Compendium

IPCC Scenarios and Integrated Assessment Models

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IPCC Scenarios and Integrated Assessment Models

The Intergovernmental Panel on Climate Change (IPCC) is the United Nations body for assessing the science related to climate change. Established by WMO + UNEP in 1988 and endorsed by UNGA Resolution 43/53, it has produced six assessment cycles (AR1 1990, AR2 1995, AR3 2001, AR4 2007 — Nobel Peace Prize shared with Al Gore, AR5 2013–14, AR6 2021–23) plus three Special Reports in the AR6 cycle (SR1.5 2018, SROCC 2019, SRCCL 2019). The Panel does not conduct primary research; it synthesises peer-reviewed literature into Working Group (WG) reports adopted line-by-line by governments. Across all six cycles, scenario analysis has been the spine connecting socio-economic assumptions to emission pathways to climate response to impacts. The scenario architecture has evolved through three generations: IS92 (1992), SRES (2000), and SSP+RCP (2011–15). Integrated Assessment Models (IAMs) generate the scenarios by coupling energy-economy + land + climate modules; their outputs underpin mitigation cost estimates, carbon prices, and 1.5 °C / 2 °C feasibility analyses. This note documents the IPCC structure, the scenario lineage, the major IAMs and their development teams, the social-cost-of-carbon framework, and the policy interface through UNFCCC NDCs and the Global Stocktake.

1. IPCC structure

1.1 Working Groups

  • WG1 (Physical Science Basis): atmospheric physics, observations, paleoclimate, climate model projections. AR6 WG1 report “Climate Change 2021: The Physical Science Basis”, 3 949 pages, 234 authors from 66 countries, 14 000+ cited papers, approved Aug 2021 (co-chairs Masson-Delmotte + Zhai). Headline finding: “It is unequivocal that human influence has warmed the atmosphere, ocean, and land.”
  • WG2 (Impacts, Adaptation, Vulnerability): sectoral and regional impacts, adaptation strategies, ecosystems, food/water/health. AR6 WG2 “Climate Change 2022: Impacts, Adaptation and Vulnerability”, 3 675 pages, Feb 2022 (co-chairs Pörtner + Roberts).
  • WG3 (Mitigation): greenhouse-gas inventory methodology, mitigation options across sectors, integrated assessment, sustainable development. AR6 WG3 “Climate Change 2022: Mitigation of Climate Change”, 2 913 pages, Apr 2022 (co-chairs Shukla + Skea + Reisinger).
  • Synthesis Report: integrates WG outputs + special reports. AR6 SYR Mar 2023, 85-page SPM + Longer Report.
  • Task Force on National GHG Inventories (TFI): maintains the 2006 IPCC Guidelines (refined 2019) used by Parties to UNFCCC for reporting.

1.2 Bureau

Chaired since 2023 by Jim Skea (UK), succeeding Hoesung Lee (South Korea, 2015–23) and Rajendra Pachauri (India, 2002–15). Sister-organisation IPBES (biodiversity) follows similar structure.

1.3 AR7 cycle

Launched Jan 2024 at IPCC-60 (Istanbul); WG schedules — WG1 due 2028, WG2 + WG3 by 2029. AR7 SYR planned ~2029. Special Reports approved for AR7: Cities and Climate Change (target 2027), Climate Change and Carbon Dioxide Removal Technologies + CCUS (target 2027). Methodology Report on Short-Lived Climate Forcers (target 2027). Updated 2006 Guidelines for CDR/CCUS accounting in preparation.

2. Scenario generations

2.1 IS92 (1992)

First IPCC scenarios — six pathways (IS92a–f) used in SAR 1995. IS92a was business-as-usual reference; only crude link to socio-economic assumptions.

2.2 SRES (Special Report on Emissions Scenarios, 2000)

Nakicenovic-Davidson-Davis-Grübler-Kram-LaRovere-Metz-Morita-Pepper-Pitcher-Sankovski-Shukla-Swart-Watson-Dadi 2000. Six “marker” scenarios from four families:

  • A1 (rapid econ growth + global convergence). Sub-variants A1FI (fossil-intensive), A1T (technology), A1B (balanced).
  • A2 (heterogeneous, regional, slow tech).
  • B1 (sustainability, service economy, global).
  • B2 (sustainability, regional).

No probability assigned. A2 + A1FI + A1B + B1 + B2 widely used in AR3 + AR4. SRES was hand-built without explicit policy intervention (no climate policy scenarios) — limitation that motivated RCPs.

