This is the first edition of a new experimental release designed to provide partial estimates for stocks and flows of selected categories of carbon within the UK.
There was an estimated 143 billion tonnes of carbon stored in the UK’s fossil fuel (coal, oil and natural gas) stocks at the end of 2013.
Materials extracted from UK fossil fuel stocks during 2014 contained the equivalent of 62 million tonnes of carbon (MtC).
There was an estimated 4,266 MtC of recorded biocarbon in the UK in 2007, of which 94.2% (4,019 MtC) was contained in soil stocks and 5.8% (247 MtC) in vegetation stocks.
Between 1998 and 2007, UK biocarbon stocks declined by approximately 19.9 MtC (-0.5%), on the back of a fall in the volume of carbon stored in soil stocks.
Carbon contained in UK vegetation rose by 1.3 MtC (+0.5%) during the decade to 2007, driven by an increase in categories of forest tree cover.Back to table of contents
This article sets out preliminary physical stock and flow accounts for geocarbon over the 2013 to 2014 period and for biocarbon carbon between 1998 and 2007. In addition, it presents possible methods for estimating the physical biocarbon flows over the 2008 to 2014 period. Some discussion is also given on valuing the ecosystem services related to carbon sequestration.
The estimates are for particular types of carbon stocks, namely: geocarbon (coal, oil, gas) and biocarbon (in soil and vegetation). Notable omissions at this stage include estimates of limestone and other carbonate rocks, inorganic soil carbon, and carbon stored in urban habitats.
The carbon accounts and ecosystem accounts provide evidence to inform and improve decision making by integrating environmental and economic information. In particular, the carbon account supports the identification of links between the ecosystem and the benefits which humans receive from the natural environment.
The methodology to develop the stock and flow estimates for carbon remains under development and the estimates reported should be considered experimental. Feedback from experts in the various disciplines covered in the article will be essential for the successful development of the carbon stock accounts. All feedback is welcome and can be sent to email@example.com.Back to table of contents
This article has benefitted from the comments of: Rocky Harris and Colin Smith (Defra); Emily Conners, Suzanne Fry, Geoff Bright and Freddie Haslehurst (ONS); David Robinson (CEH); Ruth Greg (Natural England); Michael Vardon (Australian National University); and Peter Comisari (Australian Bureau of Statistics). We are also grateful to our colleagues within the ONS Office of the Chief Economic Adviser for their earlier carbon accounting work.Back to table of contents
Natural capital can be thought of as the stock of our physical natural resources and the ecosystem services that they provide. The Natural Capital Committee’s State of Natural Capital Report (2013) defines natural capital as: “the elements of nature that directly or indirectly produce value to people, including ecosystems, species, freshwater, land minerals, the air and oceans, as well as natural processes and functions”.
In 2011, the UK Government committed to working with ONS to incorporate natural capital in UK Environmental Accounts by 2020. This work is being completed in partnership with the Department for Environment, Food and Rural Affairs (Defra). For more information on the programme of work see our Environmental Accounts publications and the Natural Capital Accounting Roadmap.
The roadmap identified carbon as an important characteristic of habitat types and proposed the development of a cross-cutting carbon account to enable changes in the UK stocks of carbon to be monitored over time. Through this, a comprehensive overview of the role of carbon in the environment and the economy could be developed.
The carbon accounts, and ecosystem accounts in general provide evidence to inform and improve decision making by integrating environmental and economic information (i.e. through environmental-economic accounts). For instance, the aim will be for the carbon stock accounts to complement the existing annual greenhouse gas flow accounts produced by the Department of Energy and Climate Change (DECC), by providing consistent opening and closing stock balances.
Furthermore, the carbon stock accounts can be a tool to help decision makers understand the trade-offs between different ecosystem services and between alternative land uses. By providing a link between the ecosystem and the benefits which we receive from the natural environment, the accounts help us to understand the contribution the environment makes to economic activity and our well-being.
In general the elements of the individual natural capital accounts provide the basis for the cross-cutting carbon account. For instance, habitat based estimates of biocarbon1 could ultimately be drawn from the data presented in the ecosystem accounts for woodland, coastal margins, and freshwater. Similarly, the geocarbon2 elements of the carbon account can be populated using adjusted data contained within the ONS energy and minerals asset accounts.
