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

Since 2015, the Paris Agreement’s long-term temperature goal (LTTG) has driven governments, agencies, and private entities to develop mitigation strategies and long-term low greenhouse gas development plans. Authoritative institutional scenarios (e.g., IEA, Shell, BP) influence national targets and investor decisions, yet their temperature implications are often opaque. The Agreement strengthens the prior “below 2 °C” aim to “well below 2 °C” while pursuing 1.5 °C and requires net-zero GHGs in the second half of the century. Based on IPCC AR6, Paris-compatibility implies: (1) not exceeding a >66% probability of overshooting 1.5 °C and achieving ≤1.5 °C in 2100 with ≥50% chance; (2) holding warming below 2 °C with close to or more than 90% likelihood; and (3) achieving net-zero GHGs in the second half of the century. The study asks whether influential institutional decarbonization scenarios are consistent with these criteria and develops a transparent, uniform assessment framework to compare them directly with IPCC-assessed IAM pathways.

The paper situates its work within prior assessments of mitigation pathways by the IPCC SR1.5 and AR6, which categorized IAM scenarios by temperature outcomes and overshoot characteristics. It references Skea et al.’s typology of scenarios (outlooks, exploratory, normative) and notes that non-IAM institutional pathways in SR1.5 had self-assessed temperature statements. It reviews the evolution from the Cancun “below 2 °C” framing to the Paris Agreement’s strengthened goals, and the associated probabilistic interpretations used by the scientific community. It also draws on literature defining Paris-consistent pathway categories, greenhouse gas accounting metrics (GWP100), and discusses uncertainties and differences among reduced-complexity climate models (MAGICC, FaIR).

The authors develop a transparent framework to assess institutional scenarios against the Paris LTTG on equal footing with IPCC IAM pathways. Steps: (1) Harmonize reported emissions to a common historical dataset and base year to ensure consistency across species and sectors. (2) Extend scenarios that end before 2100 to end-of-century using the Constant Quantile Extension (CQE) method, which preserves each scenario’s relative position within a distribution of end-century IAM pathways (from SR1.5). (3) Infer missing greenhouse gases and aerosol precursor emissions using the Quantile Rolling Windows (QRW) infilling method (a quantile regression approach) with energy and industrial CO2 as the lead gas; alternative infilling methods are tested in sensitivity analyses. Where reports discuss non-CO2 but lack data, QRW infilling is still applied for transparency. (4) Run multi-gas trajectories through the reduced-complexity carbon cycle–climate model MAGICC6 in probabilistic mode (consistent with SR1.5 configurations) to derive peak and end-of-century warming distributions and exceedance probabilities for 1.5 °C and 2 °C; cross-checks with FaIR are provided in supplementary material. (5) Classify scenarios according to SR1.5/AR6 categories (e.g., Below-1.5 °C, 1.5 °C low-overshoot, 1.5 °C high-overshoot, Lower 2 °C, etc.). Net-zero GHG timing is assessed using CO2e based on GWP100 (AR4), consistent with UNFCCC reporting. The dataset includes institutional scenarios from Shell (Sky 1.5), BP (Rapid, Net Zero), IEA (SDS 2020, NZE 2021), and Equinor (Rebalance), noting gaps in time horizon and gas coverage; the framework addresses these via harmonization, extension, and infilling. Additional methods include robustness checks for CQE and assessment of model and infilling sensitivities.

