Institutional decarbonization scenarios evaluated against the Paris Agreement 1.5 °C goal

The Paris Agreement's 1.5°C long-term temperature goal (LTTG) necessitates scientifically rigorous guidance for policymakers. Various institutions, including governments, international agencies, and private entities (like Shell and BP), produce emission scenarios influencing policy and investment decisions. The Hague District Court's ruling against Shell, which referenced scenarios from the IEA and Shell alongside IPCC's Special Report on 1.5°C (SR1.5), highlights the impact of these scenarios. Energy system scenarios can be categorized as outlooks, exploratory, or normative; normative scenarios explicitly aim to achieve a desired end state (like the 1.5°C goal), typically modeled by the IAM community. While IAM scenarios have been extensively assessed in SR1.5, institutional scenarios often lack comprehensive coverage of greenhouse gases and lack transparency in their climate assessments. The Paris Agreement's requirements include limiting warming well below 2°C and pursuing efforts to limit it to 1.5°C, along with achieving a balance between anthropogenic emissions and removals. Three criteria for Paris Agreement compatibility are established: 1) less than 66% probability of exceeding 1.5°C, achieving 1.5°C by the end of the century with at least a 50% chance; 2) keeping warming below 2°C with at least a 90% chance; and 3) achieving net-zero greenhouse gas emissions in the second half of the century. This study aims to provide a transparent temperature assessment of institutional decarbonization scenarios, comparing them to IAM scenarios and identifying key energy system features driving their alignment (or misalignment) with the Paris Agreement LTTG.

The authors review existing literature on energy system scenarios and their categorization (outlooks, exploratory, and normative). They highlight the assessment of IAM scenarios in the IPCC's SR1.5, noting the limited inclusion and self-assessment of non-IAM pathways. They discuss the historical context of the Paris Agreement's temperature goal, emphasizing the strengthened target and the increased likelihood of staying below 2°C and achieving net-zero emissions. The existing criteria for Paris Agreement compatibility of pathways are described, which is adopted in the study with minor relaxation of one criterion. The study sets the stage by discussing previous assessments of IAM scenarios and the lack of a consistent methodology for evaluating institutional scenarios.

The authors address three key challenges in assessing institutional emission pathways: limited time horizons (mostly extending to mid-century), incomplete representation of greenhouse gases and aerosol emissions (often focusing only on CO2 from energy and industrial sectors), and a lack of transparency in climate impact quantification. To overcome these challenges, they developed a consistent framework. This involved harmonizing emissions to a common base year (2010), extending scenarios to 2100 using the Constant Quantile Extension (CQE) method, and inferring missing emission species using the Quantile Rolling Windows (QRW) infilling method. The methodology leverages existing emission pathways from SR1.5 as a reference for extension and infilling. The resulting multi-gas emission trajectories are input into the MAGICC6 reduced complexity climate model for probabilistic temperature assessments, allowing direct comparison with SR1.5's approach. This allows for classifying scenarios according to the same scheme employed by SR1.5, enabling consistent comparison. The framework also includes analyses of key energy system features driving emissions pathways and their relationship to the Paris Agreement temperature goals. The study acknowledges alternative infilling methods and discusses the sensitivity of results to these choices, providing supplementary information comparing results from the QRW method and other methods.

The analysis reveals that most institutional scenarios, except for the IEA Net Zero 2050 scenario, fail to meet the established criteria for Paris Agreement compatibility. Comparison of CO2 emissions from energy and industrial processes shows that only the IEA NZE scenario falls within the range of 1.5°C compatible SR1.5 pathways. In 2030, the other institutional scenarios exhibit significantly higher CO2 emissions than the low-overshoot pathways from SR1.5. Furthermore, other greenhouse gases (CH4 and N2O) also exceed levels consistent with the Paris Agreement's goals in most scenarios. The study analyzes the different emission sectors showing that CO2 emissions from AFOLU (Agriculture, Forestry, and Other Land Use) in the Shell Sky 1.5 scenario show a significant outlier, which points to a high reliance on land-based CDR. The climate categorization analysis shows that only four scenarios (IEA Net Zero, BP Net Zero, IEA SDS, and Shell Sky) meet the criteria for a balance between sources and sinks in the second half of the century. The temperature trajectories demonstrate that most scenarios significantly overshoot the 1.5°C warming limit. Only the IEA NZE scenario is categorized as a 1.5°C low-overshoot scenario, nearly meeting the 'very likely' less than 2°C requirement (>90% likelihood). Analysis of mitigation levers (changes in carbon intensity of final energy, changes in final energy demand, and relative reductions in non-CO2 emissions) reveals that only the IEA NZE scenario aligns with low-overshoot pathways in both 2030 and 2050. Examination of technology preferences shows that the IEA NZE scenario exhibits the most rapid decrease in coal and natural gas shares in electricity generation, aligning closely with 1.5°C compatible IAM scenarios. However, there's a significant range in natural gas use across scenarios, highlighting uncertainties regarding its role in the energy transition. The study also observes a trend in some institutional pathways indicating a potential for higher and faster renewable energy penetration than shown in many IAMs.

The findings highlight the inadequacy of most institutional pathways in achieving the Paris Agreement's LTTG, primarily due to continued reliance on fossil fuels. The analysis underscores the importance of not only long-term temperature goals but also intermediate conditions such as limiting temporary overshoots of 1.5°C and the importance of mitigation levers and energy system transformations. The study emphasizes the need for transparency in reporting, including complete pathways for all GHGs across sectors to enable accurate temperature assessments. While some institutional scenarios show potentially higher renewable energy uptake than IAMs, meeting the LTTG requires comprehensive evaluations including all GHG emissions and a rapid transition away from fossil fuels. The role of natural gas remains uncertain across different scenarios, with those aligning with the Paris Agreement goals generally showing a more rapid decrease in natural gas use compared to those failing to meet the criteria. The authors suggest further research to fully understand the complexities and trade-offs related to natural gas and other bridging fuel technologies within the context of ambitious climate targets.

This study provides a comprehensive framework for evaluating the climate compatibility of institutional decarbonization scenarios against the Paris Agreement's 1.5°C goal. The authors emphasize the need for transparency and completeness in reporting GHG emissions from all sectors to allow accurate climate assessments. The analysis indicates that most evaluated scenarios fail to meet the agreement's criteria, primarily due to continued reliance on fossil fuels. While some scenarios show promise in renewable energy penetration, a rapid phase-out of fossil fuels is crucial for achieving the Paris Agreement's climate objectives. Future research should focus on refining emission scenario modeling, integrating energy-system transformation analysis, and improving uncertainty quantification.

The study acknowledges limitations in the methodology, particularly concerning the reliance on infilling methods for missing emission data. While sensitivity analyses are conducted, uncertainties remain due to the assumptions inherent in these methods. The study focuses on a specific set of institutional scenarios and might not be fully generalizable to all such scenarios. Furthermore, the study uses simplified climate models (MAGICC6 and FaIR), which may not capture the full complexity of the climate system. The choice of GWP100 as the greenhouse gas accounting metric influences the results, and other metrics might yield different interpretations. Finally, the analysis does not explicitly assess carbon dioxide removal (CDR) deployment separately for each scenario (except for IEA Net Zero data), making a comprehensive assessment of CDR reliance more difficult.