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Greenhouse Effect

Teachers' Information Sheet

Introduction

The composition of the Earth's atmosphere is undergoing an unprecedented change, largely as a result of human activities. Industrial development, fossil fuel burning, deforestation and agricultural practices have led to an increase in the atmospheric concentration of gases such as carbon dioxide and methane, the gases that are responsible for the greenhouse effect. The increase in concentration of these gases could have far reaching consequences. Scientific estimates indicate a rise in the global mean temperature of between 1.4 and 5.8C over the next 100 years, and it is obvious that a temperature rise of this magnitude, over such a relatively short space of time, would have major impacts.

This lesson aims to introduce pupils to the concepts of the greenhouse effect and global warming, and can be used as an introduction to the issues, preceding the more comprehensive discussions contained in lessons 2 to 5. Alternatively, it can be used as a 'one off' lesson to provide a brief overview of the subject.

The lesson format is based on a series of questions:

  • What is the atmosphere?
  • What is the natural greenhouse effect?
  • What is the enhanced greenhouse effect?
  • What are the greenhouse gases?
  • Where do greenhouse gases come from?
  • What is wrong with global warming?
  • What can be done to stop global warming?

What is the atmosphere?

This section introduces the structure and importance of the atmosphere. It includes a diagram of the structure of the atmosphere.

The atmosphere is a 500 km thick layer, composed of a mixture of gases, which protects the planet from the Sun's harmful radiation, allows us to breathe and helps maintain the temperature of the planet.

The atmosphere has several different layers. The troposphere is the lowest layer of the atmosphere, and extends to an altitude of between 10 - 20 km from the Earth's surface. It is within the troposphere that clouds and weather systems occur. Approximately 80 - 90% of the mass of the atmosphere is in the troposphere. The temperature decreases with height in the troposphere, until a minimum temperature is reached at the tropopause (the boundary between the troposphere and the stratosphere).

The stratosphere reaches from the top of the troposphere to about 50 km above the surface of the Earth. Concorde and other supersonic aircraft fly within the lower part of the stratosphere. The ozone layer is also found in the stratosphere. It protects us from the Sun's ultra-violet (UV) radiation. The ozone layer is located between 15-35 km above the Earth's surface. Unlike the troposphere, the temperature within the stratosphere increases with altitude, primarily as a result of the absorption of UV radiation by ozone.

The mesosphere is the layer of the atmosphere between 50-80 km above the Earth's surface. It is within this layer that satellites orbit the Earth. Temperatures within the mesosphere decrease with altitude, to a minimum of about -80C at the boundary with the thermosphere.

The thermosphere is the upper layer of the atmosphere between 80-500 km above the surface of the Earth. Temperatures within this layer can reach as high as 700C, and all particles within the layer are either positively or negatively charged. It is this ionisation of atmospheric gases that leads to the phenomena of the Aurora Borealis and Aurora Australis. The ionosphere and magnetosphere are parts of the thermosphere.

What is the natural greenhouse effect?

This section describes how the greenhouse effect is a natural phenomenon. It also explains how it is similar to the effect the heating of a greenhouse on a sunny day. The experiment associated with this section illustrates, in simple terms, the effect of a 'greenhouse' on air temperature. A copy of the experiment is included with this lesson in the pupils' notes.

The greenhouse effect is a natural occurrence that allows the Earth to maintain a temperature suitable for supporting life. The greenhouse gases within the atmosphere behave in a way similar to a pane of glass in a greenhouse.

In a greenhouse, shortwave (visible and UV) radiation passes into the greenhouse through the glass. This shortwave radiation is absorbed and re-radiated by objects in the greenhouse as longwave (IR) radiation, or heat. However, most longwave radiation is reflected by the glass. This traps the heat inside the greenhouse and causes the temperature within the greenhouse to rise. Similarly, shortwave radiation can pass through the Earth's atmosphere easily. This radiation is then absorbed by the Earth's surface, and re-radiated as longwave radiation. The greenhouse gases in the atmosphere absorb or reflect some of the reflected longwave radiation, preventing it from escaping back into space. The effect of this is to increase the temperature of the lower atmosphere.

Without the greenhouse effect the temperature of the Earth would be approximately -18C, about the temperature at the North Pole.

What is the enhanced greenhouse effect?