2.3 RCPs (Representative Concentration Pathways, 2009–11)

Moss-Edmonds-Hibbard-Manning-Rose-vanVuuren-Carter-Emori-Kainuma-Kram-Meehl-Mitchell-Nakicenovic-Riahi-Smith-Stouffer-Thomson-Weyant-Wilbanks 2010 (Nature). Process: WGCM-IPCC parallel-process inverted scenario logic — pick four target radiative-forcing levels (W m-2 at 2100), reverse-engineer concentration + emission pathways.

RCP2100 forcing (W m-2)2100 CO2-eq (ppm)Description
RCP 2.62.6 (peak 3.0, decline)~450Aggressive mitigation, negative emissions late century. vanVuuren-Stehfest-denElzen-Kram-vanVliet-Deetman-Isaac-Klein-Goldewijk-Hof-Mendoza-Beltran-Oostenrijk-vanRuijven 2011 (IMAGE).
RCP 4.54.5~650Moderate mitigation. Thomson-Calvin-Smith-Kyle-Volke-Patel-Delgado-Bond-Adam-Frumhoff-Edmonds 2011 (GCAM).
RCP 6.06.0~850Slow mitigation. Masui-Matsumoto-Hijioka-Kinoshita-Nozawa-Ishiwatari-Kato-Shukla-Yamagata-Kainuma 2011 (AIM).
RCP 8.58.5~1370High emissions. Riahi-Rao-Krey-Cho-Chirkov-Fischer-Kindermann-Nakicenovic-Rafaj 2011 (MESSAGE).

CMIP5 (Coupled Model Intercomparison Project Phase 5) used these — Taylor-Stouffer-Meehl 2012 BAMS.

2.4 SSPs (Shared Socioeconomic Pathways, 2014–17)

O’Neill-Kriegler-Riahi-Ebi-Hallegatte-Carter-Mathur-vanVuuren 2014; O’Neill-Kriegler-Ebi-Kemp-Benedict-Riahi-Rothman-vanRuijven-vanVuuren-Birkmann-Kok-Levy-Solecki 2017 (Global Environ Change). Five narratives along challenges-to-mitigation vs challenges-to-adaptation:

  • SSP1 (Sustainability — “Taking the Green Road”): low population (~7 B by 2100), high education + tech innovation, global cooperation, low fossil fuels, low resource intensity. Low M-low A.
  • SSP2 (Middle of the Road): extrapolation of current trends. Medium M-medium A.
  • SSP3 (Regional Rivalry — “A Rocky Road”): nationalism, conflict, slow growth, high population (~12 B by 2100), low tech transfer. High M-high A.
  • SSP4 (Inequality — “A Road Divided”): stratified society, elite + tech-rich vs disadvantaged majority, low resource availability for majority. Low M-high A.
  • SSP5 (Fossil-Fueled Development — “Taking the Highway”): rapid growth, high consumption, fossil-fuel intensive but high adaptive capacity. High M-low A.

SSP narratives were quantified by IAM teams producing “marker” scenarios per SSP × RCP combination.

2.5 SSP × RCP matrix and ScenarioMIP

Combined into matrix of feasible combinations. CMIP6 ScenarioMIP (O’Neill 2016 GMD) ran climate models on emissions pathways:

TierScenario2100 forcing (W m-2)Description
1SSP1-1.91.91.5 °C consistent with overshoot, requires major NETs
1SSP1-2.62.62 °C consistent, moderate NETs late century
1SSP2-4.54.5Middle, current-policies plausible
1SSP3-7.07.0High-emissions, fragmented world
1SSP5-8.58.5Fossil-fuelled, very high emissions
2SSP4-3.43.4Inequality, moderate mitigation
2SSP4-6.06.0Inequality, slow mitigation
2SSP5-3.4-OS3.4 overshootOvershoot 4.5 then decline
2SSP1-1.9-LowNTCF1.9Low short-lived climate forcers

2.6 The RCP/SSP 8.5 plausibility critique

Hausfather-Peters 2020 (Nature) argued RCP 8.5 / SSP5-8.5 is “increasingly implausible” — assumes 5× expansion of coal use globally, which contradicts observed decoupling of GDP from coal. Recommended SSP2-4.5 or SSP3-7.0 as current-policies baselines. Strong commentary debate (Schwalm-Glendon-Duffy 2020 defended 8.5 as historically tracking cumulative emissions); IPCC AR6 acknowledged the issue but retained SSP5-8.5 for upper-bound risk assessment + high signal-to-noise model evaluation. Burgess-Pielke-Ritchie 2020 also critiqued.

