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Introduction
Climate change is the most serious environmental challenge currently confronting society. The ongoing COVID-19 pandemic has tested global resolve to address and mitigate threats to the planet and society. At the same time, the pandemic has shown how science and evidence-based policies can help us confront major threats to human well-being. Current and future changes in the Earth’s climate depend extensively on changes in atmospheric concentrations of radiatively important gases (greenhouse gases [GHGs]) and particles (also called aerosols). In turn, the concentrations of these gases and particles—collectively referred to as GHGs in this report—are driven by natural and human-related (anthropogenic) emissions and by largely natural removal processes.
Anthropogenic emissions have resulted in large changes in the concentrations of GHGs. Carbon dioxide (CO2) has the largest impact on current and projected climate and is long-lived in the atmosphere, meaning that human-related emissions of CO2 cause increases in atmospheric CO2 concentrations lasting decades to millennia. Nitrous oxide (N2O) and various fluorinated gases are also important long-lived GHGs. Methane (CH4) is another major GHG but has a shorter atmospheric lifetime of ~10 years. The methane lifetime is still longer than the time scales of atmospheric mixing so it is relatively well mixed in the troposphere, as are the other long-lived GHGs. As discussed below, anthropogenic emissions of shorter-lived radiatively important gases and aerosols (also called short-lived climate forcers) are also important in determining future climate changes (see Figure 1-1).
GHG emissions information is being used to plan, track, and assess national, subnational, local, and corporate emissions and emission mitigation efforts, often in the context of GHG reduction targets and pledges. In this study, the Committee examines existing and emerging approaches used to generate and evaluate anthropogenic GHG emissions information. 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 1-1.
In this report, we will make a distinction between GHG emissions information, GHG inventories, and GHG emissions (see Box 1-2). GHG emissions are the sum of emission or removal values of GHGs. GHG emissions inventories are tools to estimate emissions, often by individual sectors for a specific period of time. GHG emissions inventories can be thought of as a primary subset of GHG emissions information. We define GHG emissions information in this report to include a compendium of emissions-related information content beyond what is often traditionally included in most GHG inventories. GHG emissions information, therefore, might include data used to calculate emissions, an archive of emission factors, or additional attributes of emitters such as ownership, spatial extent, technological characteristics, and other factors such as sociodemographics.
Quantifying Greenhouse Gas Emissions
GHG emissions can be estimated and presented in many ways: by individual GHG, source (e.g., fossil fuel use), geographical region (e.g., city, province, country), economic sector (e.g., transportation, food), or individual emitting infrastructure (e.g., facilities, buildings, roads). Evaluation of GHG emissions information can help determine which policies, technologies, or other interventions could be most effective at reducing emissions. For example, Figure 1-2 shows global GHG emissions (for the well-mixed gases, excluding halocarbons covered by the Montreal Protocol and GHG precursors and aerosols) by gas from 1990 to 2019. It shows that when using 100-year global warming potentials, CO2 from fossil fuel use and industry contributes about 64 percent of total global emissions and methane emissions account for 18 percent of total global GHG emissions as of 2019, making these sources a priority for reducing emissions. In general, the emissions of CO2 from fossil fuel burning are well known, whereas there is more uncertainty associated with other sources and gases (as shown by the error bars in the right panel of Figure 1-2).
Anthropogenic GHG emissions are receiving increasing attention, particularly at the national, subnational, and corporate levels, driven by the needs of decision makers to effectively plan and achieve emissions reduction commitments. The scientific community has actively studied GHG emissions and atmospheric concentrations to better understand climate forcing and biogeochemical cycles, examining sources and sinks of GHGs that are both anthropogenic and natural. GHG emissions are highly related to the parallel challenges of air pollution, biodiversity loss, and other convergent issues for decision makers and scientists (Box 1-3).
