National Physical Laboratory

Understanding the UK Greenhouse Gas Inventory

The National Physical Laboratory (NPL) has produced a report assessing how the UK greenhouse gas inventory is calculated and the implications of uncertainty.

Understanding the UK Greenhouse Gas Inventory (NPL Report CCM 2) front cover

Every year the Department for Business, Energy and Industrial Strategy (BEIS) compiles a Greenhouse Gas Inventory which quantifies the UK's total annual emissions and sinks of greenhouse gases. This is used to track progress against emission reduction policies, including international treaties such as the Kyoto Protocol.

The Committee on Climate Change uses the inventory to recommend future carbon budgets, as well as to report to Parliament on whether enough is being done to meet existing budgets.

Producing the national inventory involves collecting and analysing data from every potential source and sink of greenhouse gas in the UK. This ranges from national energy statistics, to data collected from individual industrial plants.

NPL has produced a report to review the methodology for building the UK inventory. It provides an overview of greenhouse gas estimates and associated uncertainties across each sector, and highlights time-series trends and revisions over time. It draws out key insights around issues such as robustness, reliability and uncertainty of the inventory data and what these imply for both setting carbon budgets and monitoring progress.

As well as helping the Committee on Climate Change to understand the implications for its own work in the future, the report also informs funders and researchers about where to focus their efforts to have the biggest impact on the reduction of uncertainties in the inventory.

Key insights are:

  • The UK Government and international community can have confidence in the UK Greenhouse Gas inventory. The overall uncertainty is 3%, the third lowest by international comparison to other major emitting countries.

  • Overall uncertainty has fallen over the last years, as the UK inventory methodology has improved. This is particularly true for non-CO2 gases.

  • To calculate the uncertainty of the total inventory emissions, the uncertainty of the individual sources are combined as the square root of the sum of squares. This means that the sectors with the largest individual uncertainties have a proportionally much greater impact on the overall uncertainty compared to sectors with small uncertainties.

  • Agriculture, land use and waste contribute the largest sources of uncertainty to the UK inventory:
    • Agriculture accounts for only 9% of the total CO2 equivalent emissions, but contributes to 36% of uncertainty in the total inventory emissions. Uncertainties around the emission factors and activity data are high.
    • Land use change contributes 8% of the total emissions, but is responsible for 32% of the total uncertainty in the UK inventory. In this case emission factors are the main sources of uncertainties.
    • The waste sector constitutes 3% of the total CO2 equivalent emissions yet contributes 18% to the total uncertainty. Emission factors are the main issues for the sector uncertainties with activity data also playing a role.
  • External validation of the inventory through atmospheric measurements is happening and is important. Improvements in monitoring and modelling are resulting in convergence of emissions estimates and are highlighting inconsistencies in the inventory, which reflects the value of this activity. The UK's lead here has been recognised by international committees to further improve validation methods such as the work of the Intergovernmental Panel on Climate Change's Task Force on National Greenhouse Gas Inventories.
  • The Key Category Analysis (KCA) ranking system – which helps to prioritise improvements to the inventory – only assesses the contribution of a source and its rate of change, and does not take into account the uncertainty in specific sources. As a result, it is difficult to decide where improvements are best delivered where two sources have similar emissions. Therefore, the method could be improved by taking into account the associated uncertainty.

Download the full report from the Committee on Climate Change

Read the supplementary information for the report

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Last Updated: 3 Jul 2017
Created: 28 Jun 2017

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