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A new scientific synthesis of European greenhouse gases

21 March 2023
A team of international researchers, with VU Amsterdam as a main participant, has carried out a synthesis of the emissions of greenhouse gases – methane and laughing gas – from human and natural sources in Europe. They found that emission reductions in the European Union and the United Kingdom show steady decreasing trends since 1990, that are consistent across the different methods, despite methodological uncertainties. The overall match with official reported estimates confirms the promising potential of atmospheric-based methods for evaluating and improving emissions inventories of methane, laughing gas and other greenhouse gasses.

The VU Amsterdam team, represented by Professor of Earth Sciences Sander Houweling and environmental researcher Roxana Petrescu, compared UNFCCC national reported greenhouse gas emissions from European Union’s Member States to estimates from scientific methods, including those of atmospheric observations. All results were published today in the journal Earth System Science Data.

Main findings
This study strengthens the conclusions of previous studies regarding the need for better quantification and allocation of human (anthropogenic) and natural sources and sinks of greenhouse gases (GHGs) in Europe. Improved GHG emission estimates of important sectoral activities (e.g., cattle farming for methane and fertilizer application for laughing gas) and natural sources (e.g., wetlands for methane) support emission mitigation policies, and thereby help reaching Europe’s climate targets. The accuracy of the inverse modeling technique is currently still limited due to the restricted availability of measurements and the density of the current measurement network. Petrescu: “We also want to inform policy makers that extensions of the surface observation network are needed to further improve the independent evaluation of GHG emission estimates on national as well as within country’s city-scales”

Annual updates
The plan is to update the data annually, similar to the annual assessments of the Global Carbon Project. Petrescu explains: “Our analysis shows what is officially reported by European countries and how these estimates relate to alternative methods (process modelling and inverse modelling) developed by the scientific community. Knowledge of GHG fluxes, their spatial and temporal variability and attribution to natural or anthropogenic sources is essential for monitoring emission mitigation efforts under the Paris Agreement and to inform its Global Stocktake. This, and previous studies conclude that anthropogenic emissions from agriculture, waste and land-use forestry are the most uncertain in all emission estimation methods. As the EU develops policies and procedures to limit its GHGs, these uncertainties should be further reduced”.

Different reporting method
All countries in the world report their GHGs annually to the Climate Secretariat of the United Nations Framework Convention on Climate Change (UNFCCC) in Bonn. This is implemented in accordance with the guidelines laid down by the Intergovernmental Panel on Climate Change (IPCC), which give countries the freedom, within limits, to choose their own methods and the degree of detail they wish to provide in their reports. These methods, called Tiers, make use of statistical information on certain activities that cause emissions, for example, the number of landfills or cows that emit methane. These so-called activity data can be determined reasonably accurate. The simplest method to translate activity data into emissions is to multiply the activity by an emission factor. Different emission factors may be chosen depending on the reporting Tier. A default value from the IPCC guidelines may be used that applies globally or a more representative value for a certain region within a country based on specific data or expert judgment.

For processes such as land-use, a better, but more complex, method is to model the geochemical processes based on physical knowledge and parameters that are region and time specific. Alternatively a so-called inverse modelling approach can be adopted, which makes use of atmospheric observations (e.g., from surface networks or satellites) to infer the emissions of a gas. This approach has large uncertainties, for example, due to uncertainties in the atmospheric transport model that is used. Furthermore, atmospheric measurements provide little process specific information, complicating the attribution of emissions to specific processes. The main challenge of the process modelling and inverse modelling approaches is to compare them ‘apples to apples’ to official country reports due to differences in temporal and spatial resolution, temporal variability and the definition of emission categories.

VERIFY, CoCO2 and follow-up projects
This analysis, carried out as part of the Horizon Europe 2020 funded VERIFY project, will continue in the Horizon Europe CoCO2 and AVENGERS projects. The VERIFY project integrated the knowledge from the research community, national inventories, and international organization, in a concerted effort to build an operational system for evaluating national GHG emission reports using independent observations. The goal was to monitor anthropogenic GHG emissions in support of the EU commitment to reduce its GHG emissions by 55 percent in 2030 compared to 1990 and reaching neutrality by 2050. In 2021, the European Commission approved a new project to prepare for a series of new satellites that are to be launched in the Copernicus (EC/ESA) initiative’s Sentinel programme in 2026. This EU funded project, CoCO2, will deliver a prototype monitoring system that makes use of data from a new European anthropogenic CO2 satellite mission (CO2M).

In 2020, the first scientific synthesis of European greenhouse gas emissions by Petrescu and colleagues was published. This was followed up by the 2021 scientific publications on methane, laughing gas and carbon dioxide (CO2) emissions and this study. The AVENGERS project, co-ordinated by VU Amsterdam and Houweling, allows Petrescu to continue her annual series of greenhouse gas syntheses.

Image: this illustration was created by Roxana Petrescu. Image credits: Horizon 2020 Framework Programme VERIFY project (grant no. 776810).