Decision makers need clear, timely, reliable, and complete information about emissions (and removals) that contribute to climate change in order to develop policies that address societal concerns and direct or indirect health and economic impacts. Improving the characterization of anthropogenic GHG emissions could improve predictions of climate change and its impacts, inform decision making at all levels, and enable a more focused and rigorous global response (Gurney and Shepson, 2021). GHG inventories—tools that quantify GHG emission and removal totals or emissions by economic and industrial sectors for a specific place and time—have been constructed for local, corporate, and national-scale decision making. GHG inventories have largely been constructed using activity-based approaches, often referred to as bottom-up analyses (see Box 1-2). Much of this GHG inventory development has been carried out by policy practitioners adhering to standard protocols and approaches. In recent years, the scientific community has developed a wider variety of activity-based approaches and methodologies availing of new data and modeling algorithms (e.g., Bun et al., 2019; Gurney et al., 2009). Examples include the use of “big data” and machine learning, often incorporating very high-resolution imagery and other highly granular techniques (Gurney et al., 2019; Kaack et al., 2022; Rolnick et al., 2022). However, novel approaches from the research community have had little uptake by the decision-making community, due to a number of challenges, described below.
While activity-based analyses are critically important, there have been challenges and limited focus on how information from atmospheric observations can be integrated to further improve the characterization of the GHG emissions and sinks on national, subnational, and local scales to inform specific mitigation strategies. Monitoring and high-resolution models of GHG concentrations in the atmosphere offer the potential to complement and further constrain emissions and budgets independent of activity-based data sources. Most importantly, this atmospheric-based (“top-down”) approach to assessing GHG emissions can be integrated with advanced activity-based approaches to potentially provide a more accurate, actionable, and policy-relevant view of GHG emissions relative to changing concentrations of GHGs worldwide.
Defining Anthropogenic Greenhouse Gases for This Study
The most significant anthropogenic contributions to present-day temperature forcing are the major well-mixed gases (CO2, CH4, N2O, fluorinated gases), which account for about 82 percent of present-day warming, or 1.49°C (0.87–2.45°C) out of 1.83°C (0.90–3.12°C), compared to pre-industrial temperatures (IPCC, 2021; see Figure 1-3). Approximately 12 percent of warming is due to emissions of precursor gases and particles: non-methane volatile organic compounds and carbon monoxide (CO), especially because of their effects on the concentration of atmospheric ozone (O3), and 5 percent due to black carbon emissions. Importantly, about 0.79°C (0.13–1.55°C) of present-day warming is masked by reflective aerosols and aerosol precursors (SO2, NOx, organic carbon) and changes in albedo due to land use and irrigation. Definitions of radiatively important gases, particles, and related terms are provided in Box 1-4 and Appendix A.
Future radiative forcing will depend on shorter-lived forcers, including gases and particles, as well as well-mixed greenhouse gases; thus, the definition of GHGs in this report encompasses all
of these forcers. Emissions of short-lived climate forcers have important implications for mitigation strategies and limiting warming in the near term to below 1.5°C with no or limited overshoot (e.g., Dreyfus et al., 2022; Dvorak et al., 2022). For example, a recent analysis of satellite and other records shows that net aerosol forcing since 2000 has reversed sign from negative to positive and is contributing to accelerating warming (Quaas et al., 2022). Some of these gases and particles have been the focus of air quality emission inventories but have largely not been included in GHG emission inventories. Further consideration of shorter-lived forcers in GHG emissions information may be needed. Additional gases, particles, or indirect GHGs—for example, hydrogen, which can increase concentrations of other GHGs—may also warrant consideration in future GHG inventories if there are sufficient human-related emissions.
A category that may be challenging from the perspective of decision makers is managed vegetation. In general, for mitigation purposes, GHG emissions from human-managed ecosystems have been considered similarly to fossil fuel-related emissions because they are both a consequence of human activities. Managed vegetation refers to planted areas, such as crops, forests, and urban landscapes. Since national and regional mitigation planning is also accounting for these sectors,
careful consideration is needed to develop accurate estimates of these human-driven emissions from managed ecosystems. These components are important across a range of scales, from city to regional to national. Improved characterization of managed vegetation would impact estimates of net fluxes, our understanding of the impacts of extreme events, and the overall fidelity of Earth systems models.