These accounts utilise both the System of Environmental-Economic Accounting Central Framework (SEEA-CF) 3 , an international standard for valuing physical environmental assets, and the System of Environmental-Economic Accounting–Experimental Ecosystem Accounting guidelines (SEEA-EEA)4, international guidance endorsed by the UN Statistical Commission.
What is carbon? — Carbon as an element and the carbon cycle
Scientifically carbon (C) is a chemical element — a basic substance that cannot be broken down. Carbon is also an incredibly versatile element. Arrange carbon atoms in one way and they become soft, pliable graphite. Reorganise the arrangement and the atoms form one of the hardest materials on Earth: diamonds. The highly versatile nature of carbon results in it being present in multiple forms across the Earth’s spheres.
Carbon is also the key ingredient for most life on Earth, with the compounds formed by carbon and other elements such as hydrogen, oxygen and nitrogen essential for life. Carbon exists in all living matter, in vegetation and organisms, and the soils that support life (Ajani & Comisari, 2014).
The global carbon cycle refers to the cycle by which carbon flows between and inside the Earth’s various geographic spheres. Broad definitions of these are presented in Table 1. In addition to the movement and storage of carbon due to natural processes, carbon accumulations can also be found within the economy in the products produced through human activities.
Table 1: The geographical spheres where carbon is held and exchanged
|Geosphere:||Carbon contained in the solid part of the Earth consisting of the crust and outer mantle|
|Biosphere:||The global ecological system integrating all living beings, consisting of the carbon in soil, plants, animals and other life forms (living and dead).|
|Atmosphere:||The blanket of gases surrounding the Earth including carbon dioxide and trace carbon gases|
|Oceans:||The carbon dissolved in ocean water|
|Source: Office for National Statistics|
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It is the combination of natural processes, which may occur over very long periods of time, and human activity, which generally occur over relatively short periods of time, which change the size of the respective carbon stores (Ajani & Comisari, 2014). Furthermore, it is these stocks and flows that give the underlying context for carbon accounting (Vardon, 2014).
Accounting for carbon
The accounting framework adopted in this work is based upon SEEA-EEA carbon stock account. The high level structure of a carbon stock account is shown in Table 1 of the reference tables. It provides a complete articulation of carbon accounting based on the carbon cycle. (SEEA-EEA, p.89).
The stocks and flows of the carbon cycle give the underlying context for carbon accounting in the carbon stock account. A stock represents the total quantity of a category of carbon at a given time. Physical flows represent changes in the level of a given carbon stock between one period and another. Annex 1 provides a detailed overview of the types of additions to, and reductions in, the carbon stock account.
Only geocarbon and biocarbon accounts are presented in this article. Carbon estimates relating to the economy, the oceans and the atmosphere have not been included at this time, but are discussed later.
- Carbon contained in the solid part of the Earth consisting of the crust and outer mantle.
- Carbon contained in the solid part of the Earth consisting of the crust and outer mantle.
- The United Nations was the lead author of the SEEA-CF, with other joint authors being: European Union, Food and Agriculture Organization of the United Nations, International Monetary Fund, Organisation for Economic Co-operation and Development, and The World Bank.
- The United Nations was the lead author of the SEEA-EEA, with other joint authors being: European Union, Food and Agriculture Organization of the United Nations, Organisation for Economic Co-operation and Development, Eurostat, and The World Bank.
Geocarbon refers to the carbon stored in the geosphere – the solid part of the earth consisting of the crust and outer mantle. It is disaggregated into: oil, gas, coal resources, rocks (primarily limestone), and minerals, e.g. carbonate rocks used in cement production, methane clathrates and marine sediments (SEEA-EEA, 2012).
The high commercial interest in certain forms of geocarbon, notably fossil fuels, means data availability is good relative to the other carbon pool categories. However, there remains a high degree of technical uncertainty surrounding estimates of geocarbon reflecting uncertainty around the location, size and types of geocarbon stocks, as well as the respective average carbon contents contained within them.
