FOX-UK: Fossil-fuel CO2 emissions from OXygen measurements in the UK

To reduce our carbon dioxide emissions from fossil fuels (known as ffCO2), we need to know how much we are emitting accurately and quickly. Currently, ffCO2 emissions are reported with a ‘bottom-up’ approach, using accounting methods that combine emission factors with energy statistics, compiling the data into national emissions inventories. But these inventories are often inaccurate, especially for countries with emerging economies (uncertainties of ~10%; Friedlingstein et al., Global Carbon Budget, 2023). In the UK, the national CO2 inventory is quite sophisticated. The Department for Energy Security and Net Zero (DESNZ) have stated that the UK national CO2 inventory is accurate to within a few percent at the country scale on a yearly basis (DESNZ, 2024). But it takes two years for the UK’s annual CO2 inventory to be calculated, and the uncertainty of emissions increases at finer spatiotemporal scales. This makes it hard to monitor the effectiveness of new environmental policies or to mitigate the impacts of unforeseen events, such as war or economic recession, on CO2 emissions. A ‘top-down’ approach The UK also uses a ‘top-down’ approach to calculate some greenhouse gas emissions – a method that combines atmospheric measurements and ‘inverse’ transport modelling (i.e., assimilating atmospheric concentration data into models to infer surface-level emissions) – which has the potential to provide more frequent emission estimates and at finer spatial scales than bottom-up inventories. While the top-down method is successful for methane, nitrous oxide and some hydrofluorocarbons, it has not been possible to develop a top-down approach for ffCO2, for two reasons: 1) ffCO2 emissions are hard to detect in the atmosphere, due to large emissions of CO2 to and from land plants, meaning that additional measurements of ‘atmospheric tracers’, which can distinguish ffCO2 from plant CO2, are required; and 2) there are currently insufficient tracer measurements to quantify ffCO2 using top-down methods: in the UK, there are not enough tracer measurements to determine emissions from Scotland, Northern Ireland and the north of England, preventing a country-wide top-down assessment. What will the FOX-UK project deliver? FOX-UK will address this critical knowledge gap using atmospheric measurements of oxygen (O2), an emerging tracer for ffCO2 (Pickers et al., 2022). Our UK-based study will produce the first top-down assessment of ffCO2 in the UK, and become a blueprint for using a top-down O2-based method more widely, such as in emerging economies, where top-down methods have the potential to outperform inventory approaches on timeliness, cost-effectiveness, and accuracy. FOX-UK will: Establish a new atmospheric O2 measurement site in Scotland, filling a critical data gap in ffCO2 tracer data coverage in the UK. Produce the world’s first high-density ffCO2 tracer dataset, based on in-situ atmospheric O2 measurements from multiple sites, for country and sub-country scale top-down estimates. Calculate high-frequency UK ffCO2 emissions at country and sub-country scales by carrying out state-of-the-art inverse atmospheric transport modelling, using a multi-model approach. Why should we do this now? The 2020s is a critical decade for curbing ffCO2 emissions if we are to avoid global warming in excess of 2°C. Successful implementation of the Paris Agreement relies on countries’ emissions being accurately known and immediately available, as countries work towards implementing net-zero emission policies. We need reliable ffCO2 emissions estimates quickly and at policy-relevant, sub-national scales.