While it is well recognized that understanding natural GHG emission sources and sinks (i.e., not managed by humans) in the land biosphere and oceans is essential to quantifying net emissions, the Committee has limited its focus to anthropogenic emissions (defined in Box 1-2).
Many of the methods used to generate GHG inventories can also provide information on natural sources and sinks. The pillars that underpin the assessment framework developed by the Committee are also relevant to information on natural sources and sinks but the primary focus of this report in developing the framework is on the evaluation of GHG information on anthropogenic emissions (and removals). A comprehensive framework with full consideration of approaches specific to natural sources and sinks was beyond the scope of this study.
Defining Greenhouse Gas Emissions Information Scales for This Study
The development of GHG emissions information has occurred at many different scales including global, national, regional, urban, and facility spatial scales and annual to hourly temporal scales, discussed below. This report focuses on information at the global scale for which global GHG emissions information and related supporting information are the primary target. However, throughout the report, we include reviews and examples of GHG emissions information at finer spatial scales. The inclusion of this “subnational” GHG emissions information is not exhaustive or comprehensive. Indeed, the sheer volume of GHG emissions information produced at subnational scales has exploded in recent years and attempting to comprehensively capture and reflect on this work would be both challenging and outside the Committee’s Statement of Task (Box 1-1).
Nevertheless, we include examples of subnational work throughout the report because many of these efforts offer considerable insight into new inventory techniques, approaches, examples of decision-maker engagement, and lessons learned. Hence, they are included to provide insight and practical examples of particularly successful GHG emissions information elements. This distinction is important because in Chapter 4, an evaluation framework is built to assess current global inventories. The assessment of current capabilities in Chapter 4 is limited to global GHG emissions information and does not reflect the full range of subnational examples and case studies found throughout the report.
User and Decision-Maker Needs for Greenhouse Gas Emissions Information Across Scales
Historically, the need for GHG emissions information began at the global and national scales as a necessary ingredient in the international policy-making process. Today, as nations continue to report and use GHG emissions information as part of that process, many have simultaneously used national emissions information domestically, often to inform and support climate-related objectives and targets. In turn, this has stimulated national governments, regional governments, cities, and nongovernmental and private entities to become producers and consumers of GHG emissions information. This section introduces the full range of spatial scales considered in this report—global, national, regional, local, and facility—and the associated context and needs of decision makers and stakeholders for GHG emissions information.
Global and National Scales
Adopted in 1992, the United Nations Framework Convention on Climate Change (UNFCCC) went into force in 1994, establishing the need for GHG emissions information at the global/national scale as part of a formal international treaty (UNFCCC, 1992). GHG emissions information is used to inform GHG policy and mitigation considerations, monitor progress toward commitments, and support the scientific community and public assessment of GHG emissions and removals (ClimaSouth, 2014; Lindroth and Tranvik, 2021; Yona et al., 2020). The UNFCCC includes a formal process whereby UNFCCC Parties develop and update GHG inventories based on guidelines produced by the Intergovernmental Panel on Climate Change (IPCC). Tracking GHG inventories over time and continually assessing progress toward agreed upon mitigation targets was of primary importance and emphasized regular, continual GHG budget quantification. GHG emissions information has also been used within the international negotiating process itself with countries discussing targets and mitigation options contextually grounded with ongoing numerical assessment of national emissions and their sector composition.
