Accurate and transparent reporting of greenhouse gas (GHG) emissions is a cornerstone of international climate agreements, particularly under the Paris Agreement’s Enhanced Transparency Framework (ETF). Each country must compile a national greenhouse gas inventory (NGHGI), usually based on “bottom-up” methods that calculate emissions from activity data (such as fuel use, land use, and industrial activity) combined with emission factors.

While these inventories are essential for tracking progress and guiding policy, they are not without limitations. To strengthen their robustness, scientific initiatives are developing complementary methods that draw on atmospheric observations.

One such initiative is the PARIS Project, a European effort funded under the EU Horizon programme. It responds to the call on “Verification and reconciliation of estimates of climate forcers” by combining bottom-up inventories with “top-down” approaches such as inverse modelling. These methods use measurements of atmospheric GHG concentrations to infer emissions at the surface, offering an additional line of evidence that can support and improve national reporting.

Bottom-Up Inventories: Strengths and Challenges

National Inventory Teams (NITs) compile greenhouse gas inventories primarily through bottom-up approaches. These methods are well established, internationally guided, and form the official basis for reporting to the United Nations Framework Convention on Climate Change (UNFCCC).

However, like all statistical methods, they face some challenges:

• Incomplete or uncertain activity data in certain sectors.
• Complex processes, such as land-use change or fugitive emissions, that are difficult to quantify precisely.
• Methodological differences across countries and time periods, which can make direct comparisons less straightforward.

Despite these limitations, bottom-up inventories remain the foundation of national climate reporting and are critical for shaping mitigation strategies and fulfilling international commitments.

Top-Down Approaches and Inverse Modelling

Top-down approaches start with the atmosphere itself. By measuring concentrations of greenhouse gases from satellites and ground-based networks, scientists apply inverse modelling to infer the underlying emissions.

This approach offers key benefits:

• It provides spatially integrated estimates across large regions.
• It can help identify sources that may be underestimated or missing in bottom-up inventories.
• It relies directly on atmospheric measurements, making it less dependent on national activity data or emission factors.

Top-down methods are not a replacement for inventories. They also come with uncertainties — for example, related to atmospheric transport models or the resolution of observational data. But when combined with bottom-up methods, they create a more complete picture, reducing uncertainties overall and strengthening confidence in reported emissions.

The Paris Project: Linking Science, Policy, and Practice

The Paris Project aims to make top-down methods more accessible and relevant for national reporting. It brings together inventory compilers, atmospheric scientists, government agencies, and non-governmental organizations (NGOs) to exchange knowledge and apply inverse modelling in practical contexts.

Its main contributions include:

1. Capacity Development – Building understanding of inverse modelling techniques among inventory teams.
2. Collaboration – Supporting case studies in specific countries and regions, focusing on key gases such as methane (CH₄), nitrous oxide (N₂O), and fluorinated gases (F-gases).
3. Access to Tools and Data – Sharing outputs from inverse modelling through open platforms and partnerships.
4. Reconciliation Frameworks – Developing systematic approaches for comparing and combining bottom-up and top-down estimates, documented through annexes to National Inventory Reports.

Reconciling Different Approaches

Reconciliation between bottom-up and top-down estimates involves comparing results, identifying discrepancies, and understanding their causes. For example:

• Bottom-up inventories may lack updated data for certain sectors.
• Top-down models may need finer regional calibration.
• Additional field studies can improve both approaches.

Rather than highlighting weaknesses, reconciliation is a process of continuous improvement. It helps refine data, strengthens transparency, and provides independent verification that increases trust in reported emissions.

Progress to Date

The Paris Project has already delivered draft annexes for several European countries’ National Inventory Reports and initiated stronger dialogue between researchers, policymakers, inventory teams, and NGOs. These efforts are laying the groundwork for integrating atmospheric science into regular reporting.

Looking Forward

Combining bottom-up and top-down methods marks an important step in monitoring, reporting, and verifying greenhouse gases. By drawing on both statistical data and atmospheric observations, countries can develop inventories that are more transparent, comparable, and scientifically robust.

As climate reporting under the Paris Agreement evolves, collaboration across disciplines will remain essential. The Paris Project demonstrates how science and policy can work hand in hand, providing tools that help countries not only meet international requirements but also strengthen the evidence base for effective climate action.