2.7 Storyline approach

Shepherd 2016 + Shepherd-Boyd-Calel-Chapman-Dessai-Dima-West-Fowler-Goodess-Hall-Hallegatte-Hegerl-Knutti-Kretschmer-Lenton-Pinto-Raible-Shaw-Sillmann-Stainforth-Stott-Sutton-Woollings-Zappa 2018 storyline approach: ask “given an event with these large-scale dynamical features, what is the magnitude under climate change?” complements probabilistic scenarios. Used heavily in WG2 chapter on key risks.

3. Integrated Assessment Models (IAMs)

IAMs couple energy-economic + land-use + simple climate modules to compute cost-effective emission pathways under policy constraints. Two families:

  • Cost-benefit IAMs (CB-IAMs): optimise welfare = damages avoided − mitigation cost. DICE, FUND, PAGE. Output: social cost of carbon, optimal trajectory.
  • Process-based IAMs (PB-IAMs / detailed-process IAMs): bottom-up energy + land + agriculture modelling under exogenous targets or policy scenarios. MESSAGE, REMIND, GCAM, IMAGE, AIM, WITCH. Output: technology deployment, sectoral emissions, costs.

3.1 DICE (Dynamic Integrated Climate-Economy)

William Nordhaus, Yale. Original Nordhaus 1992 (Science), DICE-1994, DICE-2007, DICE-2013R, DICE-2016R, DICE-2023 latest. Nobel Prize in Economic Sciences 2018 (shared with Paul Romer) for “integrating climate change into long-run macroeconomic analysis”. Aggregated Cobb-Douglas production with damage function. RICE (Regional Integrated Climate-Economy, Nordhaus-Yang 1996) is regional disaggregation.

Damage function (DICE-2016R): D(T) = 0.00236 T² (fractional GDP loss at T °C above 1900). At 3 °C, ~2.1 % global GDP loss — long criticised as too low (Stern Review 2006; Burke 2015).

3.2 FUND (Climate Framework for Uncertainty, Negotiation, and Distribution)

Richard Tol (Sussex / VU Amsterdam). FUND-3.x current. 16 regions; sector-specific damages — agriculture, sea-level rise (gross migration model), tropical/extratropical storms, heat/cold mortality, disease.

3.3 PAGE (Policy Analysis of the Greenhouse Effect)

Chris Hope (Cambridge Judge). PAGE09, PAGE-ICE 2015. Used in Stern Review (Stern 2006). Stronger tail-risk modelling than DICE.

3.4 MERGE (Model for Evaluating Regional and Global Effects)

Manne-Mendelsohn-Richels-Edmonds 1995; updated 2010. Equilibrium framework with backstop technologies.

3.5 REMIND (Regionalised Model of Investment and Development)

Potsdam Institute for Climate Impact Research (PIK). Luderer-Pietzcker-Bertram-Kriegler-Meinshausen-Edenhofer 2013. CGE + energy-system hybrid. Combined with MAgPIE (land) for AR6 SSP scenarios. Open source.

3.6 MESSAGE-GLOBIOM

International Institute for Applied Systems Analysis (IIASA, Laxenburg, Austria). MESSAGE (Model for Energy Supply System Alternatives and General Environmental impact) coupled with GLOBIOM (Global Biosphere Management Model). Riahi-Rao-Krey 2011 RCP 8.5; Krey-Luderer-Clarke-Kriegler 2014 model description.

3.7 GCAM (Global Change Assessment Model)

Joint Global Change Research Institute (Pacific Northwest National Laboratory / University of Maryland). Edmonds-Reilly 1985 origin; Calvin-Patel-Clarke-Asrar-Bond-Cui-DiVittorio-Dorheim-Edmonds-Eom-Hartin-Hejazi-Horowitz-Iyer-Kyle-Kim-Link-McJeon-Smith-Snyder-Waldhoff-Wise 2019 (GMD) current GCAM v5 description. Open source. 32 regions. Used for RCP 4.5.

3.8 WITCH (World Induced Technical Change Hybrid)

Bocconi University / Fondazione Eni Enrico Mattei (FEEM). Bosetti-Carraro-Galeotti-Massetti-Tavoni 2006. Game-theoretic.

3.9 IMAGE (Integrated Model to Assess the Global Environment)

Netherlands Environmental Assessment Agency (PBL). Stehfest-vanVuuren-Kram-Bouwman-Alkemade-Bakkenes-Biemans-Bouwman-denElzen-Janse-Lucas-vanMinnen-Müller-Prins 2014 description. Used for RCP 2.6.

3.10 AIM (Asia-Pacific Integrated Model)

National Institute for Environmental Studies (NIES, Japan). Kainuma-Matsuoka-Morita 1999. Used for RCP 6.0 (Masui 2011).

