Summary
Current and future changes in the Earth’s climate are driven by changes in atmospheric concentrations of radiatively important gases (greenhouse gases [GHGs]) and particles (also called aerosols)—collectively referred to as GHGs in this report. Understanding the emissions of GHGs from human-related (i.e., anthropogenic) and natural sources is critical for national, regional, local, and corporate decision makers seeking to reduce GHG emissions and thwart the most disastrous impacts of climate change. Since 2015, when nations adopted the Paris Agreement to limit global temperature rise this century, governments and the private sector have made commitments to reduce GHG emissions. More than 136 countries, accounting for about 80 percent of total global GHG emissions, have committed to achieving net-zero GHG emissions. GHG emissions information is already being used to plan, track, and assess GHG reduction targets and pledges, and holds the potential to help users make informed mitigation decisions and increase public awareness and accountability.
Three converging trends motivated this report: (1) rapidly increasing demand from a range of users for trusted information about GHG emissions across multiple sectors and geographic scales; (2) development of many new approaches for quantifying GHG emissions that aim to address this increasing demand; and (3) a growing and rapidly evolving institutional landscape, including public, private, and academic entities seeking to provide better GHG emissions information. These converging trends motivated a need for basic criteria or principles that users and decision makers could use when evaluating different types of GHG emissions information.
In this study, the Committee examines existing and emerging approaches used to generate and evaluate anthropogenic GHG emissions information at global to local scales. Ultimately, this report develops a framework for evaluating GHG emissions information to support and provide guidance for policy makers about the use of GHG emissions information in decision making. The Committee’s complete Statement of Task is provided in Box S-1.
Approaches for Quantifying Anthropogenic Greenhouse Gas Emissions
Anthropogenic contributions to the current warming trend globally are driven mainly by the well-mixed GHGs (carbon dioxide [CO2], methane [CH4], nitrous oxide [N2O], fluorinated gases), which account for about 82 percent of present-day warming. About 12 percent of warming comes from non-methane volatile organic compounds and carbon monoxide, which can increase concentrations of atmospheric ozone. About 5 percent of warming is from black carbon. Quantifying emissions of these gases (and particles) is challenging because there are many types of sources and removal processes that have different characteristics, vary over time and space, and encompass countless individual emitters. Some emissions can be measured directly at their source, such as power plants or industrial facilities, whereas other sources are more distributed in space, requiring that emissions be estimated or inferred from other data.
GHG inventories are tools that quantify GHG emissions (or removals) often divided into economic and industrial sectors for a specific place and time. They are developed and used by a range of stakeholders including policy makers, the scientific community, businesses, media, nongovernmental organizations, and the general public. GHG inventories allow policy makers to identify key GHG-emitting sectors and make informed decisions by setting emission baselines, tracking emission changes over time, and assessing emission mitigation efforts. GHG inventories are constructed using a wide range of approaches:
- Activity-based approaches (often referred to as “bottom-up” approaches) generally utilize activity data—a term referring to representative indicators or drivers of GHG emissions such as fuel consumption statistics, traffic counts, population, or land area. To achieve an estimate of GHG emissions, these activity measures are transformed using a conversion factor such as an emission factor—the emission or removal of a GHG per unit of activity. For example, the miles of natural gas pipelines would be multiplied by an emission factor representing the methane emissions per mile of natural gas to quantify emissions from a natural gas distribution pipeline system.
- Atmospheric-based approaches (often referred to as “top-down” approaches) use atmospheric measurements of GHGs and an understanding of atmospheric transport and chemical processes to infer information on GHG fluxes (emissions and sinks). Surface-, aircraft-, and space-based observations are combined with analysis approaches and models to transform measurements of atmospheric concentrations into estimates of emissions. For GHGs that have both anthropogenic and natural sources, and/or overlapping sources in space and time, attributing estimated emissions to a particular sector or source may require more complex analysis.
- Hybrid approaches generate GHG emissions information through the combination and more complete integration of activity- and atmospheric-based approaches, and/or other data sources, data assimilation, or emerging digital technologies (Figure S-1). For example, an activity-based approach using multiple overlapping core datasets could be further constrained by atmospheric-based estimates. Hybrid approaches are nascent and hold the possibility of combining multiple measurement streams and atmospheric- and activity-based approaches to produce more complete and accurate estimates of GHG emissions and sinks.
