“Roughly 70 percent of the world’s greenhouse gas emissions comes from cities. These emissions are much more difficult to monitor in real time compared to other areas, such as forests or agricultural land. In close collaboration with cities around Europe, we have now gained a lot more knowledge about these emissions and how to measure them”, says Claudio D’Onofrio, project manager for ICOS Cities.
Greenhouse gas (GHG) inventories provide a comprehensive accounting of all GHG emissions and removals by an entity, such as a country, city, or company, over a set time period (typically a year). While traditional GHG inventories are the global standard for emission reporting, they face a 10-30% uncertainty margin and reporting lags of up to two years. This prevents city planners from seeing the immediate impact of climate policies. The ICOS Cities - PAUL project aimed to revolutionise urban climate governance by establishing high-density atmospheric sensor networks in European cities. By providing an evidence-based "ground truth" from monitoring the atmosphere, the project supports the 100 EU Mission Climate-Neutral and Smart Cities in moving from self-reported estimates to near-real-time, empirical verification of their net-zero progress.
ICOS Cities - PAUL’s core success is the development of a monitoring framework which verifies a city’s greenhouse gas emissions by combining traditional statistical inventory calculations with real-time atmospheric observations. Traditional emission reporting has been limited by a lag problem, where city planners were making 2026 decisions based on 2024 data. This two-year lag made it impossible to see if a specific climate-focused policy, such as a new low-emission zone, was working in real-time. To address this, the ICOS Cities - PAUL project successfully engineered a high-density sensor network that reduced reporting lags from years to weeks. This has allowed the project to offer cities rapid feedback on municipal climate action decisions. In cities such as Paris, the project’s physical sensors verified a reported 25% reduction in CO2 emissions, proving that the city’s climate efforts are paying off in concrete terms.
The high temporal and spatial resolution of the project’s sensor network helped to identify hotspots and leaks, and in cities like Munich, the system allowed the team to pinpoint previously unknown emission sources that were overseen by traditional statistical methods. Furthermore, the project also assisted cities in attributing emissions to specific sources and sectors.
Highlights
• Near-real-time verifiability: Reduced emission reporting lags from years to weeks, allowing for agile policy adjustments.
• High resolution: Achieved street-level data (100m–500m resolution) capable of identifying specific GHG hotspots and infrastructure leaks.
• Policy assessment: Enabled city planners to observe the impact of specific interventions, such as new low-emission zones or heating policies.
• Accessibility for all cities: Developed a downscaling tool that allows cities with limited resources to create their first emission inventory without extensive prior knowledge.
• Sector prioritisation: For the 15 European cities in the ICOS Cities network, the project developed city emission factsheets to help identify the biggest emission sectors (typically buildings, energy, and road transport), which often account for over 80% of urban emissions.
Outputs
ICOS-Cities PAUL has successfully established high-density networks of sensors to monitor atmospheric GHG emissions at the local scale Paris, Zurich, and Munich. Thanks to its experience with the pilots, the project has also has contributed to the development of World Meteorological Organisation (WMO) Urban Emission Guidelines - library.wmo.int, a new global standard for urban greenhouse gas monitoring co-authored by ICOS Cities scientists. For deeper insight, the project has published an interview with a Lead Author on its website - icos-cp.eu.
In addition, the project developed the report “Guidelines for European cities: a flexible and efficient approach for creating city emission inventories" - icos-cp.eu. This report provides a step-by-step roadmap for cities to build robust inventories regardless of their prior knowledge.
Impact
The measurement networks set up by ICOS-Cities PAUL help to verify GHGs reported by cities in fulfilment of national efforts for the European Climate Law - climate.ec.europa.eu. The law requires the EU and its Member States to report their annual GHG emissions to monitor alignment with global emission reduction obligations. The sensors help to prove whether cities are on track with the European target of a 55% reduction in GHG emissions by 2030. Furthermore, by reducing the margin for error in GHG inventories, the project has helped to build political and public confidence in net-zero policies. The project’s network of sensors improves GHG monitoring for sectors, such as urban transport and residential heating. The project makes creating a GHG inventory easily achievable for all European cities and empowers local stakeholders to make informed, data-driven decisions.
Surveys and teaching
The scope of the project goes beyond just measuring greenhouse gas concentrations, working with political and social scientists. Three surveys were done together with the European Social Survey (ESS), focusing on environmental attitudes in urban spaces.
“One result that I found particularly interesting from the surveys was that people had a higher trust in climate policies implemented in their city, compared to policies on a regional or national level”, says Claudio D’Onofrio.
“The respondents seem to believe that local governments are the most likely to take action on climate change since many environmental public policies are implemented at the city or municipal level. The residents are also aware of how these policies affect their day-to-day lives and, therefore, hold more confidence in their municipalities when it comes to environmental action,” explains Dr Diana Zavala-Rojas, Deputy Director (External relations) at the European Social Survey European Research Infrastructure Consortium (ESS ERIC).
Educational materials were also developed as part of the project and are now being used by teachers and pupils in primary school in Germany, Switzerland and France. Educational modules are available on the ICOS website – icos-cp.eu.
Lessons learned
• Atmospheric monitoring serves as a vital verification step that identifies where emissions inventories are accurate and where they require correction.
• The most effective strategy for cities is combining traditional ‘bottom-up’ activity data with ‘top-down’ atmospheric measurements.
• Standardising measurements via the WMO guidelines ensures that data from different European cities are comparable, creating a unified carbon map.
• By prioritising sectors, for example by targeting the top 80-95% of emitters, cities can achieve high-accuracy results without being overwhelmed by data collection.
All resources and guidelines that came out of the project are available on the ICOS Cities website - icos-cities.eu.