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Greenhouse Effect

Pupils' Information Sheet

What is the atmosphere?

The atmosphere is a mixture of gases and particles which form a blanket around the Earth. The atmosphere contains just the right amount of oxygen to allow us to breathe; it protects us from the Sun's radiation and helps maintain the temperature of our planet at a level which is comfortable for living things.

The atmosphere stretches for over 500km up from the Earth's surface and is made up of several layers, shown in Figure 1 overleaf. The average composition of the atmosphere is approximately 78% nitrogen (N2) and 21% oxygen (O2). The other 1% consists mainly of argon and carbon dioxide with very small amounts of other gases such as methane, nitrous oxide and CFCs.

The Troposphere
is the lowest layer of the Earth's atmosphere and spans from the Earth's surface up to about 15km. About 80-90% of gases in the atmosphere are found within this layer. The temperature within the layer decreases the higher one climbs. All of the Earth's weather systems are contained within the troposphere.

Figure 1: The structure of the atmosphere

The Stratosphere
Supersonic aircraft, such as Concorde, fly in this level of the atmosphere. Unlike the troposphere, the temperature in the stratosphere increases with altitude, and there is little or no water vapour. The ozone layer is found in this layer of our atmosphere.

The Mesosphere
Many satellites orbit the Earth within this layer, and the Aurorae (the Northern and Southern Lights) also occur in this layer. The lowest temperatures in the whole of the atmosphere are in the mesosphere.

The Thermosphere
This layer absorbs energy from the Sun and temperatures within the layer can reach as high as 700C. There are very few gases or particles in this region of the atmosphere, and it is the last layer that space rockets have to travel through before they reach the near vacuum of space.

What is the natural greenhouse effect?

The greenhouse effect is a natural phenomenon which allows the Earth to be warm enough to support life. Without the greenhouse effect the average temperature of the earth would be -18 degrees Celsius (C), about the temperature at the North Pole!

The greenhouse gases in the atmosphere act in a similar way to panes of glass in a greenhouse (see Figure 2 below). Radiation from the Sun (consisting mainly of visible and ultraviolet (UV) radiation) can travel through glass into the greenhouse. When this radiation is absorbed by objects in the greenhouse, it is re-radiated as infrared (IR) radiation, or heat. This heat cannot escape through the glass, so the greenhouse warms up.

Figure 2: The greenhouse effect in a greenhouse

The greenhouse gases in the atmosphere produce a similar effect. The Sun's radiation passes through the atmosphere, is absorbed by the surface of the Earth, and is re-radiated as infrared radiation. This is then absorbed or reflected by the greenhouse gases, rather than escaping to space, therefore warming the atmosphere. Figure 3 below shows this effect.

Figure 3: The greenhouse effect of the Earth

What is the enhanced greenhouse effect?

Within the past few hundred years human activities have increased the concentrations of greenhouse gases in the atmosphere. Scientists believe that the addition of greenhouse gases from these activities has thrown the natural greenhouse effect out of balance, and that the atmosphere is trapping too much heat and causing the temperature of the Earth to rise. This is known as the enhanced greenhouse effect or global warming (Figure 4).

Figure 4: The enhanced greenhouse effect

What are the greenhouse gases?

Most of the gases (apart from ozone) which give rise to the natural greenhouse effect let through ultraviolet and visible radiation, but absorb infrared radiation. The main greenhouse gases are:

  • Water Vapour (H2O)
  • Carbon dioxide (CO2)
  • Methane (CH4)
  • Nitrous oxide (N2O)
  • Chlorofluorocarbons (CFCs)
  • Hydrochlorofluorocarbons (HCFCs)
  • Hydrofluorocarbons (HFCs)
  • Ozone (O3)

Greenhouse gases make up a very small percentage of the concentration of the gases in the atmosphere; nitrogen and oxygen make up 99%! Carbon dioxide, for example, makes up only 0.037% of the gases in the atmosphere.

Figure 5 shows how the various greenhouse gases have contributed to the enhanced greenhouse effect or global warming during the last 200 years.

Figure 5: Contribution of greenhouse gases to global warming

Where do greenhouse gases come from?

Water vapour is the most important greenhouse gas, and occurs naturally in the atmosphere because of evaporation from the oceans and by a process known as transpiration in plants. We have very little or no control over the amount of water in the atmosphere.