Data and information products developed using the above approaches have been widely utilized by the scientific and regulatory communities to support emissions reporting, but a number
of challenges have limited the usefulness of emissions information to support decision making. For example, activity-based approaches depend on activity data and emission factors that may be out of date or may not be appropriately representative for the system boundary of interest. One of the most widely relied on atmospheric-based approaches, atmospheric inverse modeling, still faces limitations in attributing fluxes to specific sources or sectors to inform mitigation decisions at relevant spatial and temporal scales. While the proliferation of “big environmental data” has considerably advanced the spatiotemporal coverage of atmospheric information, interoperability and transparency are key challenges that can limit the utility of this information, and more research and development are needed to incorporate these data into hybrid approaches.
Framework for Evaluating Greenhouse Gas Emissions Information
As more GHG emissions information becomes available and as more decision makers use this information, a common evaluation framework can help users determine what information products best meet their needs and understand the limitations of that information. A common framework can also provide guidance to researchers for designing more useful and trusted data and information. The Committee has identified six criteria or “pillars” that form a common framework to evaluate current and future GHG emissions information:
- usability and timeliness: information is comparable and responsive to decision-maker needs and available on time scales relevant to decision making;
- information transparency: information is both publicly available and traceable by anyone;
- evaluation and validation: review, assessment, and comparison to independent datasets;
- completeness: comprehensive spatial and temporal coverage of GHG emissions information for the relevant geographic boundary;
- inclusivity: who is involved in GHG emissions information creation and who is covered by the information; and
- communication: methodologies and assumptions are described in understandable forms, well documented, and openly accessible.
In developing this framework and these pillars the Committee seeks to initiate a discussion on the approach and criteria. One of the challenges in developing a framework for assessment is recognizing the many different scales, drivers, and intended uses for GHG emissions information. Figure S-2 summarizes how current capabilities of the three approaches, described above, generally perform relative to the pillars. The evaluation in Figure S-2 focuses on national to global scale GHG emissions information. The qualitative rankings are useful to compare the approaches to each other to identify strengths and opportunities for improvement. Overall, the performance of activity-based approaches spans a wide range but the data tend to be weaker for evaluation and validation. Inclusivity is generally lower, though governmental activity-based inventories tend to perform better for inclusivity and communication. Atmospheric-based and hybrid approaches also tend to rank low for inclusivity and communication, but higher for evaluation and validation. There is room to improve usability and timeliness across all approaches, information transparency for hybrid approaches, and completeness for activity- and atmospheric-based approaches. However, the performance of any given approach will vary depending on the design, implementation, objectives of the effort, and the needs of the information users, as explored in specific case studies in the report.
Drawing on the assessment of activity-based, atmospheric-based, and hybrid approaches against the pillars, the Committee identified needed improvements in current GHG emissions information development capabilities, as described in the following sections. The Committee also
recommends striving for hybrid approaches that optimize the integration of individual activity- and atmospheric-based approaches, as a way to provide the best available comprehensive GHG emissions information for users. The Committee’s recommendations are intended to apply to the full spectrum of spatial scales relevant to users of GHG information—global, national, regional, local, and facility—and provide clear directions applicable to multiple audiences, empowering different entities to identify specific implementation steps within their means and to meet their needs.
Greenhouse gas emissions information development and evaluation should strive to align with the six pillars: usability and timeliness, information transparency, evaluation and validation, completeness, inclusivity, and communication.
The “pillars” introduced above provide a way to evaluate individual emissions datasets and approaches. Thus, the pillars provide guidance for improving GHG emissions information development and products. These same pillars embody the desired attributes for the institutions that develop GHG emissions information and the broader aspirations for the global, multiscale endeavor of understanding the sources and sinks of GHGs. The application of the six pillars of the framework to both individual datasets and approaches as well as the structures that support the development, provision, and exchange of GHG emissions information would advance the current complex GHG emissions information landscape toward one that could more comprehensively meet the needs of users and decision makers. Strengthening GHG emissions information to satisfy these pillars at local and subnational levels would help to build international coordination and support.
