuric acid. Apart from forming H2SO4, SO2
can directly dissolves in rain water to form sulphurous
acid.
2. Nitrogen oxides are another cause of acid rain. The
burning of fossil fuels in motor vehicles and power stations are mainly
responsible for the release of nitrogen oxides into the atmosphere. Once
nitrogen monoxide is formed, it will combine with atmospheric oxygen to form
nitrogen dioxide. Following a series of chemical reactions, NO2
reacts with water and oxygen to form nitric acid.
Acid rain is usually regarded as a regional problem rather than a global problem. It is because the acidic components remain in the atmosphere only for a short time period. However, acid rain is a serious problem in many places. The harmful effects are as follows:
1. Acid rain causes damage to buildings, statues and
monuments. Marble and various metals are corroded by acid rain.
2. Acid rain causes human respiratory diseases such as
bronchitis and asthma.
3. Acid rain can damage tree foliage directly and weaken
the trees so that they become more susceptible to diseases, insects and
drought.
4. Acid rain increases the acidity of soil. Soil has a
natural capacity to neutralize some inputs of acids. This normal soil-buffering
capacity will be depleted in the presence of acid rain. When the soil is below
4.5, most valuable nutrients are rapidly lost and bacterial activities will be
greatly reduced. This affects the soil fertility and in turn, affects the
growth of forest trees and crops.
5. Acid rain can induce the release of aluminium ions from soil particles. Once released, the
water-soluble aluminium ions may reach a
concentration that is poisonous to plants. This can damage the roots of trees.
Once the aluminium ions are washed into the water
bodies, they can also kill aquatic organisms.
6. Acid rain can acidify lakes. With an increased
acidity, moderately toxic inorganic mercury compounds in lake-bottom sediment
are converted into methylmercury, which is highly
toxic and can accumulate in the fatty tissues of animals. The compound can
reach high concentrations in food chains and food webs.