Catch it and lock it away: carbon sequestration takes carbon dioxide out of the atmosphere, or stops it getting there, and stores it as a solid or a liquid.
The points to remember
- Carbon sequestration captures CO₂ gas, from the atmosphere or from a chimney.
- It stores the carbon in a solid or liquid form, so it stays out of the air.
- It can be natural (plants, soils, oceans) or artificial (carbon capture and storage).
- The longer the carbon stays locked away, the better it is for the climate.
Remember it as: Catch the gas, lock it up as a solid or liquid.
Real example: an oak tree is natural sequestration you can touch. About half the dry weight of its wood is carbon, taken from the air as CO₂ gas and stored for as long as the tree lives.
Say both halves: A definition needs the capture of the CO₂ gas AND its storage as a solid or liquid. 'Storing carbon' alone is too vague.
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Nature has sequestered carbon for billions of years. Living things build carbon into their bodies, and some of it is buried before it can rot.
The points to remember
- Trees absorb CO₂ by photosynthesis and turn it into biomass: wood, roots, leaves.
- Dead plant matter builds up in soils and peat where it decomposes slowly.
- In the sea, seagrass, kelp and plankton take up CO₂; some carbon sinks into the sediment.
- Over millions of years, buried organic matter is fossilised into coal, oil and natural gas.
| Natural store | How the carbon gets there | Form |
|---|---|---|
| Trees | Photosynthesis builds wood and roots | Solid biomass |
| Soils and peat | Dead leaves and roots rot slowly | Solid organic matter |
| Seagrass and sea floor | Plants and plankton sink and are buried | Solid sediment |
| Coal, oil and gas | Buried remains fossilised over millions of years | Solid, liquid, gas |
Real example: between 2001 and 2019 the world's forests absorbed about 16 billion tonnes of CO₂ a year, about twice what clearing and fires released from them (Harris and others, 2021).
Fossil fuels count too: Coal, oil and gas are carbon that plants and plankton sequestered long ago. Burning them undoes that sequestration in a few seconds.
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People now copy nature with carbon capture and storage, often shortened to CCS.
How it works, step by step
- Capture: CO₂ is removed from the waste gas of a power station or factory.
- Compress: the CO₂ is squeezed into a liquid, so it can be piped or shipped.
- Store: it is pumped over 1 km down into salty sandstone, old oil or gas fields, or coal seams.
- An impermeable cap rock above stops it escaping; in basalt it can turn into solid rock.
- Direct air capture filters CO₂ straight out of the air, then stores it the same way.
Real example: since 1996, Norway's Sleipner gas field has removed CO₂ from its natural gas and pumped about 1 million tonnes a year into salty sandstone under the North Sea.
Reading a CCS diagram: Follow each numbered flow: where the CO₂ comes from, which rock layer it goes into, and what (if anything) comes back up, such as oil or methane.
Artificial sequestration can help, but it is not a simple fix. Weigh it up with points for and against.
For and against
- For: it removes CO₂ from the biggest sources before it reaches the air.
- For: it lets countries keep using fossil fuels while they switch to cleaner energy.
- Against: CO₂ pumped into an oil field pushes out more oil, which releases more CO₂ when burned.
- Against: it is costly and uses extra energy; a power station burns more fuel to run it.
- Against: it is small: about 50 million tonnes a year, far under 1% of the CO₂ we emit.
- Against: it treats the symptom; it does not cut the burning of fossil fuels.
Remember it as: Stores the waste, but can pump out more oil.
Real example: the Gorgon gas plant in Australia was the world's biggest CCS scheme when it opened in 2019, but in its first years it stored only about a third of the CO₂ it had promised.
The oil-field trap: CO₂ pumped into an oil field is stored, but the extra oil it pushes out is burned. Say both halves: some carbon is stored, yet more fossil carbon is released.
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How this comes up: Paper 2, Section A: a drawing of a carbon capture and storage scheme with numbered flows. Outline how two flows capture and store carbon [2].
Figure 1 shows a carbon capture and storage scheme at a power station.
Outline how Flows 1 and 2 shown in Figure 1 may contribute to the capture and storage of atmospheric carbon.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.