Scottish Greenhouse Gas Statistics 2024

This section examines key revisions in estimated source emissions between the latest inventory (1990-2024) and the previous inventory (1990-2023). 

Compilation of the Greenhouse Gas Inventory

The greenhouse gas inventory covers a wide variety of anthropogenic sources of greenhouse gas emissions.   There is therefore a wide variety of emissions sources which require different approaches to their estimation. There are a large number of data sources used in its compilation, obtained from Government statistics, regulatory agencies, trade associations, individual companies, surveys and censuses. The methods used to compile the greenhouse gas inventory are consistent with international guidance on national inventory reporting from the Intergovernmental Panel on Climate Change.

Most emission estimates are compiled by combining activity data (such as fuel use) with a suitable emission factor (such as amount of CO2 emitted per unit of fuel used). Estimates of emissions from the industrial sector are often compiled based on plant-specific emissions data. Emissions from some sectors are based on more complicated models - such as the model used to estimate emissions from landfill, and the model used to estimate the carbon dynamics in soils when trees are planted. Much of the data on net emissions from ‘agriculture’ and  ‘land use, land use change and forestry emissions’ are based on modelled data for Scotland, which are consistent with, but not constrained to, the UK totals and thus are known as “bottom up” estimates.

Many of the remaining emissions sources within the inventory have been collated on a “top down” approach where estimates of emissions have been apportioned to Scotland using proportions of energy use in the Department of Business, Energy and Industrial Strategy (BEIS) Publication “Digest of UK Energy Statistics (DUKES)”. This approach is prompted by data availability on emissions being more limited at the sub-UK level.

Impact of Revisions

Revisions between the 1990-2023 and 1990-2024 inventories

Charts 14 to 16 illustrate the impacts of revisions between the 1990-2023 and 1990-2024 inventories.  This is followed by a discussion of the reasons for the key revisions.

Chart 14.  Scottish Greenhouse Gas Emissions.  Comparison of the net greenhouse gas emissions for the 1990-2023 and 1990-2024 Inventories.

Chart 15 shows revisions to the baseline period, between the two most recent inventories.

Chart 15.  Revisions to emissions in 1990, from the 1990-2023 inventory to the 1990-2024 inventory, by Territorial Emissions Statistics Sector.

Chart 16 shows revisions to the data for the year 2023, between the two most recent inventories.

Chart 16.  Revisions to emissions in 2023, from the 1990-2023 inventory to the 1990-2024 inventory, by Territorial Emissions Statistics Sector.

Details of Main Revisions and Interpretation of Revisions to the Inventory

Revisions to emission inventory estimates reflect the continuous development of scientific understanding of emissive processes, and the improvement to underlying data and methods to generate accurate emission estimates; few revisions to the Greenhouse Gas Inventories arise as a result of 'errors' in the popular sense of the word. The compilation of the inventory is governed by a rigorous quality assurance process and is subject to a great deal of third party scrutiny, such as annual reviews by the UNFCCC of the UK inventory.

The latest published Scotland greenhouse gas inventory (currently 1990-2024) represents the best available data at the time and these supersede any previous data, which should be disregarded.

A complete list of the revisions between the previous and latest inventories can be found in the National Atmospheric Emissions Inventory report Greenhouse Gas Inventories for England, Scotland, Wales and Northern Ireland: 1990 - 2024.  Details of the most notable revisions are listed below:

• Agriculture
  • There are updates to dairy cattle emissions due to a re-parameterisation of the dairy cattle growth curve using slaughter-weight data resulting in an increase in liveweight of all dairy animals and enteric CH4 emissions. 
  • For sheep, there was a downward adjustment in the assumed metabolisable energy density of grazed grass, to better match survey evidence, that directly resulted in an increase in total dry matter intake to meet the energy requirements of an animal.
  • Manure management estimates were also impacted by the re-parameterisation of the dairy cattle growth curve using slaughter-weight data resulting in an increase in liveweight of all dairy animals and therefore changes to the volatile solids (VS) and nitrogen excretion.  
  • Manure management estimates were also impacted by the updated the metabolisable energy content of grass in preferential grazing areas resulting in an increase in nitrogen excretion.  
  • Small additional manure management specific changes arise from an update to the percentage of manure diverted to anaerobic digestion, with minor revisions from around 1991 onwards, increasing toward the end of the time series.
• Buildings and product uses
  • Updates were made to the mapping grids for oil in Commercial/Institutional stationary fuel use and Residential stationary fuel use from 2005. 
  • Note there are also UK level revisions due to splitting the natural gas data from DUKES based to quantity of biogas injected into the grid, which results in minor sector specific changes in emissions from natural gas.