Table 2 presents information on the conversion factors applied when determining the carbon content of the respective carbon stocks. The set of geocarbon stock and flow accounts by geocarbon category are also provided in reference Tables 2 to 4.
Table 2: Estimated carbon content of UK geocarbon resources
|Geocarbon resource||Carbon Content||Source|
|Oil||0.85%||Biomass Energy Centre|
|Source: Office for National Statistics|
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This article provides UK carbon stock estimates for the geocarbon categories of oil, gas and coal for the period 2013 to 2014. Data limitations mean estimates of carbon contained in UK shale deposits are not included at this time. Stock and change estimates for the high level geocarbon categories can be found in reference Table 1.
Approximately 143,095 million tonnes of carbon (MtC) relating to UK oil, gas and coal deposits was stored in the geosphere at the end of 2013. Of this, coal stocks accounted for 98% of total stocks. During 2014, extraction processes resulted in a combined total of 62 MtC carbon being removed from the respective categories of geocarbon.
The carbon contained in UK coal resources totalled 140,523 MtC at the end of 2013. Coal types presented are for hard coal and lignite. Hard coal represents almost the entire share of UK coal stocks (99.8%). Stock data for coal is only available at the start of the period providing an opening balance figure. We have assumed no significant changes to have occurred in coal stocks over the year.
Oil and Natural Gas
The UK Oil and Gas Authority (OGA) provides the most comprehensive range of data on UK oil and gas stocks. The estimates for oil and gas are split into reserves, possible, potential additional resources (PARs) and undiscovered resources. While the OGA categorisation is different to SEEA-CF, SEEA Class A reserves could broadly be categorised as proven and probable reserves. This article uses the SEEA definition of reserves. Table 3 below provides an explanation of the classification terms used in this section.
Table 3: Definitions of geocarbon deposit types
|SEEA Reserves||Proven||Virtually certain to be technically and commercially producible i.e. have a better than 90% chance of being produced.|
|Probable||Not yet proven, but have a more than 50% chance of being produced|
|Possible||Cannot be regarded as probable, but which are estimated to have a significant – but less than 50% – chance of being technically and commercially producible.|
|Potential additional resources (PAR’s)||Not currently technically or commercially producible.|
|Undiscovered||Provide a broad indication of the level of oil resources which are expected to exist. However, they are subject to higher levels of uncertainty than reserves and PAR’s.|
|Source: SEEA Central Framework, OGA|
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The total amount of carbon stored in UK natural gas deposits was 739 MtC in 2014. Of this, undiscovered gas contained the largest stock of carbon followed by proven plus probable gas reserves. Figure 1 presents the breakdown by deposit type.
For proven plus probable reserves, estimates can also be disaggregated by type of gas for: dry gas; condensate gas; and associated gas. A full breakdown of the carbon contained in each natural gas category is presented in reference Table 3.
In terms of changes in natural gas carbon stocks, the resource category recorded a fall of 32 MtC over the 2013 to 2014 period. The majority of this reduction can be attributed to natural gas proven plus probable reserves, which recorded a managed contraction (i.e. due to extraction) in its carbon stock equal to 19 MtC, as well as a downward reappraisal equal to 5 MtC. The only stock categories to increase over the 2014 period were probable condensate gas resources and undiscovered natural gas resources owing to a reappraisal, which found stocks to be higher than previously estimated.
No flow information was available at the proven plus probable reserve category levels for dry gas, condensate gas and associated gas.
Total UK stocks of oil held a carbon mass equal to 1,796 MtC. Of this, undiscovered deposits held the largest proportion of total oil carbon stocks, followed by proven plus probable reserves.
Between 2013 and 2014, the total carbon stored within UK oil stocks fell by 3 MtC. At the sub-category level, oil proven plus probable reserves recorded a net reduction in stocks. Of this, a decline in proven reserves through extraction was offset by an upward reappraisal to the geocarbon stock category. PAR oil reserves were also subject to an upward reappraisal equal to 23 MtC.