Advancing Greenhouse Gas Emissions Information Capabilities, Trust, and Accessibility
Greenhouse gas emissions information should be better coordinated (e.g., through the creation of a coordinated repository or federation of repositories) across the global community, enabling adherence to a set of minimum common pillar attributes.
A coordinated repository or federation of repositories where GHG emissions information can be hosted, documented, and clearly characterized would be a critical step forward in maximizing use and understanding of GHG emissions data products. A mechanism that brings different types of information together could facilitate the integration of multiple types of data at various spatial scales and make the information accessible to decision makers and users in ways that meet their needs. Such a clearinghouse or federated data center could establish standards and practices, aligned with the pillars, which would enable decision makers and other data users to clearly and quickly grasp individual characteristics and quality of the wide range of GHG emissions information. To maximize adoption and equity, such a coordinated effort should support and integrate information from existing national and international programs to leverage, rather than replace, both established and emerging efforts.
Critical characteristics and functions of a coordinated repository or clearinghouse would operationalize each of the six pillars by including
- Timely information that is transparent and traceable to primary, supporting, and derived datasets;
- Standardization of data formats and metadata to facilitate comparability and interpretability across scales;
- Descriptive documentation of models and estimation procedures in nontechnical language, and in multiple languages, that can be used to enable the use of diverse information, including novel observations and methods from the research community;
- Qualitative (e.g., caveats and limitations) and quantitative (e.g., uncertainties, error characterization) evaluation metrics;
- Databases of key input data and information (e.g., emission factors, activity data, atmospheric observations, models) that would be regularly updated and widely accessible to facilitate information exchange;
- Governance mechanisms that are coordinated, trusted, and designed to be inclusive of the global community and built on the best practices of data governance and information quality;
- Education modules and capacity building for using GHG emissions information and contributing data and estimation results; and
- Mechanisms to support stronger collaborations between GHG, air quality, and meteorological science communities and stakeholders.
Greenhouse gas emissions information providers should clearly communicate underlying data, methods, and associated uncertainties.
While the information clearinghouse or federated repository effort described above would be a longer-term undertaking for the global community, actionable steps can be taken by data providers in the short term to enhance the transparency of GHG emissions information where feasible. Focused resource allocation or government purchases aimed at bringing data and methods into the public domain with standards on transparency and open principles (e.g., FAIR data principles [findable, accessible, interoperable, and reusable]) could have substantial near-term impact on the utility of GHG emissions information. By following many of the same guidelines outlined for a clearinghouse and aligning with the pillars, data providers have the opportunity to facilitate comparability and verification of their data and methods to foster trust between information providers and users.
Addressing Key Data and Information Gaps and Uncertainties
Greenhouse gas emissions information (e.g., observations, data analysis, activity data, emission factors) development at more granular temporal and spatial scales with source-level detail should be accelerated to meet the rapidly increasing needs of cities, states, and provinces for managing their emissions.
Cities, states, provinces, landowners, and the business community, among others, are organizing collectively, enacting mitigation policies, and are in critical need of consistent, standardized, trusted GHG emissions information. Where possible, there are substantial gains associated with extending GHG emissions information development to finer space and time scales, enhancing completeness. Currently, data available on granular spatial and temporal scales are insufficient and there is a need to expand the necessary data resources; this includes activity data, emission factors, and atmospheric observations. Furthermore, enhancing source-level detail—for example by characterizing the entire distribution of emission sources—would strengthen completeness of GHG emissions information and better inform mitigation decisions.
The accuracy and representativeness of all underlying data used to estimate greenhouse gas emissions (e.g., emission factors, activity data) should be further improved.
Many of the data elements, observations, and models used to estimate GHG emissions rely, sometimes by necessity, on large spatial averages or averages that represent well-observed or high-capacity parts of the globe. The Committee recognizes the need to improve both the specific representativeness and resolution across the globe of these key underlying data drivers to strengthen the completeness and accuracy of GHG emissions information. Examples include emission factors that are often fuel averages (i.e., missing true coal quality variation or biomass variation) or tied to countries with well-quantified fuel characteristics; activity data collected for particular countries but used to calculate emissions for other countries even if the data are unrepresentative; and atmospheric monitoring that may only reflect large-scale integration of information or is biased to locations with high scientific capacity.