Outside of the UNFCCC reporting framework, GHG emissions information development at the global and national scale has a long history dating back to the 1970s (Keeling, 1973; Marland and Rotty, 1984). While the development began for primarily scientific research reasons (e.g., understanding the global carbon cycle, climate change projections), the outcome of the early GHG inventory efforts has provided methodological foundations to the emission calculations and approaches suggested in IPCC guidelines and is increasingly needed as both alternative and complementary information to the UNFCCC process. At a national level, decision makers need national quantification of GHG emissions and removals to understand their national GHG landscape, plan mitigation targets and actions, and place their national budgets within the global context to make measurable progress on limiting climate change.1 In addition, various nongovernmental organizations, the media, think tanks, the business community, and others have long used and archived national GHG emissions information to track progress and maximize emissions transparency (Yona et al., 2020).
Subnational Government Scales
Since the 2015 Paris Agreement formally recognized the contributions of “all levels of government and various actors” to achieve global climate mitigation goals, the number of cities, state and regional governments, and private actors pledging and implementing their climate actions has grown (Hale, 2020; Hsu et al., 2018; IPCC, 2022c). A need for GHG emissions information at these smaller subnational scales also originated mostly independent of the international policy process as subnational entities have taken on mitigation planning and bi/multilateral policy arrangements (Rosenzweig et al., 2010). The UNFCCC’s Global Climate Action Portal recorded 11,355 city and 270 regional governments pledging some form of individual climate mitigation pledge or participation in a transnational climate network, such as the Global Covenant of Mayors for Climate and Energy, which is the largest city-actor initiative with nearly 12,000 cities participating as of July 2022.
Urban-scale (i.e., neighborhoods, communities, metropolitan areas) emissions occupy a unique niche among the subnational decision makers and stakeholders. First, cities account for almost three-quarters of energy and transportation-related atmospheric CO2 emissions and that share is projected to grow substantially in the coming decades (Gurney et al., 2022; Seto et al., 2014). This reality in combination with the acknowledgment that low-emission development is often consistent with a variety of other co-benefits (e.g., air quality improvement) has pushed cities to independently pursue GHG emissions mitigation (Hsu et al., 2015; Rosenzweig et al., 2010; Watts, 2017).
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1https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement
Within these networks and cooperative climate initiatives, subnational governments are frequently required to regularly report emissions inventories detailing progress toward climate mitigation goals. Subnational characterization of GHG emissions also finds use among a larger swath and complex array of the decision-making world. For example, citizens, local environmental organizations, and local media are active in using GHG emissions information in planning and strategizing climate action activity (Hsu et al., 2020).
Despite these efforts, however, existing subnational estimates of GHG emissions can be incomplete, contain potential flaws, be difficult to intercompare, and are primarily skewed toward cities in the Global North (i.e., the United States, Canada, United Kingdom, European Union, Singapore, Japan, South Korea, Australia, and New Zealand) (Gurney et al., 2021; Ibrahim et al., 2012; Wei et al., 2021). One challenge of estimating emissions at disaggregated spatial scales is that many of the methods were developed to calculate GHG emissions and removals at the national level. Subnational decision making needs subnational GHG emissions information, but at these scales, there are reporting challenges associated with system boundaries (e.g., emissions leakage, lateral transfers, scope definitions) and multigovernance arrangements, among others (Gurney et al., 2015; Hutyra et al., 2014). While there have been some efforts to standardize subnational emissions reporting and align guidance with national inventory methodologies such as the IPCC’s 2019 refinement of the 2006 national inventory guidelines (e.g., Fong et al., 2014), subnational governments have discretion in the ways that they both estimate and report GHG emissions.
Facility and Corporate Scales
The facility or individual business scale has also emerged as an important consumer of GHG emissions information, often because of corporate energy transition strategies and project financing related to climate impacts in alignment with the Paris Agreement. Over 40 percent of the world’s 2,000 largest publicly traded companies have committed to net-zero or other climate targets.2 Over 90 percent of the S&P 500 and 80 percent of the Russell 1000 companies issue corporate environmental, social, and governance reports with GHG and carbon footprints accounting for emissions reductions as key performance indicators.3 The success of these commitments relies on accurate GHG inventories and transparent reporting of their progress. Contrasting governments’ geographic focus, this scale places particular emphasis on supply-chain approaches and ownership boundaries. For example, the Task Force for Climate-related Financial Disclosures provides a guide for firms to properly disclose their climate-related risks and opportunities and calls for businesses and financial services companies to regularly evaluate and update their analyses of these risks and opportunities (e.g., UNEP, 2020). Corporate or facility-level accounting forms the underlying basis to conduct such analysis.