A full breakdown of the carbon contained in each deposit category of oil is presented in Table 2 of the reference tables.Back to table of contents
The carbon stored in plants, soils, animals and ecosystems as a whole are all components of the biocarbon stock. Biocarbon reservoirs can be separated by type of ecosystem, which at the highest level are terrestrial, aquatic and marine (SEEA-EEA, 2012).
The primary focus of this article is on terrestrial (land) habitat based ecosystems. Coastal margin carbon storage data is presented separately. This is because the intertidal characteristics1 of coastal margins habitat ecosystems can create potential issues with double counting. Furthermore, a lack of data on the carbon stored within, and sequestered by, open water (aquatic) ecosystems means they are excluded from the analysis at this stage.
The carbon content of animals living within the ecosystems are also absent from current estimates due to a lack of available data. A more detailed discussion on the potential scope of biocarbon estimates is contained in the Biocarbon: background and methods section.
Finally, urban green space is often overlooked in terms of carbon storage. However, this land class could potentially be a significant store stock of carbon (Natural England, 2016). Research is very limited and the habitat class is not covered in the carbon accounts at this stage. However, the inclusion of biocarbon data for urban habitats could be very useful to urban planners, particularly when designing future green infrastructure projects. We will look to investigate this during the development of an Urban Ecosystem Account.
Approximately 4,266 million tonnes of carbon (MtC) was stored within the UK’s land based biocarbon reservoirs2 in 2007. This is an underestimate since the vegetation carbon data excludes data for the Fen, marsh and swamp habitat classification, as well as the carbon content of animals living within the ecosystem habitats. Reference Table 5 provides a breakdown of UK biocarbon stocks by SEEA-EEA habitat classes and at subcategory level based on the Countryside (CS) Survey Broad habitat classes as at 2007.
As Figure 3 shows, the carbon stored in UK soils is by far the largest component of the biocarbon stock containing approximately 4,019 million tonnes of carbon (MtC), or 94.2% of the total3. The amount of carbon stored in UK terrestrial vegetation was considerably lower containing an estimated stock of 247 MtC, or 5.8% of the total.
The carbon stored in Open wetlands (peat soils) makes up the largest portion of soil carbon stocks (57.3 %), followed by Improved grassland habitat (9.0%). The volume of carbon stored in the latter is primarily down to the wide extent of this habitat class rather than its capacity to store carbon i.e. carbon density. Soil carbon contained in Forest tree cover habitats also makes a significant contribution to total soil carbon stocks (16.7%).
In terms of the carbon stored in UK vegetation, forest tree cover habitats had the largest proportion of total stocks (91.4%). Vegetation carbon in Forest tree cover can be further disaggregated into Coniferous Woodland habitat, and Broadleaf, mixed and yew woodland habitat, which contain 48.0% and 43.4% of total stocks respectively.
Figure 4 presents the total (soil carbon plus vegetation carbon) change in biocarbon stock by habitat over the 1998 to 2007 period. UK biocarbon stocks4 recorded a net decline of 19.9 MtC (-0.5%) during the period 1998 to 2007. A net rise in the carbon stored in UK vegetation over the period (+1.3 MtC), was offset by a larger net decline in UK soil carbon stocks (-21.2 MtC).
At habitat level, the most significant falls were recorded in the soil carbon stocks held in Rainfed and irrigated herbaceous cropland. The decline was due to the combined effect of a decrease in the habitat’s land cover extent and a fall in its average soil carbon content (-4.5 tonnes of carbon per hectare, 1998 to 2007). Most increases in soil carbon stock by habitat were due to a rise in their respective extents over the 1998 to 2007. These included: Shrubland, bushland and heathland; and Pastures/Improved grassland. The rise in the soil carbon stocks within the Semi natural grassland resulted from increases in the habitat’s extent and average carbon content per hectare.
The most notable changes within vegetation carbon stocks came from the forest tree cover habitats. A 6.1 MtC in Coniferous woodland habitat was offset by a 7.4 MtC increase in Broadleaf, mixed and yew woodland habitat.
For a more complete picture of the changes in biocarbon stocks see reference Table 6.
