Introduction

1. This paper reviews the various options for the sequestration of carbon dioxide for the British Government Panel on Sustainable Development. The report consists of an overview and three technical annexes. Annex A covers sequestration of carbon dioxide by vegetation and was prepared by the Institute of Terrestrial Ecology in Edinburgh. Annex B covers sequestration by ocean fertilisation and was prepared by the Centre for Coastal and Marine Sciences, Plymouth Marine Laboratory. Annex C covers the industrial capture and disposal of carbon dioxide and was prepared by the International Energy Agency Greenhouse Gas R&D Programme.

Carbon Dioxide and Climate Change

2. The concentration of carbon dioxide in the atmosphere depends on the rates of exchange of carbon dioxide between the atmosphere and the terrestrial biomass, the oceans, and the carbon in deposits of fossil fuels and other geological formations.
3. Economic growth and population pressure, especially since the industrial revolution, have greatly increased the input to the atmosphere of carbon from fossil fuel deposits and from terrestrial biomass, by deforestation and other land use changes. The processes which remove carbon from the atmosphere have not increased proportionately. Consequently, atmospheric concentrations of carbon dioxide have risen to levels which are unprecedented in the recent geological history of the earth. The current level is about 360ppmv compared to a pre-industrial level estimated to be about 280ppmv. This has given rise to concern about climatic change.
4. This concern has produced policy responses in the form of the United Nations Framework Convention on Climate Change (UNFCCC) and its Kyoto Protocol. These agreements aim to limit the accumulation of carbon dioxide and other greenhouse gases in the atmosphere and to stabilise concentrations at levels which would prevent dangerous anthropogenic interference with the climate system.
5. In principle concentrations may be stabilised by any combination of policies which decrease emissions of carbon dioxide to the atmosphere, or increase rates of removal and storage, also called sequestration. The initial policy debate concentrated mainly on emissions reduction, but since the Kyoto Protocol there has been increased emphasis on the contribution that sequestration could make.

Types of Sequestration

6. The biosphere, geological formations and the oceans are the three main reservoirs which sequester carbon from the atmosphere. Each of these reservoirs is in principle accessible to policy intervention. Biospheric carbon may be increased by policies which promote afforestation or increase soil carbon content. Carbon dioxide may be captured and sequestered by injection into geological formations including depleted oil and gas fields, saline aquifers or unmineable coal seams. The ocean reservoir can be increased by fertilising plankton to increase the rate of transfer across the sea surface, or by direct injection of carbon dioxide at depth.
7. Carbon sequestration from afforestation (Annex A) occurs while trees are growing and ceases when they mature, although sequestration of carbon in the forest soil may continue at a much lower rate. In productive forests, trees are often harvested before they mature so that sequestration is not halted. The sequestration effect is potentially reversible due to the vulnerability of forests to fire, pests etc. Additional benefits can arise, e.g. from improved biodiversity and new recreational opportunities, but increased water uptake by trees can in some situations (e.g. in dry tropics) have impacts on supplies of groundwater and river flow. Indicative cost estimates vary from £50 to £80/tonne of carbon (tC) in the UK and substantially less (about £5/tC) for developing countries, but these are sensitive to assumptions about land values. Interest in sequestration in soils by reducing tillage or increasing the input of organic matter is growing internationally. However the stability of this sink can be affected by climate change and future human activities.
8. Understanding of the impact on greenhouse gas balances through ocean fertilisation by the addition of iron or nutrients, such as nitrates and phosphates, is still limited (Annex B). With regard to iron fertilisation, there are substantial uncertainties about the overall response of the Southern Ocean ecosystem, and it is possible fertilisation could lead to increased emissions of other greenhouse gases, nitrous oxide and methane, significantly offsetting any increased uptake of carbon dioxide. Estimated costs of ocean fertilisation are highly uncertain. One source estimates are £3 to £37/tC but this may be rather optimistic due to uncertainties over the effectiveness of the process. The other option, ocean fertilisation by nutrients appears to be even less practical than that for iron. Costs are also uncertain but are likely to be higher at £30 to £120/tC.
9. Technology for the removal and subsequent storage of carbon dioxide from the flue gases of large stationary sources such as power stations is available (Annex C). It is currently relatively inefficient, but further development could reduce costs and energy consumption. It is not deployed commercially at present, although Norway recently began operating a large pilot facility to store carbon dioxide separated from natural gas in a deep saline reservoir under the North Sea. Slow leakage from such saline aquifers is a potential problem in the longer term and would require monitoring; disused oil and gas reservoirs may be more secure, providing the injection of carbon dioxide does not fracture the reservoir cap. Estimated costs of capture and storage are £60 to £120/tC.
10. Captured carbon dioxide could also be injected into the deep ocean for disposal, at mid-depths (greater than 1500m), or at the sea bed (at depths greater than 3000m), where a lake of liquid carbon dioxide would form. This technology is not yet demonstrated and research is required into possible impacts on marine life, and in particular, deep ocean species in and below the sea floor.