Operationalizing Current Capabilities
Greenhouse gas emissions estimation research efforts should transition with urgency to operational capabilities with institutions to maintain and ensure longevity.
As the urgency to immediately reduce GHG emissions is increasing, decision makers likewise need the best-available, comprehensive information about emissions as soon as possible. The current timeline and processes to operationalize new data, technologies, or approaches to enable decision-useful strategies is misaligned to meet the above climate goals in a timely manner. Accelerating the transition of research to operations will require scientists, research funders, and data users to identify ways to lower existing barriers to that transition and ways to make new data products more immediately usable. The clearinghouse and other coordination mechanisms recommended above, along with alignment with the pillars, should help make new GHG emissions information usable more quickly.
Some of the approaches for generating GHG emissions information that have been developed within the scientific community are, or are approaching, a level of readiness that would allow for operationalization, resulting in consistent, comprehensive, reliable information that could be better embraced and exploited by decision makers. Much like weather forecasting systems, optimal approaches will combine a more complex suite of observed data to drive optimal estimates of GHG
emissions. Iteration between scientists and decision makers is needed to prioritize the continuity and deployment of observing systems and to ensure these approaches are integrated with the decision-making process.
Striving for Hybrid Approaches
Greenhouse gas emissions data collection, modeling, and information development should be designed and implemented to enable a fuller integration and “hybridization” of information and approaches.
Most of the current GHG inventory and information development to date has tended to use single methods or approaches with single-technique observations or data sources. While this was warranted during the development of many of the state-of-the-art estimation techniques, going forward, a “cross-technique” or hybridization of (traditional) approaches and datasets would provide more accurate and comprehensive GHG emissions information by integrating different types of information with more granularity. Some of this work has begun and includes data assimilation and data fusion as well as new machine learning and other nonparametric numerical techniques that leverage new data from private and public satellites, sensors, and other types of activities. Greater synergy between air quality, meteorology, and GHG data collection and analysis efforts would facilitate the development of these hybrid approaches.
Efforts that more fully integrate the traditional activity- and atmospheric-based approaches present a path toward a more complete, complementary approach that overcomes gaps and weaknesses in each approach when used in isolation. Enabling the development of greater integration and hybrid approaches would require designing data collection to fill the most needed gaps. To strive for hybridization is to holistically improve GHG monitoring across scales, approaches, and capacity. Hybrid approaches that mix complementary approaches, datasets, and models and that integrate the needs of end users promise to offer richer, more usable data outputs for decision making.
Ensuring Usability, Timeliness, and Effective Communication of Greenhouse Gas Emissions Information
Greenhouse gas emissions information generators, decision makers, and global stakeholders should engage in an iterative process in a timely manner to ensure the information provided is relevant and useful.
Incorporating decision maker input is critical for information developed to respond to the policy needs of stakeholders and decision makers. The time lag to integrate relevant findings from new research into developing empirical- or measurement-informed inventories limits development and execution of sound mitigation policy, and delays transmittal of appropriate market signals for investments and technology development related to mitigation programs by various stakeholders. Usability and timeliness of GHG emissions information can be enhanced if data producers and users engage in an iterative process, which the clearinghouse or federated repository could support, to facilitate investments in systems that are focused on providing decision support and responsive to an evolving policy-making landscape.
As more users are utilizing and communicating information about GHG emissions, and as more data sources become available, clear expectations about how to evaluate information will help build literacy and shared understanding. Drawing on the pillars in the Committee’s framework, Box S-2
outlines key questions that users and communicators of GHG emissions information should pose as they consider how to interpret and use the data.
This report provides a framework for evaluating anthropogenic GHG emissions information that can be adapted as information systems become more complex and to serve a range of decision-making needs. Case studies are provided in the report for several existing approaches to GHG emissions information development, including discussion of advantages and areas for potential improvements. Although it was not the focus of this report, much of the discussion on evaluation and inventories could also be applied to removal processes.
The Committee has made a number of recommendations toward advancing the accuracy and usability of GHG emissions information. By examining existing capabilities and future directions, the hope of the Committee is that this report will help push the global community forward in GHG mitigation decision-making processes.
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