Coastal margins habitats
Natural England (2012) publishes carbon stock average estimates by broad habitat. The report gives a figure of 48 tonnes of soil carbon stored per hectare of Coastal margins habitat. Combining this with the 2007 SEEA-EEA Land Cover estimate for Coastal margins habitats of 153,000 hectares produces an estimate of 7.3 MtC stored in the respective soil assets. Estimates of vegetation carbon contained within this habitat class could not be produced owing to data limitations. The Coastal margins habitat account is given in reference Table 7.
Other approaches to the recording of biocarbon stock changes
DECC reports on flows of carbon from land use, land use change and forestry (LULUCF) as part of the annual UK Greenhouse Gas Inventory. The LULUCF classification does not include marine ecosystems including those intertidal areas recorded within the Coastal Margins Land Cover category. The flow data are reported on a net (emissions less removals) basis and calculated based on annual stock changes. Carbon uptake by UK forests is calculated by using a carbon accounting model, CARBINE. These are recorded as gains and losses in pools of carbon in standing trees, litter and soil in conifer, broadleaf trees, and in harvested wood products (DECC, 2014). It is not clear how changes in vegetation across the other habitat classes are calculated. Changes in soil carbon content due to land use change are estimated using dynamic models. The models combine soil carbon density information (Milne and Brown, 1997, Cruickshank et al. 1998, Bradley et al, 1995) with land use change matrixes, of which information from the CS Survey is a key input. DECC data does not distinguish between soil and vegetation carbon at this stage, so change statistics are presented on an aggregate basis.
There is still considerable work to be done if the if the carbon information available at DECC is to be optimally utilised to bring it into a consistent, compatible format for use within the carbon stock and flows account. Further research is required in the future, particularly relating to vegetation carbon beyond forest habitat and, more broadly, soil content by habitat. Even so, Table 4 presents an attempt to gauge an indication of net changes in carbon stocks using the current available data. It should be emphasised that this work is highly experimental and will be subject to significant revisions as the developmental process progresses.
DECC data are compiled based on a land use Classification basis, which accords to the Intergovernmental Panel on Climate Change (IPCC) reporting guidelines. IPCC land use Classifications were mapped to the SEEA-EEA habitat classes using guidance provided by Weber (2014) (see Annex 2). Data relates only to carbon dioxide emissions, which are then converted into carbon equivalents5. Emissions are generally recorded in CO2 equivalent (CO2e) terms, whereas the stocks are in terms of carbon. This is because most of the matter in carbon stocks is held in non-oxidised form. A lack of detailed information on the emissions of other trace carbon gases, such as methane from within the LULUCF classification, means we have been unable to include them in the calculations. Carbon flows presented in Table 4 are based on the aggregated net changes between 2008 and 2014.
Table 4: Estimated net additions to (+) / reductions in (-) biocarbon stocks within UK terrestrial habitats
|SEEA-EEA Habitat classes (1)||IPCC Land Use (2)||2008||2009||2010||2011||2012||2013||2014||Change in carbon stock, 2008-14|
|Broadleaved, mixed and yew woodland||Forest land||+4.9||+4.9||+4.9||+4.9||+4.8||+4.8||+4.7||+33.9|
|Rainfed and irrigated herbaceous cropland||Cropland||-3.5||-3.5||-3.5||-3.4||-3.4||-3.3||-3.2||-23.8|
|Permanent crops, agriculture plantations|
|Semi natural grassland|
|Shrubland, bushland, heathland||Other||+0.3||+0.3||+0.3||+0.3||+0.3||+0.3||+0.3||+2.1|
|Source: Department of Energy and Climate Change|
|(1) System of Environmental-Economic Accounting — Experimental Ecosystem Accounting|
|(2) Intergovernmental Panel on Climate Change|
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The carbon stock change data presented in Table 4 relates to net flows of carbon resulting from the regulatory ecosystem service of carbon sequestration. In order to present a more comprehensive picture, the changes in carbon stocks relating to provisioning ecosystem services need to also be included. Provisioning ecosystem services in this context relate to the carbon contained in timber harvested (removed) from Forest Land. In addition to annual estimates of UK timber production, the Forestry Commission produces annual statistics on changes to UK timber stocks, which includes information on new plantings, removals and felling residues. Furthermore, the annual timber stock estimates published in the ONS Environmental Accounts factor in various entries relating to additions and reductions to stock when calculating stock change estimates. While both represent valuable data inputs, further work is required before the respective data sets are in a suitable format that can be used in the carbon stock account.
