Earth science for schools by Moorland School

The Earth's Atmosphere

The atmosphere is a thin layer of gas which surrounds the Earth. This picture shows the two most important layers known as the troposphere and the stratosphere. The air gets thinner and thinner the higher you go, 90% of all the molecules in the atmosphere are in the troposphere. Air is a mixture of various gases, information on the uses of some of these gases can be found here.

The present composition of the atmosphere is:
21% OXYGEN (O2)
78% NITROGEN (N2)
0.04% CARBON DIOXIDE (CO2)
~0.9% ARGON (Ar)

Besides water vapour, several other gases are also present in much smaller amounts:

  • Carbon monoxide (formula CO)
  • Neon (Ne)
  • Oxides of nitrogen
  • Methane (CH4)
  • Krypton (Kr)

Concentrations of these gases are measured in parts per million (ppm)

The atmosphere has changed a lot compared to the Earth's early atmosphere, but for the last billion years it has remained pretty constant. We now need to look at 3 very different atmospheric problems:

1) The Greenhouse effect

The earth is surrounded by a blanket of gases. This blanket traps energy in the atmosphere, much the same way as glass traps heat inside a greenhouse. This results in an build up of energy, and the overall warming of the atmosphere. The greenhouse effect is a natural process which made life on Earth possible. Without naturally occurring greenhouse gases such as water vapour, carbon dioxide, methane and nitrous oxide, the Earth's surface temperature would be 33C cooler, a chilly -18C rather than the tolerable 15C.

When we talk about the greenhouse effect we mean the ENHANCED effect which is caused by the increase of greenhouse gases from human sources. Since the beginning of industrialization, 200 years ago, concentrations of these gases have increased . It is estimated that the Earth's average temperature has risen by 0.6C since 1880 because of emissions of greenhouse gases from human activity.

The main sources of these emissions, particularly carbon dioxide, methane and nitrous oxide, are:

  • the combustion of large amounts of fossil fuels (producing CO2)
  • deforestation (less trees mean that less CO2 is being mopped up)

A increase in global temperatures may seem great, you might even think of 'Costa del Blackpool'. Unfortunately global warming will probably result in big swings in weather patterns across the world. Summers will become dryer and hotter, Winters will be wetter and colder. Other things will start to happen:

  • Thermal expansion of the water and melting of continental glaciers would cause sea levels to rise, possibly as much as two feet, by the end of next century.
  • Rising temperatures could lead to changes in regional wind systems which would influence global rainfall distribution and lead to the redistribution and frequency of floods, droughts and forest fires.
  • Increased sea temperatures would cause the destruction of coral reefs around the world.
  • Climate change would create favourable conditions for growth in insect populations. This would likely have a bad effect on agriculture and human health and result in a spread of malaria and other tropical diseases.
  • Water supplies would become disrupted and droughts would be more common

There is a lot of controversy surrounding global warming, views range from those who believe that there is nothing to worry about to those who believe that the world is heading for a global catastrophe. An edited version of a Greenpeace article on Global warning, climate change and the greenhouse effect can be found (here). Other sources of information can be found on the links page on this site

2) Damage to the ozone layer

Ozone is oxygen that contains molecules that have 3 oxygen atoms (O3). The molecule is triatomic instead of the usual O2 molecule which is diatomic. There is a layer of ozone high up in the atmosphere which shields the Earth from the sun's harmful UV rays, these rays can lead to an increase in skin cancer. The ozone is present in very small quantities but it is enough to absorb the UV rays preventing them reaching the surface.

Scientists began to investigate the ozone layer in the 1970's, it wasn't until the mid 1980's that alarm bells started to ring. Concentrations of ozone appeared to be dropping in certain areas of the world (the layer was starting to thin-out). The cause of this reduction was thought to be man-made. The images below highlight the Ozone depletion over the Southern Hemisphere 1980-1991:-

(image used by permission from the Centre for Atmosphere Science, Cambridge University, for more details click here)

In 1985 over 60 countries pledged to phased out a group of chemicals called CFC's. These very stable chemicals were once widely used in aerosols and refrigerators. It was thought that their release into the atmosphere produced chlorine radicals which reacted with O3 to produce O2. The emission of CFC's into the environment is now greatly reduced, unfortunately the damage has already been done and the CFC molecules, thanks to their stability, are still causing ozone depletion.

 

3) Acid rain

Rain water is naturally acidic due to carbon dioxide which partially reacts with water to give carbonic acid (H2O + CO2   ->  H2CO3). When we talk about acid rain we mean the ENHANCED effect which is caused by other gases released when fossil fuels are burnt. Two gases are the main culprits:

  • Sulphur dioxide - Fossil fuels often contain a lot of sulphur impurities which burn to give sulphur dioxide. The SO2 reacts with water in the atmosphere to from a weak solution of sulphuric acid.
  • Nitrogen oxides - Under normal conditions nitrogen and oxygen don't react together. At very high temperatures (in an engine) a small proportion of oxygen reacts with nitrogen to give nitrogen oxides. These oxides react with water in the atmosphere to from a weak solution of nitric acid acid.
The dilute acid falls to ground as acid rain which causes the following problems:
  • Lakes become acidic and plants and fishes die as a result
  • Tree growth is damaged, whole forests can die as a result
  • Acid rain attacks metal structures and also buildings made of limestone

One method of reducing the amount of SO2 that gets pumped into the atmosphere  is to remove the sulphur impurities from the fuel. Can you think of any others? ANSWER.

Don't get global warming, ozone depletion and acid rain confused. They are all different.

Back to the Earth Science zone     

 

• The Rock Cycle 
• Plate Tectonics
• Earth Structure
• Earth Origin
• Volcanoes
• Earth's Atmosphere
• Fossil Fuels
• Polymers
• home

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Clitheroe,
Lancashire
BB7 2AJ
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