This section briefly introduces the concept of an anthropogenically induced climate change, i.e. one that is changed by mankind. It is important to emphasise the fact that both the natural and man-made greenhouse effects involve the same process, but that the anthropogenic greenhouse effect is an unnecessary enhancement of the natural effect.

Over the past few hundred years, human activity has led to large amounts of greenhouse gases being released to the atmosphere. Scientists believe that the resulting increase in the atmospheric concentrations of these gases has caused an increase in the natural greenhouse effect. This means that the atmosphere is trapping too much heat, leading to an increase in the temperature of the Earth. The enhanced greenhouse effect is therefore that part of the greenhouse effect that can be attributed to anthropogenic emissions of greenhouse gases. It is this enhanced greenhouse effect which may be causing global warming and climatic change.

What are the greenhouse gases?

This section explains what the greenhouse gases are, and how most of them have both natural and man-made sources. A list of the main greenhouse gases is included.

The gases which create the greenhouse effect by absorbing radiation in the atmosphere are called the greenhouse gases. They are generally transparent to shortwave radiation, but opaque to longwave radiation. The greenhouse gases are produced through both natural and anthropogenic or man-made processes. Greenhouse gases comprise a very small percentage of the atmosphere, which consists mainly of nitrogen and oxygen. Water vapour is the most important greenhouse gas, and usually makes up between 1 - 4% of the atmosphere. Carbon dioxide makes up only 0.037% of the atmosphere. The main greenhouse gases are:

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

Where do the greenhouse gases come from?

This section discusses the natural and man-made sources of the major greenhouse gases in more detail.

Water vapour is the gas responsible for most of the natural greenhouse effect. Its concentration in the atmosphere is almost entirely due to evaporation and transpiration from the surface of the Earth. Hence, its atmospheric concentration cannot be controlled by human intervention.

Carbon dioxide is the most important greenhouse gas with significant man-made sources. It is produced naturally through respiration of plants and animals, the decay of plant and animal matter and forest fires. There are also many man-made sources of CO2. The burning of the fossil fuels: oil, coal and natural gas emits large quantities of carbon dioxide into the air every year. Deforestation and other land-use changes are further sources of increased emissions. Trees and plants absorb carbon dioxide into the tissues and wood fibre through the process of photosynthesis. In recent years, large areas of land have been cleared of trees to make way for agricultural systems and housing. This means that the carbon dioxide that was stored within the trees is released into the atmosphere. Other man-made sources of CO2 include biomass burning and the manufacture of cement (lesson two discusses the role of CO2 in more detail).

Methane is formed naturally in wetland regions through the anaerobic decay of organic material. Man-made emissions of methane include the cultivation of rice, biomass burning, coal mining and landfill sites. The emission of methane from landfill sites is an increasing problem and is the result of anaerobic decay of domestic waste. 90% of the UK's domestic refuse is put into landfill sites, this accounts for 29% of the total man-made methane emissions within the UK.

Nitrous oxide is naturally produced by the oceans and rainforests. Man-made sources of nitrous oxide include: the production of nylon and nitric acid; agricultural practices (especially the use of artificial fertilisers) and cars with three-way catalytic converters.

Chlorofluorocarbons (CFCs) do not exist naturally; they are man-made compounds containing chlorine, fluorine and carbon. CFCs have been responsible for about a quarter of the enhanced greenhouse effect. Production of CFCs began in the 1930s when they were developed as refrigerants. After the Second World War many other uses for CFCs were discovered including propellants in aerosols, blowing agents in foam rubber and takeaway food cartons, and in air conditioning systems. CFCs are however very powerful greenhouse gases and cause considerable damage to the ozone layer. Consequently, CFCs are being phased out under the Montreal Protocol (more information on the Protocol can be found in lesson three: Ozone).

Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are also totally man-made compounds; they were developed in the 1980s as a replacement for CFCs. HCFCs are scheduled to be phased out by the year 2029, but there are currently no restrictions on HFCs.

Ozone occurs mainly in the stratosphere, where it does not contribute to the greenhouse effect. However, it is also present in very low concentrations in the troposphere, where it does contribute to global warming. Detailed information can be found in lesson three: Ozone.

What is wrong with global warming?

This section outlines the potential impacts of climate change and discusses how they are likely to affect various aspects of our environment.

