Multi-decade evaluation of the UK’s greenhouse gas emissions

Top-down evaluation of the UK’s methane, nitrous oxide and halogenated greenhouse gas emissions has been possible since the 1990s, firstly due to measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE), combined later with the UK national-scale Deriving Emissions linked to Climate Change (UK DECC) network. Here, we show that carbon dioxide-equivalent emissions inferred from observations of these gases have declined more slowly than stated in the UK’s National Inventory Report (NIR); a decline of approximately 50 Tg/yr CO2-e is found between 1990 and 2021, compared to approximately 100 Tg/yr CO2-e in the inventory. This difference, which is driven largely by a smaller-than-reported reduction in methane emissions, suggests that the UK may be approximately 3 years behind its stated progress toward net-zero. This paper will describe the evolution of greenhouse gas monitoring in the UK, including an overview of the first decade of national-scale emissions estimation from the UK DECC network. It will show how top-down emissions are calculated, and how atmospheric observation-based estimates are used, in close collaboration with inventory teams, to improve the national inventory. Finally, it will discuss the UK’s plans for a prototype “operational” emissions evaluation system, the Greenhouse gas Emissions Measurement and Modelling Advancement (GEMMA).