Carbon dioxide is the most important greenhouse gas which has man-made sources. Carbon dioxide is released by animals (including humans) when they breathe, by burning fossil fuels (coal, gas and oil) or wood, and through the cutting down of trees and plants which take in carbon dioxide.

Methane is produced naturally when vegetation is burnt, digested or allowed to rot without oxygen being present. Methane also comes from rice fields, grazing cattle, landfill sites full of household rubbish, and coal mines. 90% of rubbish in the UK is disposed of in landfill sites, making up 29% of total man-made methane emissions in the UK.

Nitrous oxide is produced naturally by the oceans. Man-made sources of nitrous oxide include the use of fertilisers on farms and the production of nylon.

CFCs do not exist naturally; they are man-made chemicals used in air conditioning, fridges and polystyrene foam. CFCs are also very powerful ozone depleters.

HCFCs and HFCs are also man-made chemicals; they are used as replacements for CFCs.

What is wrong with global warming?

Although global warming may mean warmer weather, it could also lead to many changes in our normal lives. Nobody is sure how severe the effects will be.

Sea levels may rise; as warmer weather would melt the polar ice caps, parts of the world may disappear, for example the Maldives, Bangladesh and even East Anglia!!

Water Supplies might be affected. In some areas there will be more rain, whilst other areas might experience drier weather and eventually droughts.

Agriculture could be affected. Warmer climates might mean longer growing seasons. On the other hand, crops might be affected by less water and an increase in the number of pests and parasites.

'Tropical' diseases might become a widespread problem in a warmer world.

What can be done to stop global warming?

There are many ways we, individually and as a family, can help stop global warming. All involve reducing our energy use, or energy used to make products which we buy, which reduces the amount of greenhouse gases released

  • Use less electricity gas and oil.
  • Turn off unnecessary lights.
  • Walk, cycle or catch the bus to school instead of going in the car.
  • Turn the thermostat of your central heating down by 1C. This can help you save up to 10% on your family's fuel bills.
  • Recycle your household rubbish; items made of glass, metal plastic or paper can all be recycled.
  • Do not buy products which use too much packaging.
  • Re-use plastic carrier bags when you go the supermarket.
  • Only buy wood and wood products grown in sustainable forests.
  • Become involved with a local environmental group.

Questions/further work

1. Can you think of other ways in which the individual may help reduce the emission of greenhouse gases into the atmosphere?

2. If you were taking a trip in a NASA space shuttle, describe the conditions and views you would experience as you travel through the Earth's atmosphere.

3. Can you list ten good and ten bad points about a warmer world?

4. If the Earth did not have a greenhouse effect, describe what life would be like on the surface. Which planet would the Earth be most like: Venus, Jupiter or Mars?

5. How do cars, buses and trains enhance the natural greenhouse effect?


EXPERIMENT

Introduction

The greenhouse effect is a very important natural phenomenon. Without the greenhouse effect life on Earth would be very different. The greenhouse gases trap heat energy allowing the Earth to keep warm. If they were not there, the temperature of the Earth would be about 30C colder than it is now.

Mankind's activities have led to an increase in the concentrations of greenhouse gases in the atmosphere, and if this continues it could have many effects on the Earth.

Aim

To make a 'greenhouse' and to test the effect it has on air temperature.

Equipment

  • One 2 litre clear plastic drinks bottle.
  • One nail.
  • Two thermometers.
  • One piece of black paper or card.
  • Blu-tac.

Method: Creating a Greenhouse

1) Carefully make a hole in the top of the drinks bottle with the nail. Place a piece of black paper in the bottle. This will help to absorb sunlight and re-radiate it as infra-red radiation.

2) Insert one of the thermometers into the hole so that the bulb of the thermometer is in the middle of the bottle. If necessary, secure the thermometer with blu-tac to ensure an airtight seal. Make sure both thermometers are at the same temperature.

3) Place the bottle in sunlight. The experiment will work best on a sunny day.

4) Put the second thermometer next to the bottle, make sure that it is receiving the same amount of sunlight as the bottle.

5) After 20 minutes read and record the temperatures on both the thermometers.

Results

Initial temperature of both thermometers: =   C

Final temperature of thermometer in bottle: =   C

Final temperature of exposed thermometer: =   C

Which thermometer recorded the highest temperature?

Why do you think this is?

Conclusion

The plastic drinks bottle works in the same way as a greenhouse; it allows sunlight to enter the bottle but it does not allow the heat radiation to pass back out of the bottle, causing the temperature within the bottle to rise.