• Coverage and transparency gaps: Most institutional scenarios end by 2040–2050 and report only energy and industrial CO2, lacking full multi-gas coverage; Shell Sky 1.5 is the exception with broader species reporting (though limited aerosol precursors).
• Emission levels vs SR1.5 ranges: For energy and industrial CO2 in 2030, SR1.5 no-overshoot pathways have 13.6 GtCO2 [13.2–16.1 IQR], low-overshoot 21 GtCO2 [18.6–22.6 IQR]. Only IEA NZE lies close to the low-overshoot median; others are above. By 2050, IEA NZE lies below the low-overshoot median; Shell Sky 1.5 remains higher than even high-overshoot ranges.
• Infilled non-CO2 differences: 2030 methane emissions in SR1.5 are 156 Mt CH4 (no-overshoot, IQR 129–248) and 236 Mt CH4 (low-overshoot, IQR 189–257). Shell Sky 1.5 reaches 426 Mt CH4, above even high-overshoot ranges. N2O patterns are similarly elevated for institutional scenarios when infilled.
• AFOLU CO2: Infilled AFOLU CO2 for most institutional scenarios clusters around high-overshoot/2 °C ranges; Shell’s reported 2050 AFOLU CO2 implies heavy land-based CDR reliance (near the 22nd percentile of joint low/no/high-overshoot categories), beyond most 1.5 °C-consistent pathways.
• Temperature outcomes (MAGICC6, QRW infilling; Table 2): • Equinor Rebalance: peak median 1.73 °C (2060), category Lower 2 °C; P(>1.5 °C) max 78% (2100: 64%); P(>2 °C) max 23% (2100: 20%). • Shell Sky 1.5: peak median 1.81 °C (2059), Lower 2 °C; P(>1.5 °C) 86% (60%); P(>2 °C) 29% (17%). • BP Rapid: peak median 1.73 °C (2058), Lower 2 °C; P(>1.5 °C) 78% (61%); P(>2 °C) 23% (18%). • BP Net Zero: peak median 1.65 °C (2049), 1.5 °C high-overshoot; P(>1.5 °C) 71% (36%); P(>2 °C) 16% (9%). • IEA SDS: peak median 1.68 °C (2056), Lower 2 °C; P(>1.5 °C) 73% (55%); P(>2 °C) 19% (14%). • IEA NZE: peak median 1.56 °C (2045), 1.5 °C low-overshoot; P(>1.5 °C) 58% (18%); P(>2 °C) 11% (4%).
• Paris Agreement alignment: Only IEA NZE satisfies all three criteria: close to ≥90% likelihood of staying below 2 °C throughout, ≤50% chance of exceeding 1.5 °C by 2100 with low overshoot, and achieving net-zero GHGs in the second half-century. BP Net Zero reaches ~1.5 °C by 2100 but with high overshoot; other scenarios are Lower 2 °C and not Paris-compatible under the adopted criteria.
• Energy-system levers: IEA NZE aligns with 1.5 °C low-overshoot IAMs in both reduced final energy (Et ~90% of 2010) and very low carbon intensity (CIt ~5–10% of 2010 by 2050). Equinor Rebalance and Shell Sky 1.5 show higher final energy and higher CI, resembling higher-temperature pathways.
• Technology shares: PA-compatible IAMs reduce coal in power rapidly; IEA NZE matches these coal reductions by 2030 and 2050. Scenarios closest to PA-compatibility also reduce natural gas shares fastest; large uncertainty remains due to varying CCS assumptions. Institutional scenarios often show higher wind/solar shares (~60–65% by 2050) than IAM medians (~45%), reflecting IAMs’ historical underestimation of VRE potential.

The analysis demonstrates that most influential institutional decarbonization scenarios are inconsistent with the Paris Agreement’s LTTG primarily due to higher near-term fossil fuel use, slower reductions in carbon intensity, and insufficient mitigation of non-CO2 gases, leading to substantial overshoot of 1.5 °C. While some scenarios signal rapid renewable deployment, this alone does not ensure Paris-consistency without comprehensive multi-gas reductions and stringent energy-system transformations. The single Paris-consistent case (IEA NZE) combines strong demand reductions with rapid decarbonization and early coal and gas phase-down, resulting in low overshoot and high probability of staying below 2 °C. The findings underscore the need for full-century, multi-gas transparent reporting by institutions to enable robust temperature assessments. They also highlight policy-relevant uncertainties, especially regarding the purported bridging role of natural gas and the risks of relying on large-scale CDR to compensate for delayed mitigation. Reducing non-CO2 emissions (notably methane and N2O) alongside CO2 is critical to meeting the LTTG.

This study provides a transparent, uniform framework to assess institutional decarbonization scenarios against the Paris Agreement using harmonization, end-century extension, multi-gas infilling, and probabilistic climate modeling consistent with IPCC assessments. Applying this framework, only the IEA Net Zero by 2050 scenario is Paris-compatible (1.5 °C low-overshoot) under the adopted criteria; other assessed institutional scenarios either remain in the Lower 2 °C category or achieve ~1.5 °C by 2100 with high overshoot. The analysis reveals that inadequate reductions in fossil fuel use, higher carbon intensity, and insufficient non-CO2 mitigation drive misalignment with the LTTG. The authors call for institutions to publish complete, transparent, end-of-century multi-gas pathways, including separate reporting of emission reductions and removals, to enable consistent climate outcome assessments. Future research should improve multi-gas modeling consistency with energy and land systems, constrain feasible CDR and BECCS deployment, refine reduced-complexity climate model calibrations, and investigate the system-wide implications of natural gas transitions.

Key limitations include reliance on extension (CQE) for scenarios ending before 2100 and infilling (QRW and alternatives) for missing gases, which introduces uncertainty into non-CO2 trajectories, AFOLU CO2, and resultant temperature outcomes. Climate categorization can shift under alternative infilling methods (e.g., IEA NZE becomes high-overshoot under an RMS infilling in sensitivity analysis). Differences between reduced-complexity models (MAGICC6 vs FaIR) add model uncertainty, although generally consistent with SR1.5-assessed differences. The study infers rather than directly reports CDR components for most institutional scenarios, limiting explicit assessment of removals and sustainability constraints. Historical harmonization may not fully capture short-term anomalies (e.g., COVID-19 effects around 2020), though impact on long-term assessments is judged minimal.