Exercises: The carbon cycle

Name the process by which each of the following removes or adds carbon dioxide to the atmosphere:

1. a) Green plants in a forest in Wicklow during the day.

1. b) A car engine burning petrol.

1. c) Bacteria decomposing a dead tree.

1. d) A turf fire in a home.

Name four places (reservoirs) where carbon is stored in the carbon cycle, for example the atmosphere. For each, state the form in which the carbon is found.

1. a) Name three fossil fuels.

1. b) State how fossil fuels formed.

1. c) State the gas released when fossil fuels are burned that adds carbon to the atmosphere.

A peatland in County Offaly stores carbon in dead plant material that has not fully decomposed because the ground is waterlogged.

1. a) Explain why decomposition is very slow in waterlogged conditions.

1. b) Explain what happens to the stored carbon if the bog is drained and the peat is dried and burned.

1. c) Explain why a healthy peatland is described as a carbon sink.

A farmer in Tipperary burns $2,000$ litres of diesel in a year. Each litre of diesel releases about $2.7$ kg of carbon dioxide.

1. a) Calculate the mass of carbon dioxide released in a year.

1. b) One tree absorbs about $22$ kg of carbon dioxide per year. Calculate how many trees would be needed to absorb the farmer's emissions.

1. c) Give two other ways, apart from planting trees, in which the farmer could reduce carbon dioxide emissions.

Describe the path of a carbon atom that starts as carbon dioxide in the air, is taken in by a grass plant in a Kerry field, is eaten by a cow, and later returns to the air. Name each process involved, and include at least one route that returns carbon to the air by a different process.

Over the last 150 years the concentration of carbon dioxide in the atmosphere has risen from about $280$ ppm to over $420$ ppm.

1. a) Calculate the percentage increase in concentration.

1. b) Explain two human activities that have contributed to this increase, in terms of the carbon cycle.

1. c) The oceans absorb a large amount of carbon dioxide. Explain one consequence of this for ocean water and for marine organisms with shells.

A forest of fast-growing trees is planted in Co. Leitrim. A student states: "Planting forests completely solves the problem of carbon dioxide in the atmosphere."

1. a) Evaluate this statement, giving two arguments in support of forests as carbon sinks.

1. b) Give two reasons why forests alone cannot completely solve the problem.

1. c) Explain what happens to the carbon in the trees if the timber is burned as fuel rather than used to build houses.

The balanced equation for the complete combustion of methane in natural gas is:

1. a) Explain how this equation shows that burning a fossil fuel transfers carbon from a long-term store to the atmosphere.

1. b) The equation shows that each methane molecule ($\mathrm{CH_4}$) produces one carbon dioxide molecule ($\mathrm{CO_2}$). A $\mathrm{CH_4}$ molecule has a relative mass of $16$ and a $\mathrm{CO_2}$ molecule has a relative mass of $44$. Use these values to work out the mass of carbon dioxide produced when $16$ g of methane burns completely.

1. c) Compare the time scale on which fossil fuels form with the time scale on which they are burned, and explain why this makes them a non-renewable source of energy.