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What is carbon sequestration?

Carbon sequestration is the storage of carbon dioxide (CO₂) to prevent its release into the atmosphere where it contributes to global warming.

Although trees, plants and crops naturally sequester CO₂ by converting the gas to biomass and soils, the term carbon sequestration generally refers to either the intentional creation of additional biomass for sequestration or for methods of storage that don't naturally occur.

The stores of CO₂ are often referred to as carbon 'sinks'.

Is this the same as 'carbon capture and storage'?

Nearly - but not quite. Sequestration is effectively the storage part of carbon capture and storage.

What methods of carbon sequestration exist?

The main types of carbon sequestration include: increased storage in vegetation and soils, geological storage, ocean storage and mineral storage. Some methods are currently being used and others are propositions and have yet to be applied.

• Vegetation and soils
  The most obvious means to significantly increase vegetation is through afforestation (planting trees where there were none previously) or reforestation (replanting trees in areas that have been deforested). Critics of this method claim that it is only effective when the vegetation is alive and that old trees emit more CO₂ than they take up.

  Altering farming practices to plant crop and grass species with extensive root systems can increase soil carbon content significantly. It is claimed that improving the soil carbon content by 0.5 per cent in the top 30 centimetres of just two per cent of Australia's agricultural land would sequester all of our annual CO₂ emissions. This doesn't mean that it would be easy to achieve or that it could continue year on year, however.

  Other related ways of sequestering carbon in vegetation include restoring natural carbon sinks, such as degraded wetlands.

• Geological storage

  This method is also known as geo-sequestration. It involves locking CO₂ below ground in geological formations such as oil fields, gas fields, aquifers and coal seams that cannot be mined. Typically the source of the CO₂ is from major industrial and energy related sources like power stations.

  Proponents of geo-sequestration claim that this method stores carbon indefinitely. They also point to the potential benefits it offers such as aiding the recovery of oil from ageing oil fields and releasing methane from coal beds that can be captured for use.

  On the other hand, critics are concerned by the energy required for the process, the risk of CO₂ leaks, and of course the fact that some of the supposed benefits include facilitating further harvesting of fossil fuels.

• Ocean storage

  To store CO₂ in oceans there are several potential options. It can be injected to a depth of approximately 1,000m in the ocean where it dissolves, or it can be deposited below 3,000m where its relative density could enable the creation of 'lakes' that slow its dissolution into the atmosphere.

  Tentative propositions have also been made to store CO₂ in carbon dioxide clathrates (ice-like structures) in the deep ocean. CO₂ can also be used to feed the growth of plants in the ocean just as it can on land.

  Uncertainty about the environmental impacts of storing CO₂ presents an obstacle to ocean sequestration. The ocean can only temporarily store CO₂ as it eventually releases it to the atmosphere. Adding CO₂ to the sea is likely to increase its acidity, also, which is already increasing due to rising atmospheric concentrations of CO₂. This is harmful to ocean ecosystems.

• Mineral storage

  It is possible to sequester CO₂ by adding it to magnesium or calcium to form carbonates. The process effectively replicates natural weathering reactions, but it controls them and speeds them up considerably. The carbonates store CO₂ permanently and can simply be buried.

  Ample reserves of appropriate minerals available for storage and the cost of processing is not prohibitive, but there are environmental drawbacks. Extensive mining operations are required to retrieve the minerals and bury the carbonates. Also, unless CO₂ production, mineral mining and the carbonation process are undertaken in close proximity then large volumes of materials need to be moved around.

So is sequestration the silver bullet?

Sadly not. Although some methods, such as sequestering in oilfields, have been used for many years, others are not fully researched and currently they all seem to have environmental or economic drawbacks.

Ultimately, carbon sequestration is of a similar nature to carbon offsetting in that it alleviates the symptoms of our problem but does not remove the cause. However, until we manage to live without using fossil fuels sequestration is set to provide some temporary relief from the effects of CO₂ pollution.