RESOURCES INDEX | HOMEPAGE | CONTENTS | PDF | HELP | BACK | NEXT | TEACHERS' NOTES
 

Carbon Dioxide

Pupils' Information Sheet

What are the sources of carbon dioxide?

Carbon Dioxide (or CO2) is created naturally by animals' breathing (respiration) and by the decay of plant and animal matter. These processes are natural sources of carbon dioxide in the atmosphere and account for about 38% of all CO2 emissions (Figure 1). Another large natural source of CO2 includes the oceans.

Carbon dioxide is also released by the burning of fossil fuels (coal, oil and gas) for power and electricity, and the production of cement. These are anthropogenic or man-made sources of carbon dioxide. Although man-made emissions of CO2 are significant, they are much smaller than natural emissions.

Another important man-made source of carbon dioxide is deforestation. Trees and plants take in carbon dioxide through the process of photosynthesis, and store it as carbon in their tissues and wood fibre. In many regions of the world, forests are being destroyed to clear land for development. This allows the large amount of carbon in the wood to be released into the atmosphere as carbon dioxide.

Figure 1: Sources of carbon dioxide

What are the sinks of carbon dioxide?

A sink is a method by which a gas can be removed from the atmosphere.

Carbon dioxide has several sinks within the atmosphere, including trees and plants (mentioned earlier in the text). Hence the process of deforestation removes an important sink for carbon dioxide. In addition to this, if the wood is then burnt as in the 'slash and burn' technique the CO2 is released through the burning process.

Another sink for carbon dioxide is the ocean (Figure 2). (The ocean is both a source and a sink for CO2.) The ocean acts as a big reservoir holding CO2 within its watery depths. A molecule of carbon may stay in the ocean for anything up to 200 years.

Figure 2: Sinks of carbon dioxide

How much carbon dioxide is in the atmosphere?

Before the Industrial Revolution, the concentration of CO2 in the atmosphere had hardly changed over hundreds of years. This was because the amount of CO2 removed from the atmosphere by the CO2 sinks equalled the amount released to the atmosphere from the CO2 sources. Human activity has resulted in CO2 being released from sources faster than it can be absorbed by the sinks, so the concentration of CO2 in the atmosphere has increased.

Since 1957 a record of the amount of carbon dioxide present in the atmosphere has been kept at Mauna Loa in Hawaii (Figure 3). This record show a clear increase in the amount of carbon dioxide in the atmosphere of about 1.2 ppmv or 0.3% per year. The sawtooth shape is due to the annual fluctuation of CO2 concentration in the atmosphere arising from the annual growth cycle.

Figure 3: Atmospheric concentration of carbon dioxide in Hawaii

Both Table 1 and Figure 4 show the concentration of carbon dioxide in the atmosphere over the past 250 years.

Table 1: Concentrations of carbon dioxide in the atmosphere since 1740

Year

CO2 Concentration (ppmv)

1740

280

1760

280

1820

285

1850

290

1890

295

1815

300

1930

305

1950

310

1960

317

1965

315

1970

325

1975

330

1980

339

1985

350

1990

353

1995

359


Figure 4: Atmospheric concentration of carbon dioxide since 1750

How long does carbon dioxide stay in the atmosphere?

All gases stay in the atmosphere for a certain length of time before they are removed by their sinks. This time is known as the atmospheric lifetime of a gas. Carbon dioxide has an atmospheric lifetime of between 50 - 200 years. This means that carbon dioxide will be present in the atmosphere for at least 50 years before it is absorbed by a sink or becomes part of another chemical reaction. Consequently, carbon dioxide emitted into the atmosphere today could cause global warming for up to two centuries to come.

What can be done to reduce the amount of carbon dioxide in the atmosphere?

A group of scientists brought together by the United Nations, called the Intergovernmental Panel on Climate Change (IPCC), estimate that in order to stabilise carbon dioxide concentrations at present day levels a 60% reduction of global CO2 emissions would be needed.