• Transport
  • There are large shipping recalculations at the DG and UK level due to updating the shipping model to use 2019 as Base Year (instead of 2014) as part of implementing the Greenhouse Gas Inventory Improvement Project shipping improvement. This includes the following updates:
  • There are substantive reductions to fuel use for Domestic Shipping due to the transition from a ‘fuel used’ approach to a ‘fuel sold’ approach (normalising modelled shipping fuel use to national energy statistics from DUKES).
  • Assumptions for fuel consumption and emissions at berth have been reviewed and revised emission factors and assumptions from the Department for Transport's (DfT) Maritime Emissions Model (MEM) have been incorporated as part of the model

• LULUCF
  • Updated new planting estimates for 2023 provisional data.  
  • Updated wood production estimates for 2023 provisional figures. 
  • Update to the total woodland area by correcting the area of unmapped woodland for Great Britain. The total ’unmapped NFI’ area found in the 2017 Forestry Commission Tree Cover Outside Woodland in Great Britain report was previously added to small woodland area. This ‘unmapped NFI’ data contains both ‘extra’ woodland area that was missed in previous surveys and ‘new’ woodland planted between 2014 and 2015.
  • Improve the species and Yield Class (growth rate) distribution for recent (post-2011) afforestation to better reflect recent planting practices. 
  • Wood Product Trade Statistics: 
    • Updated woodfuel supply estimates for 2022-2023. 
    • Updated sawmill consumption by country estimates from 1994-2022 due to error correction. 
    • Updated roundwood delivery estimates for 2023 provisional figures. 
    • Updated roundwood fencing estimates from 2006-2014 due to error correction in dataset received from Forestry Statistics. 
    • Updates to deforestation estimates for 2015-2023 based on revised figures on rewetted areas received from Peatland ACTION. 
    • Updates to CARBINE modelling: 
    • Closer match to total wood production across the time series. 
    • Include the transfer of carbon from branches on dead trees into the soil. 
  • Peatlands.  The majority of the change arises from the update to the CARBINE model to include the transfer of carbon from branches on dead trees into the soil which impacts both mineral (reported in Forestry) and organic (reported in Peatland) soil. The remaining minor changes arise from two activity data changes. The first is a recalculation of the hydrological buffer/footprint applied by Peatland Action to the length of ditches blocked, which has reduced from 50m (biodiversity and hydrological footprint) to 30m (hydrological footprint), to align with Peatland Code methodology, which slightly reduced the areas of restoration. Secondly, additional hectares of restoration have been added to the Scotland time series. 

• Waste
  • Recalculations to Managed anaerobic waste disposal sites are driven by an update to the landfill model to use gross GDP data to approximate solid waste disposal volumes where actual activity data is unavailable. Importantly, historic data on solid waste disposal at landfill sites is uncertain and unavailable before 1997. Previously, solid waste disposal volumes from 1990 to 1996 were extrapolated from 1997 data using household and employment statistics. However, an external review of this method highlighted that it is inconsistent with the IPCC Guidelines for National Greenhouse Gas Inventories. To improve consistency with IPCC guidelines, the landfill model has been updated to use GDP data as a proxy for solid waste disposal volumes. This change is exclusive to the waste sector and leads to a large decrease in landfill emissions estimates over the early portion of the historic time series. 
  • There are recalculations (Domestic wastewater treatment and discharge) due to the implementation of the new municipal wastewater model to improve consistency with IPCC guidelines and capture UK-specific wastewater treatment practices:
  • Increase to N2O emissions in all years due to estimating emissions from aerobic treatment, which has a much higher emission factor than the emission factor for N discharged to receiving waters and represents an almost tripling in N2O emissions estimates from this sector;
  • Decrease to CH4 emissions in the 1990s due to using a lower value for the maximum CH4 producing capacity;
  • Increase to CH4 emissions after ~2000 due to estimating emissions associated with Biological Oxygen Demand (BOD) remaining in effluent discharged to receiving waters; 
  • Increase to CH4 emissions in all years, but more in later years than the 1990s due to the 2019 methodology combined with UK-specific CH4 capture rates resulting in higher emissions than the previous Carbon Accounting Workbook (CAW)-based method;
  • Increase to CH4 emissions in all years due to introducing an estimate of emissions from sewers; and
  • New estimates of CO2 emissions from liming and decomposition of materials with fossil Carbon content where previously no estimate was made.
  • Updated municipal wastewater treatment mapping grid, based on the data provided into the model, that has resulted in specific recalculations across the time series; the previous driver utilised population as a proxy for distributing municipal wastewater emissions.
  • There are recalculations to 5D2 (Industrial wastewater treatment and discharge) due to:
    • Estimates for CH4 and N2O emissions from the discharge of industrial wastewater being included in this submission where previously emissions were not estimated. Inclusion of this emissions source is now justified through its addition to the common reporting table (CRT) format used to comply with international reporting commitments.
    • Estimates for CH4 and N2O emissions from industrial wastewater treatment for 5D2 being revised in this submission. This is primarily due to the adjustment to the activity data (e.g. the industrial production estimates for various products allocated to industry) revision to the approach to splicing and gap-filling of various datasets used.

Interpretation of uncertainties in the inventory

All estimates, by definition, are subject to a degree of statistical 'error' but in this context it relates to the uncertainty inherent in any process or calculation that uses sampling, estimation or modelling.

Estimates of greenhouse gases are compiled by a consortium of contractors.  The source emissions are based upon a range of data sources, ranging from model based estimates to point source emission data.  As a result, the estimates are subject to a degree of uncertainty.  Full analyses of these uncertainties are provided on the National Atmospheric Emissions Inventory website[1].

The Scottish Government previously commissioned research to overhaul and update the uncertainties model used for the Scottish greenhouse gas inventory.  A detailed study was carried out in parallel with the compilation with the 1990-2014 Scottish greenhouse gas inventory to review and improve the uncertainty calculations.  A link to this project and to the full report can be found in the Scottish Greenhouse Gas Inventory Uncertainties Project.