An inaccurate baseline or even prospective targets based on poor, inaccurate, or incomplete accounting could result in a lack of trust and reputational damage, including claims of “green-washing” or ineffective deployment of capital and resources (Pfadt-Trilling and Fortier, 2021). An analysis of public corporate reports indicates shortcomings of incomplete inventories (Day et al., 2022; Tollefson, 2022). Hence, establishing corporate commitments or assessing progress based on inaccurate or incomplete information may limit the ability of a reasonable investor and other stakeholders to make informed decisions on the GHG performance of the company (Cannon et al., 2020).
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2https://zerotracker.net/#companies-table
3https://www.yahoo.com/now/92-p-500-companies-70-140530175.html
Barriers to Widespread Use of Emissions Information
Decision makers require timely and accurate information by which to base management decisions, as well as to evaluate the effectiveness of policy responses. However, information uptake and translation of GHG emissions data and information to actionable knowledge and insight can fail to occur for a variety of reasons:
- Capacity: Decision makers must have the capacity to either collect and analyze their own emissions data or utilize datasets generated from other sources to inform action. The capacity for emissions information data collection may be limited due to a lack of resources, technical know-how, or a range of other reasons (e.g., Andres et al., 1996, 2012, 2014). In terms of the ability to utilize others’ data and information, analytical capacity, defined as the ability of decision makers to analyze information, and the application of research methods and modeling techniques (Howlett, 2009), may be a limiting factor. In the age of big data and information, decision makers can frequently become overwhelmed with the vast amount of data and information available. Without considering the analytical capacity of decision makers or end users of information, “overloading” capacity can lead not only to the failure of data and information uptake but also risks mismanagement of the original problem (Dietz et al., 2003). Decision makers may also lack relevant expertise or background to know how to analyze or utilize data or information when presented.
- Transparency: The processes and methods used in developing GHG emissions information give credibility to the dataset and are important for understanding potential uncertainties. This underlying information may not be provided to decision makers in a transparent manner or not clearly communicated and, thus, decision makers may not be comfortable using a given emissions dataset.
- Accessibility: Data may not be available in formats that are easily accessible or usable for decision makers. Documents may be hard to extract and use or can be unwieldy in size or format, making them difficult to download, format, and analyze, depending on the capacity of users. This challenge is more general to Earth science data (Carlson and Oda, 2018). The lack of a common framework for reporting and analysis contributes to this barrier because it privileges experts and prevents decision makers from learning from each other.
- Trust: Users need to find the GHG emissions information trustworthy. With competing estimates and lots of political and financial interests involved in emissions analyses, there can be a lack of trust or at least suspicion that can create missed opportunities.
- Timeliness: GHG emissions inventories developed according to the IPCC’s Tier 1 or 2 methodologies (IPCC, 2006) are primarily based on activity data (e.g., fuel consumption) that take time and resources to collect and aggregate (Andres et al., 2012). As a result, emissions inventories are generally only available with a few-year time lag and may not be indicative for decision makers requiring real-time information on which to base policy decisions, or individual entities developing their respective mitigation actions. For example, the latest national-level information for developed countries is the 2022 submission to the UNFCCC (UNFCCC, 2022a), containing data for 2020; for developing countries, the latest year of data availability varies but is never more recent than 2–3 years before the year of submission (UNFCCC, n.d.-b, c). On the other hand, near-real-time emission estimates can be more timely but less accurate (Oda et al., 2021).
