The Use of fossil fuels

Coal and oil are formed from once-living things primarily plants.

Waterlogging of soil gives rise to anaerobic conditions

If the remains of dead organisms settle in such anaerobic conditions they can, over millions of years, be converted to fossil fuels.

The formation of coal involves the conversion of the biomass to, first, peat (which can itself be used as a fuel) and then, as a result of compression by successive layers of sedimentary rock, to the rock that is coal.

Oil is formed in a similar way but is typically formed from marine (sea) biomass and, during the fossilization process, there is less compression so the end result is a liquid found in reservoirs within sedimentary rock.

The rate at which coal and oil are made is extremely slow (being measured in millions of years) but we are currently using these resources at an extremely fast rate

Result: there will come a point when the reserves of fossil fuels will be exhausted - this is currently believed to be sometime in the next century

• oil is converted to petrol for use in internal combustion engines (as found in cars)
• Coal is combusted in the generation of electricity

The combustion of oil and coal produces huge quantities of carbon dioxide, sulphur dioxide and nitrogen oxides.

• Carbon dioxide (which is released in the combustion of coal and oil) is a greenhouse gas and thus contributes to the greenhouse effect
• Sulphur dioxide is produced primarily by the combustion of coal
• Nitrogen oxides are produced mainly in the combustion of petrol in internal combustion engines

Sulphur dioxide + water --> sulphuric acids (H₂SO₄)

Nitrogen monoxide OR nitrogen dioxide + water --> nitric acid (HNO₃)

These acids precipitate back to the earth as acid rain

So carbon dioxide, sulphur dioxide and the oxides of nitrogen can be considered to be atmospheric pollutants

The Greenhouse Effect

The Natural Sequence of Events

Radiation from the sun passes through the upper atmosphere on its way to the earth

When it reaches the planet some of the sun's energy provides warmth

Some of the energy is reflected back through the atmosphere.

However when the radiation reflects off the earth its wavelength changes (it becomes longer)

Some of the gases found in the atmosphere are less transparent to this long wavelength light so absorb it

This absorbed energy radiates back to the earth warming it further.

This process is natural and necessary

Man's influence

The production of extra quantities of carbon dioxide (as a result of burning fossil fuels which, were they not actively extracted, would be hidden away) over the natural production corresponding to respiration and "accidental" combustion is of significance because carbon dioxide is a greenhouse gas.

Greenhouse gases (carbon dioxide, methane, CFCs etc) are added to the gas layer in the atmosphere increasing the degree of absorption of the reflected solar energy. Hence there is an increased warming of the earth - the Greenhouse Effect leading to global warming

One significant result of global warming will be the melting of the polar icecaps. These ice sheets contain vast quantities of trapped water and, should they melt, would result in a raising of the sea level. This would result in the flooding of low-lying land.

Falling acid rain dissolves the waxy cuticle on leaves resulting in increased transpiration and loss of nutrients

When sulphuric or nitric acid enter soil the reduction in pH causes the loss of positive ions from the soil (leaching).

These ions include calcium which is an important nutrient in plants (it is a component of cell walls) and several toxic heavy metals: including aluminium, lead and mercury

The consequences are:

Plants lacking calcium are unable to correctly synthesise cell walls so do not thrive

Lead and mercury are non-active site-directed inhibitors of enzymes so are harmful to plants and animals including decomposer organisms so there is reducing cycling of nutrients

Aluminium has specific effects such as damaging the gills of young fish

The effects of untreated (raw) sewage

Sewage is the waste flowing from homes and industries

It contains:

• urine
• faeces
• detergents
• other chemicals

Ideally sewage should be routed to a treatment works where filtration and the action of bacteria remove the harmful chemicals to produce a clean effluent that can be returned to the water system. (Note that the water you drink has been treated in this way - mmm, yummy)

However in some instances sewage is not treated and this untreated waste is allowed to flow directly into the waterways (this is a particular problem in the developing world but can also happen in developed countries)

The problems associated with raw sewage:

• faeces may contain pathogenic bacteria and viruses (for example those associated with gastro-enteritis but also the bacterium that causes cholera and the virus that causes poliomyelitis). If the untreated faeces are ingested the recipient can be infected
• industry produces toxic substances (such as heavy metals) which may have a boilogical effect (toxic metals are non-active site directed inhibitors of enzymes)
• Female urine may contain oestrogen. This can affect fish by altering their hormone levels - a process that can result in feminisation (the development of female characteristics in males) among fish. A similar process has been posited as an explanation for the falling sperm counts among human males
• Where the contents of sewage are nutrients these provide food for bacteria or algae resulting in high growth rates for these organisms. This addition of nutrients is known as eutrophication and leads to other problems described here
• Where the sewage contains high quantities of suspended particles these will act to block out light in the river. Plants in the river will be prevented from photosynthesizing and will thus die. This provides more food for bacteria - increasing the negative effects of eutrophication

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