The United Nations Framework Convention on Climate Change was signed by over 150 nations at the 1992 Earth Summit in Rio. The Convention says that all countries that sign the Treaty must produce a programme describing the steps they will take to limit the amount of greenhouse gases they produce. The programme must also state how they are going to protect sinks of carbon dioxide such as forests. The developed countries such as the UK, USA and Japan were committed to return their emissions of greenhouse gases to the levels that were emitted in 1990 by the year 2000. Developing countries were offered helped to produce their own programmes by countries like the USA, the UK and Japan.

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.

What can we do to reduce the amount of carbon dioxide that is being produced?

There are a number of steps that our families and we can take to reduce the amount of carbon dioxide being produced by being energy efficient at home. The average home creates 7.5 tonnes of carbon dioxide each year (the same weight as a full sized elephant).

  • Only use the heat, light and appliances you really need.
  • Ask your parents to lag the hot water tank.
  • Turn the thermostat of your central heating down by one or two degrees Celsius.
  • Buy energy saving lightbulbs.
  • Use public transport, walk or ride a bike to school rather than go in the car. (Did you know the average car emits more than its own weight of carbon dioxide each year.)
  • Plant trees at home and in your school grounds.
  • Don't buy over-packaged goods - recycle and reuse wherever possible.

Questions/further work

1. Using the table of CO2 concentrations found earlier in the text, plot a graph of year against carbon dioxide concentrations.

2. From the graph, can you estimate what the concentration of carbon dioxide was in the year 1880?

3. In what year was the concentration 345 ppmv?

4. In your opinion, what would be the best method of reducing carbon dioxide concentrations?


EXPERIMENT

Introduction

Carbon dioxide is an important gas in our atmosphere and is one of the main greenhouse gases. Carbon dioxide is naturally given out by people and animals when they breathe, and is also given out when coal, oil and gas are burnt. It is possible to make and test for CO2 and this is what we are going to do in this experiment.

Aims

  • To make and collect carbon dioxide (CO2).
  • To test for CO2

Equipment

  • Balloon Funnel
  • Baking Powder
  • Soft Drink Bottle
  • Vinegar
  • Straw
  • Test tube
  • Limewater (a solution of calcium hydroxide)
  • Teaspoon
  • Measuring Cylinder

Method

Making and Collecting Carbon Dioxide

Blow the balloon up and let it down two or three times.

Place the mouth of the balloon over the end of the funnel and place about 3 teaspoons of baking powder into the funnel.

Measure out 2cm3 of vinegar in the measuring cylinder; pour this into the drinks bottle.

Carefully attach the balloon to the top of the bottle, trying not to spill the baking powder.

Tip the balloon up so that the baking powder falls into the vinegar.

THIS CAUSES A CHEMICAL REACTION BETWEEN THE VINEGAR AND THE BAKING POWDER, WHERE CO2 IS GIVEN OFF. YOU WILL NOW TRAP THE CO2 GAS.

Hold the balloon and the top of the bottle tightly to trap the gas, allow the balloon to fill up with CO2.


Testing for Carbon Dioxide

Pour some fresh limewater into a test tube.

Carefully place a straw in the mouth of the balloon, and place the other end in the limewater. Slowly allow the gas to bubble through the limewater.

THE CARBON DIOXIDE REACTS WITH THE LIMEWATER, AND CHANGES ITS COLOUR. IF THE LIMEWATER IS NOW MILKY, CO2 WAS THE GAS IN THE BALLOON.


Conclusion

There are many ways in which carbon dioxide is created, both natural and man-made. Carbon dioxide is already present in the atmosphere, but over the past few centuries the amount that is there has been increasing. If the amount of carbon dioxide in the atmosphere continues to increase, the Earth will start to warm up too quickly, with possibly damaging consequences for Earth's ecosystems.

Further Investigations

Can you think of ways that CO2 is being taken out of the atmosphere naturally?

Can you make a list of ways in which the amount of carbon dioxide being released into the atmosphere (by man) can be reduced?