- Relevance: Data may not be, or not be perceived to be, relevant for a decision maker, limiting its application and uptake. There may be political ideologies that hinder decision makers from utilizing data because policy makers may interpret data based on past attitudes or beliefs that may deem certain information irrelevant (Arnautu and Dagenais,
- Awareness: Decision makers may not necessarily be aware of the data available to them to base policy decisions on. Since data collection and analysis technologies are constantly evolving, it may be challenging for a decision maker to constantly stay up to date on the latest data. The quality of the data and how it compares with other data from an agency or institution may also affect the adoption or uptake of a dataset. Limited communication between decision makers and data providers or scientists is another obstacle to awareness, trust, and transparency.
- Long-term support: Just as long-term monitoring is important to determine trends, long-term funding support for GHG emissions measurements and analyses is also important for informing decision making. The challenge is to maintain support for measurements and analyses that extend beyond political cycles.
2021). In other instances, data may not be available at a temporal or spatial scale relevant to a decision maker. For example, a national emissions inventory may not be relevant for a local government or private business to base decisions on; therefore, the data are not utilized. Projections of emissions and comparisons against pledges are often insufficiently granular to assess the relevance of the transition mitigation strategies of individual stakeholders. For example, the United Nations’ Emissions Gap Reports4 are useful in modeling scenarios and identifying gaps in pledges, but they lack granularity (e.g., by not mapping the pledges against specific sectors). Moreover, use of CO2 equivalents is an inaccurate representation of resulting climate effects and leads to ambiguity when assessing which GHGs are contributing the greatest temperature impacts. As such, annual GHG inventories reported under UNFCCC guidelines are encouraged or required to report the contribution of each gas by mass (IPCC, 2006). The reporting of GHGs separately allows GHG inventories to be used to understand the implications and efficacy of policies over different periods of time.
The Committee’s Approach to This Study
In addressing its tasks, the Committee met twice in person in Washington, DC, in June 2022. The Committee held two information-gathering meetings to solicit external input from the international community. The first meeting was held on June 2, 2022, hosted by the National Academies’ Board on Atmospheric Sciences and Climate, on Greenhouse Gas Emissions Monitoring, Inventories, and Data Integration: Understanding the Landscape with 12 presentations by scientists from the United States, Canada, and Europe. On June 27–28, 2022, the Committee held a workshop on Development of a Framework for Evaluating Global Greenhouse Gas Emissions Information for Decision Making. This workshop had 29 international experts give “lightning” presentations and invited moderators led participants through a series of World Café breakout discussions. The Committee also solicited written technical input from the community. These information-gathering sessions were followed by five virtual meetings in July and August 2022 during which the Committee deliberated and wrote the report. Following standard National Academies’ procedures, the draft report then underwent a rigorous process of external peer review before publication.
Report Roadmap
Chapter 2 describes approaches for the development of GHG inventories that are used to set emissions baselines and measure emissions changes. In this report, the Committee considers three approaches: (1) activity-based approaches that utilize activity data as representative indicators of
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GHG emissions; (2) atmospheric-based approaches that use measurements of atmospheric concentrations to infer information on emissions and sinks; and (3) hybrid approaches that functionally integrate multiple data sources and approaches.
Chapter 3 describes the institutional and technical limitations of current approaches that have inhibited their usefulness in decision making.
Chapter 4 develops the framework for evaluating GHG emissions information. The framework includes six “pillars” that serve as criteria to qualitatively assess the extent to which a given information system or inventory performs. Chapter 4 also describes how current approaches perform relative to the pillars and provides case studies exemplifying how the framework could be utilized to evaluate and improve current and future efforts.
Chapter 5 makes actionable recommendations for the short and long term. These recommendations aim to enhance the capabilities of GHG emissions information to fulfill the six pillars of the framework to provide trusted, usable, and policy-relevant information for a wide range of users and decision makers.