Sustained global warming could have many adverse effects on the climatic systems of the Earth, resulting in severe changes to the Earth's surface. In many parts of the world warmer temperatures are expected. Scientists have estimated that a rise in global mean temperature of between 1.4 and 5.8C may be experienced by 2100. The largest rises in temperature are expected to occur in the polar regions, which could have serious consequences for the rest of the planet. If the temperature within the polar regions were to rise, the ice sheets that make up these continents would start to melt, resulting in a raising of sea level. This would cause many problems. Low-lying areas of the world might become submerged, including some parts of the UK such as the Norfolk Broads; whole populations might have to move away from these areas, leading to increased pressure on land and resources elsewhere.

Water supplies would be affected; some areas may experience more rain, whilst other areas would experience a drier climate which could lead to food shortage, drought and possibly desertification of the land.

Agriculture and farming would be severely affected by temperature and other climatic changes, though no-one is sure whether there would be a net change in the availability of food resources. On the positive side, higher temperatures would mean longer growing seasons in temperate regions; increased concentrations of carbon dioxide could mean faster growth for some crop species. On the negative side, there could be a reduction in soil moisture or the availability of water for irrigation; agricultural pests, currently limited in their distribution by temperature, could thrive in warmer environments.

Disease could also be a problem. Many of the 'tropical' diseases associated with hot climates, such as malaria, may affect a larger area of the world if average global temperatures increase.

What can be done to stop global warming?

This section discusses the international response to the threat of climate change, and the options open for reducing anthropogenic greenhouse gas emissions including the international agreement on climate change signed at the Rio Summit.

The response of international leaders to the issue of global warming has been positive and very encouraging. At the 1992 Earth Summit in Rio over 160 Governments signed the Framework Convention on Climate Change. The Convention requires all countries that ratify the treaty to produce a national programme containing measures that will limit the amount of greenhouse gases produced within their national boundaries. It was agreed that developed countries should aim to return their emissions to the 1990 levels by the year 2000. In 1997, the Kyoto Protocol called for developed nations to reduce emissions of greenhouse gases by 5% by 2012. This Protocol will become legally binding when enough countries have ratified it.

The largest contribution to the enhanced greenhouse effect is from the production and consumption of energy, principally the emission of CO2 from the combustion of fossil fuels. One of the most effective ways to stop global warming is therefore to use energy sources which either release less CO2 (such as gas-fired power stations), or none at all (such as renewables or nuclear power). These options are not always feasible in the short term: for example, there is as yet no serious alternative to the petrol or diesel driven car. Another attractive option is to produce and use energy more efficiently. This is the main strategy of the UK Government for reducing CO2 emissions.

CFCs are currently being phased out under the terms of the Montreal Protocol. Though their replacements (HCFCs and HFCs) are also greenhouse gases, they stay in the atmosphere for a shorter length of time, so contribute less to the greenhouse effect over a long timescale (but not over a short timescale, i.e. 10-20 years).

Deforestation, particularly the destruction of tropical rainforests, makes a significant contribution to CO2 emissions. Sustainable forestry, where new trees are planted as older ones are felled, can help to mitigate this problem.

Questions and sample solutions

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

Sample solution: Recycling, reduce car use, energy efficiency, buying organic products.

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.

Solution: Pupils should use the description of the various layers of the atmosphere to formulate their answers.

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

Sample solution: Good: UK would be sunnier and warmer, a warmer world would need less energy for heating buildings, colder countries would be able to grow a larger range of crops, solar energy would be more prevalent.

Bad: Dry regions would become even drier, sea levels will rise and flood low-lying areas, erratic weather conditions would increase, droughts would increase, heat related diseases would become more prevalent, there would be more cases of sunburn.

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

Solution: Earth would have an average global temp of -18C which would mean the surface would be covered with ice and the majority of the seas would be frozen. A description of an ice age era would be relevant. Earth would most resemble Mars.

5. How do cars, buses and trains add to the greenhouse effect?

Solution: The burning of fossil fuels, either through petrol and diesel adds to the greenhouse effect. Electric trains do not emit greenhouse gases but they run on electricity that may have been produced at a coal, gas or oil-powered power station.


Glossary

Anaerobic: in the absence of oxygen.

Anthropogenic: man-made; resulting from human activity.

Aurora: Lights in the atmosphere seen radiating from regions of the poles.

Biomass: organic matter, generally excludes fossil fuels.

Radiation: transmission of heat, light, etc. from a body without the need for a medium (such as gas or liquid).