Guidance

Forthcoming change: the scope of international aviation and shipping greenhouse gas emissions to be reported in appraisals

Updated 5 November 2026

Description: supplementary guidance on the scope of international aviation and international shipping greenhouse gas emissions to be reported in appraisals

Unit: TAG unit A3 environmental impacts, TAG unit A5.2 aviation appraisal and value for money framework

Change announced: November 2026

Expected release date: December 2026

Description

This forthcoming change sets out updates to TAG unit A3 environmental impacts and TAG unit A5.2 aviation appraisal to include supplementary guidance on the treatment of international aviation and international shipping greenhouse gas emissions in appraisals.

This guidance will become definitive in November 2026 alongside a wider update to Department for Transport’s (DfT’s) value for money framework.

Detail

The Climate Change Act 2008 provides the UK’s legal framework for reducing greenhouse gas (GHG) emissions. Under the act, the UK is required to reduce its GHG emissions by at least 100% by 2050. To keep the UK on a pathway to achieving the 2050 target, the government is required to set and meet legally binding, 5-year caps on UK GHG emissions called carbon budgets.

In October 2025, the government confirmed that the UK’s share of international aviation and international shipping GHG emissions (IAIS emissions) will be included within the scope of UK GHG emissions for the sixth carbon budget (CB6) and all subsequent budgetary periods. Government has now enacted legislation to give effect to this policy.

The exact methodology for determining the UK’s share of IAIS emissions for the purposes of measuring and reporting UK GHG emissions from CB6 onwards will be decided before laying the first annual statement of UK emissions for the CB6 period.

In advance of this decision, for carbon budget planning purposes, only the GHG emissions from UK departing flights and 50% of the GHG emissions produced on maritime journeys between a UK port and a non-UK port are treated as UK GHG emissions. This is the analytical approach that was used in the government’s Carbon budget and growth delivery plan.

TAG guidance recommends that all GHG emissions caused by policy proposals are valued in appraisal, regardless of whether they affect GHG emissions in the UK or overseas. To date however, much of the departmental GHG analysis for international aviation and international shipping has been undertaken for carbon budget planning purposes and has therefore focused on the UK’s share of IAIS emissions, with relatively few appraisals carried out.

To align TAG guidance with the approach taken for carbon budget planning purposes, this forthcoming change recommends that, for international aviation and international shipping, an additional carbon metric should be presented for the impacts on UK GHG emissions. That is, the impacts on the GHG emissions from UK departing flights and 50% of the GHG emissions produced on maritime journeys between a UK port and a non-UK port.

This is intended to be supplementary information for decision-makers on how schemes contribute to current UK carbon budgets. This is an additional metric which does not affect core value for money (VfM) analysis, which remains global in scope.

A wider update to the DfT’s value for money framework will be published to reflect this and other updates in November 2026.

This additional metric has already been included in analysis published in the Heathrow Expansion Appraisal Report, to take into account this updated position and reflect the differential impacts caused by Heathrow expansion to the UK carbon account and global emissions.

Forthcoming updates

Edits to TAG unit A3

Introduction of text below between current paragraphs 4.1.12 and 4.1.13

Treatment of international aviation and international shipping

The UK’s share of international aviation and international shipping emissions (IAIS emissions) will be included within the scope of UK greenhouse gas (GHG) emissions for the sixth carbon budget (CB6) and all subsequent budgetary periods. The exact methodology for determining the UK’s share of IAIS emissions for the purposes of measuring and reporting emissions from CB6 onwards will be decided before laying the first annual statement of UK emissions for the CB6 period.

In advance of this decision, for carbon budget planning purposes, only the GHG emissions from UK departing flights and 50% of the GHG emissions produced on maritime journeys between a UK port and a non-UK port should be treated as UK GHG emissions. This approach is aligned with the analytical approach used in the government’s carbon budget and growth delivery plan.

However, the effects of climate change are global. In principle therefore, the full GHG emissions impact of a transport intervention should be appraised when undertaking social cost benefit analysis, regardless of where emissions occur. The 2026 HMT Green Book states that “Practitioners should consider whether a proposal might have a significant impact on emissions produced abroad”.^{[footnote 1]}

Where a transport intervention results in a net change in GHG emissions outside the scope of UK GHG emissions, it is advisable to present both the impact on UK GHG emissions and the impact on global GHG emissions. Appraisals, where proportionate, should present the quantified and monetised changes in UK GHG emissions and global GHG emissions separately. However, value for money metrics and assessments should be solely based on the impact on global GHG emissions.^{[footnote 2]}

For interventions that result in a net change in the GHG emissions from international aviation or international shipping outside the scope of UK GHG emissions, where feasible and proportionate, appraisers should produce 2 separate carbon metrics:

1. Reflecting the impacts on total, global GHG emissions: this should include all impacts on the GHG emissions from flights departing and arriving in the UK; all impacts on the GHG emissions produced on maritime journeys between a UK port and a non-UK port; as well as any wider GHG emission impacts.

2. Reflecting the impacts on UK GHG emissions only: this should include impacts on international aviation GHG emissions for UK departing flights only (i.e. excluding UK international arrivals); and only include 50% of impacts on the GHG emissions produced on maritime journeys between a UK port and a non-UK port.

In both cases, any changes in the GHG emissions from domestic aviation and domestic shipping should be included in the metric.

Edits to TAG unit A5.2

Bullet point under paragraph 3.3.3 edited:

For the appraisal of greenhouse gas (GHG) impacts for international aviation, as set out in TAG A3 section 4, where feasible, appraisers should present 2 separate carbon metrics:

1. Reflecting the impacts on total, global GHG emissions: this should include all impacts on the GHG emissions from flights departing and arriving in the UK as well as any wider GHG emission impacts.

2. Reflecting the impacts on UK GHG emissions only (in carbon budget and net zero legislation terms): this should include impacts on international aviation GHG emissions for UK departing flights only (i.e. excluding UK international arrivals).

In some circumstances, there may be evidence that a UK policy or scheme has displaced emissions from other geographies or elsewhere within the sector. Where proportionate and possible to do so, these changes in GHG emissions and associated levels of displacement should be considered in the appraisal.

If significant uncertainties exist and displacement cannot be quantified with reasonable levels of confidence, either a qualitative assessment should be made or a quantitative assessment through a sensitivity test.

Accounting for this global displacement is appropriate for carbon appraisal and VfM assessment (which reflects the impacts on total, global GHG emissions). However, for the UK-only carbon metric, where we only consider GHG emissions impacts according to the UK’s emissions accounting framework, displacement from foreign countries does not need to be adjusted for.

Contact

For further information on this guidance update, please contact:

Transport Appraisal and Strategic Modelling (TASM) Division
Department for Transport
First floor, Great Minster House
33 Horseferry Road
London
SW1P 4DR

tasm@dft.gov.uk

1. Paragraph 8.72 The Green Book – UK government guidance on appraisal ↩

2. Please contact GHGappraisal@energysecurity.gov.uk and tasm@dft.gov.uk for advice on carbon appraisal. ↩