The carbon cycle
Walk through a forest in County Wicklow and you are walking through a carbon store. The leaves, wood and soil are all built from carbon that was once carbon dioxide in the air. Burn a litre of diesel on the way home and some of that carbon goes straight back to the air. Where does carbon go, and what happens when we change how fast it moves?
Carbon is stored in reservoirs
Carbon is found in every living thing, in the air, in the sea and in rocks. A place where carbon is stored is called a reservoir (or store). The same carbon atoms are recycled again and again. This is the carbon cycle.
| Reservoir | Form the carbon is in |
|---|---|
| Atmosphere | carbon dioxide gas, $\mathrm{CO_2}$ (about 0.04% of air) |
| Living things (plants, animals) | carbon compounds such as sugars, proteins, fats |
| Oceans | dissolved carbon dioxide and carbonates (in shells) |
| Soil and dead matter, peat | dead plant and animal material |
| Fossil fuels (coal, oil, natural gas) | hydrocarbons, compounds of carbon and hydrogen |
| Rocks | carbonates such as limestone (calcium carbonate, $\mathrm{CaCO_3}$) |
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.
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.
Processes that move carbon
Arrows in the carbon cycle are processes. You need to name them.
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Photosynthesis: plants take $\mathrm{CO_2}$ from the air and make sugars. This removes carbon from the atmosphere.
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Feeding: animals get carbon by eating plants (or other animals).
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Respiration: plants, animals and microbes release $\mathrm{CO_2}$ when they break down food for energy.
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Decomposition: bacteria and fungi break down dead matter and release $\mathrm{CO_2}$ by respiration.
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Combustion: burning wood, turf, petrol, diesel or gas releases $\mathrm{CO_2}$.
Name the process by which each of the following removes or adds carbon dioxide to the atmosphere: a) Green plants in a forest in Wicklow during the day. b) A car engine…
Name the process by which each of the following removes or adds carbon dioxide to the atmosphere:
- a) Green plants in a forest in Wicklow during the day.
- b) A car engine burning petrol.
- c) Bacteria decomposing a dead tree.
- d) A turf fire in a home.
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…
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.
Fossil fuels and peat: long-term stores
Coal, oil and natural gas are fossil fuels. They formed from the remains of plants and tiny sea organisms that died millions of years ago. The remains were buried and, under heat and pressure over millions of years, changed into fuel. Their carbon was locked away from the atmosphere.
Peat (turf) is younger. In a waterlogged bog there is very little oxygen, so decomposers cannot work well. Dead plants build up as peat instead of rotting away, and the carbon stays stored.
Burning a fossil fuel is combustion of a carbon compound. For methane in natural gas:
$$\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$$The carbon atom in $\mathrm{CH_4}$ ends up in $\mathrm{CO_2}$. It has moved from a long-term store underground to the atmosphere.
The carbon cycle is linked to the weather and climate. Water evaporates, forms clouds and falls as rain, and this water cycle helps plants grow and take in $\mathrm{CO_2}$. In the atmosphere, $\mathrm{CO_2}$ is one of the gases that causes the natural greenhouse effect, which keeps Earth warm enough for life.
Fossil fuels take millions of years to form but we burn them in a few hundred years. They are used up far faster than they form, so they are non-renewable.
a) Name three fossil fuels. b) State how fossil fuels formed. c) State the gas released when fossil fuels are burned that adds carbon to the atmosphere.
- a) Name three fossil fuels.
- b) State how fossil fuels formed.
- 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. a) Explain why decomposition is very…
A peatland in County Offaly stores carbon in dead plant material that has not fully decomposed because the ground is waterlogged.
- a) Explain why decomposition is very slow in waterlogged conditions.
- b) Explain what happens to the stored carbon if the bog is drained and the peat is dried and burned.
- c) Explain why a healthy peatland is described as a carbon sink.
The balanced equation for the complete combustion of methane in natural gas is: $$\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$$ a) Explain how this equation shows that…
The balanced equation for the complete combustion of methane in natural gas is:
$$\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$$- a) Explain how this equation shows that burning a fossil fuel transfers carbon from a long-term store to the atmosphere.
- 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.
- 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.
Human activity and the balance of the cycle
Before humans, the carbon cycle was roughly balanced: the carbon taken in each year was about equal to the carbon released. Now two main activities add extra $\mathrm{CO_2}$:
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Burning fossil fuels (power stations, cars, home heating) moves carbon from a long-term store into the air.
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Deforestation removes trees, which are a sink, and burning or rotting the wood releases $\mathrm{CO_2}$.
Extra $\mathrm{CO_2}$ strengthens the greenhouse effect and warms the planet. The oceans absorb about a quarter of our emissions. Dissolved $\mathrm{CO_2}$ makes the water slightly more acidic, which makes it harder for shellfish and corals to build their shells, because shells are made of calcium carbonate.
There are many ways to reduce $\mathrm{CO_2}$ emissions: use less fuel (drive less, share lifts, use a bus or the DART), switch to electric vehicles or renewable energy such as wind, insulate buildings so less fuel is burned, and leave peatlands undrained.
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. a) Calculate the mass of carbon dioxide…
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.
- a) Calculate the mass of carbon dioxide released in a year.
- 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.
- c) Give two other ways, apart from planting trees, in which the farmer could reduce carbon dioxide emissions.
Over the last 150 years the concentration of carbon dioxide in the atmosphere has risen from about $280$ ppm to over $420$ ppm. a) Calculate the percentage increase in…
Over the last 150 years the concentration of carbon dioxide in the atmosphere has risen from about $280$ ppm to over $420$ ppm.
- a) Calculate the percentage increase in concentration.
- b) Explain two human activities that have contributed to this increase, in terms of the carbon cycle.
- 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." a)…
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."
- a) Evaluate this statement, giving two arguments in support of forests as carbon sinks.
- b) Give two reasons why forests alone cannot completely solve the problem.
- c) Explain what happens to the carbon in the trees if the timber is burned as fuel rather than used to build houses.
Back to the start: the carbon in a Wicklow forest and in a litre of diesel is the same carbon in different stores. Photosynthesis stores it; burning, respiration and decay release it.