Potential Role of Sequestration

11. The level below which atmospheric concentration of carbon dioxide should be stabilised to prevent dangerous anthropogenic interference with the climate system is not known precisely, but the European Union has suggested 550 parts per million, or about twice the pre-industrial level. Achieving this by emissions reduction alone would require a cut of about two thirds in total anthropogenic emissions. The cut by developed countries would need to be even greater, when projected economic growth by developing countries is taken into account. About 200 years would be available for these changes, depending on the emissions path followed.
12. Achieving this without additional sequestration would eventually require near decarbonisation of the world economy, probably by some combination of renewable energy technologies or nuclear power, complemented by increases in energy end use efficiency and a shift away from fossil fuels as energy carriers. This type of energy future has been discussed since the oil crisis of the 1970s. Practical achievability depends on cost and political acceptability.
13. Potentially sequestration could provide additional flexibility by removing significant amounts of carbon from flue gases or the atmosphere. This would allow fossil fuels to continue to be used for a longer period and still keep atmospheric concentrations below an agreed level. Table 1 summarises indicative estimates of the maximum potential level of sequestration achievable from sequestration options. Estimates of sequestration in soils are not included due to the uncertainties associated with this option. For comparison the estimated accumulation of carbon dioxide between 1990 and 2100 for the IPCC 1992 scenarios for anthropogenic greenhouse gas emissions range from 770GtC to 2190GtC.
14. The balance between sequestration and emissions reduction in stabilising atmospheric concentrations will depend on relative costs, environmental risks, and the uncertainties involved. The costs of the various sequestration options appear to be typically in the range £30 to £120/tC saved. (The very low estimate of £3/tC saved for iron fertilisation looks uncertain and needs to be reviewed.) This is expensive compared to most options for reducing carbon emissions.

UK Policy Position

15. The UK policy position on sinks has generally been cautious. On the one hand there is clearly potential for complementing emission reduction efforts through sequestration. On the other hand there are a number of scientific, technical and practical implementation problems which will need to be resolved before the UK could give greater emphasis to sequestration, either domestically or at an international level.
16. The 1998 Consultation Paper on the UK Climate Change Programme deals only with sequestration by forestry activity. This is consistent with Art. 3.3 of the Kyoto Protocol which says that afforestation, reforestation and deforestation since 1990 shall count towards national commitments during the first budget period, but does not specify other activities related to sequestration. The Government said in the Consultation Paper that the first priority must be to reduce emission levels as the most effective and secure way of delivering the UK's targets.
17. Some of the reasons for this are set out in Annex A to this paper: carbon sequestration by forests is reversible because of the vulnerability of forests to fires, pests, changes in land use, and, in some parts of the world, the effect of climate change itself. If carbon sequestered by forests were inadvertently returned to the atmosphere then the situation would be worse than if the sequestration had not taken place, because emissions would be higher than they would otherwise have been, as less action would have been taken on actual emissions reduction.
18. These considerations also apply to sequestration by soils (referred to in Article 3.4 of the Kyoto Protocol), which is reversible under conditions of climate change and is subject to much greater uncertainty than sequestration by forests. This uncertainty also means that it may be difficult to distinguish between natural fluxes from soils and changes in fluxes due to policies introduced. This separation is critically important for the attribution of policy response, since the natural fluxes are ten times greater than the release of carbon from fossil fuel combustion.
19. Sequestration in geological formations as described in Annex C involves removal of carbon dioxide from flue gases or by chemical shift prior to combustion. The UK view has been that this approach, which is still in a development phase, would not be the most cost effective way to meet national commitments, during the first budget period at least. However it is possible that this type of sequestration might contribute in the future, provided concerns for example about reservoir security can be met.
20. The UK view has been that ocean disposal by injection can not be considered to be a practical policy option unless the technology has been demonstrated and the environmental impacts researched. There may also be legal limitations under international agreements, particularly with regard to the UN Convention on the Law of the Sea.
21. Despite apparently low costs per tonne of carbon sequestered, ocean fertilisation (Annex B) could be counter productive when the effect on emissions of other greenhouse gases are taken into account. The effects on marine life are largely unknown. The viability of this option can not be assessed until further research is completed and the UK is contributing to this effort.

Further Research

22. All areas are the subject of active research. The DETR supports a major project to assess the nature of and potential for the use of land sinks in the UK and will play its part in a forthcoming Intergovernmental Panel on Climate Change report on sink processes. The Natural Environment Research Council supports work at the Plymouth Marine Laboratory and Southampton Oceanographic Centre and elsewhere on the issues related to ocean fertilisation. The Department of Trade and Industry funds the UK's participation in the International Energy Agency implementing agreement on greenhouse gases which includes the topic of industrial capture and disposal of carbon dioxide. It is clear that much more work needs to be done generally on understanding the natural process in the ocean and the potentially adverse side effects of fertilisation as well as the practical problems and costs.

¹For comparison, global carbon emissions are currently 6 Gt/yr and UK emissions 155MtC/yr.
MtC = million tonnes carbon; GtC = thousand million tonnes carbon.