Can encompass both marine and terrestrial habitats.
Estimate is partial owing to the exclusion of certain UK habitats, including Coastal Margins.
Total carbon in this case equates to the combined estimated value for soil carbon and vegetation carbon.
Estimates are partial owing to the exclusion of certain UK habitats, including coastal margins.
One unit of Carbon = 3.667 units of Carbon Dioxide.
Preliminary estimates for categories of UK geocarbon stocks reported that approximately 143,095 MtC was stored in UK oil, gas and coal deposits at the end of 2013 (Table 5). Of this coal stocks accounted for 98% of the total. During 2014, extraction processes resulted in a combined total of 62 MtC carbon being removed from the respective categories of geocarbon.
Approximately 4,266 MtC was stored within the UK’s land-based biocarbon reservoirs1 in 2007. The carbon stored in UK terrestrial soils is by far the largest component of the biocarbon stock containing approximately 4,019 million tonnes of carbon (MtC) or 94.2% of the total. UK biocarbon stocks declined by approximately 19.9 MtC (-0.5%) in the decade to 2007, on the back of a fall in the volume of carbon stored in soil stocks. Carbon contained in UK vegetation rose by 1.3 MtC (+0.5%) over the same period, driven by an increase in categories of forest tree cover.
Table 5: Preliminary estimates for selected categories of UK carbon stocks (MtC)
|Geocarbon (1)||Biocarbon (2)|
|Oil||Gas||Coal||Terrestial Ecosystems||Coastal Ecosystems|
|Source: Office for National Statistics|
|(1) 2013 reference year|
|(2) 2007 reference year|
|MtC. Megatonnes of carbon|
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Data limitations mean that current estimates of UK geocarbon stocks are partial due to the omission of several resource sub-classes including shale, limestone and carbonate rocks. Furthermore, the limited account given to spatial variation in the respective geocarbon deposits suggests the margins for error arising from the approach used in this article are relatively broad. In particular, further work is needed to investigate the carbon content of subcategories of geocarbon resources i.e. oil, gas, coal, etc.
The carbon stock account is missing estimates for a number of biocarbon subcategories. These include: Aquatic ecosystems; and vegetation carbon stored in Fen, marsh and swamp habitats classification. Furthermore, cross cutting carbon stock categories such as the carbon contained in animals living within the ecosystems, as well as inorganic carbon stocks contained in soils have also been excluded from current estimates. Further work to include these and other categories is required if a fully representational picture of the UK’s carbon stock is to be presented. Finally, while the urban green space habitat class is not covered in the carbon accounts at this stage, its inclusion could represent a valuable addition in the future. We will look to investigate this during the development of the Urban Ecosystem Account.
The cross cutting nature of certain ecosystems make it challenging to present carbon stock data by distinct categories of habitat. This point is particularly pertinent to Coastal margins, where the intertidal characteristics of such habitat ecosystems can create potential issues with double counting. In addition, while peatlands have been classified within the SEEA-EEA Open wetland habitat class in this article, their land use often means these such soils can also be included in Arable or Grassland habitats. This can result in their mismanagement isn’t accurately represented. The development of distinct asset accounts for both Coast margin habitats and Peatland habitats will help fill such gaps in information. This approach is supported in Defra’s Scoping the Natural Capital Accounts for Peatland (2015), while the upcoming Scoping UK Coastal Margin Ecosystem Accounts (ONS, 2016) Due for release on June 28th, 2016. will help inform on carbon stocks with this habiat class.
The aim for the carbon stock account is to complement and support the other natural capital accounts, as well as the existing annual greenhouse gas flow accounts produced by DECC. However, the current methodology used to generate the stock and flow estimates for carbon remains under development and the reported estimates should be considered experimental at this stage. Feedback from experts in the various disciplines covered in the article, as well as from stakeholders across government departments, will be essential for the successful development of the carbon stock accounts.
- Estimate is partial owing to the exclusion of certain UK habitats, including coastal margins.
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