3.11 POLES (Prospective Outlook on Long-term Energy Systems)

IEPE/Enerdata (Grenoble, France). Bottom-up energy demand + supply.

3.12 MARKAL / TIMES

ETSAP (Energy Technology Systems Analysis Programme) under IEA. Loulou-Goldstein-Noble 2004 (MARKAL); Loulou-Remne-Kanudia-Lehtila-Goldstein 2005 (TIMES). LP-based bottom-up energy technology models. Used by national energy planning (UK DECC TIMES, US EPA MARKAL).

3.13 Critique

Pindyck 2017 (“The Use and Misuse of Models for Climate Policy”, Rev Env Econ Policy) argued IAMs “should not be used in climate-policy analysis” — damage functions arbitrary, discount rates politically loaded. Stern-Stiglitz 2021 (“The Social Cost of Carbon, Risk, Distribution, Market Failures”, J Econ Methodology) developed alternative wealth-based framework with much higher SCC. Weitzman 2009 (“Dismal theorem”) on heavy-tailed damages. Pindyck 2013. Carleton-Hsiang 2016 review. Howard-Sterner 2017 quantification.

4. Social Cost of Carbon (SCC)

4.1 Definition

SCC: present discounted value of marginal climate damages from an additional ton of CO2 emitted in a given year. Inputs: emission trajectory → climate response → damages → discounting.

4.2 US Interagency Working Group (IWG) values

  • IWG 2010 (Obama): SCC $21/tCO2 at 3 % discount, year 2010, central. Used DICE, FUND, PAGE averaging.
  • IWG 2013 update: $36/tCO2 at 3 %.
  • IWG 2016 update: $42/tCO2 at 3 % (2020 emissions year, 2020 dollars).
  • Trump rescinded IWG 2017–21; reverted to domestic-only damages → ~$1–7/tCO2.
  • Biden Executive Order 13990 (Jan 2021) restored IWG, interim values ~$51/tCO2 at 3 % discount, pending update.
  • EPA Sep 2022 draft → Nov 2023 final (“Report on the Social Cost of Greenhouse Gases”, EPA 2023): SCC 80–340 across 1.5–2.5 % discount. Methodology: Rennert-Errickson-Prest-Rennels-Newell-Pizer-Kotchen-Cooke-Diaz-Ewing-Fillmore-Frase-Hardy-Hoffmann-Holzer-Howard-Kingdon-Kopits-Kruse-Liebrand-Madsen-Mahowald-Mendoza-Moore-Moyer-Nicholas-Plevin-Prinn-Reilly-Sarofim-Smith-Stehfest-Stiglitz-Toth-vanderWerf-vanRuijven-Wagner 2022 Nature “Comprehensive evidence implies a higher social cost of CO2” — central 44–413.
  • SC-CH4: 54 000/tN2O (EPA 2023).

4.3 Discount rate

Most controversial single parameter. Ramsey rule: r = delta + eta · g.

  • Nordhaus DICE: delta ≈ 1.5 % pure time preference, eta ≈ 1.45, g ≈ 2 % → r ≈ 4–5 %.
  • Stern Review (2006): delta ≈ 0.1 %, eta = 1 → r ≈ 1.4 %; produced ~$85/tCO2 in 2006.
  • IWG 2010 used 2.5 %, 3 %, 5 % range.
  • IWG 2016 added “average 95th percentile at 3 %” to capture tail risk.
  • EPA 2023 shifted to Ramsey-based endogenous discount rates calibrated to long-run interest rates (Council of Economic Advisers + OMB Circular A-4 2023 revision suggested 2 % as default for inter-generational analyses).

4.4 Tail risk and dismal-theorem objections

Weitzman 2009 (“On Modeling and Interpreting the Economics of Catastrophic Climate Change”, Rev Econ Stat): if loss distribution is heavy-tailed and marginal utility unbounded, expected discounted damage → infinity. Pindyck 2013 critique. Bressler 2021 (Nature Comms) “mortality cost of carbon” — 1 metric ton CO2 → 2.26e-4 excess deaths globally by 2100 = $258/tCO2 from mortality alone. Cromar-Markandya-vanReusel-Carlton-Hutchinson-Honda 2022 health damage update.

4.5 Damage functions

  • DICE quadratic: D = 0.00236 T².
  • Burke-Hsiang-Miguel 2015 (Nature) “Global non-linear effect of temperature on economic production” — country-level temperature-GDP relationship is concave with peak at ~13 °C annual temperature; warming reduces GDP per capita in hot countries, mildly benefits cold ones; aggregate global GDP loss ~23 % at 4 °C warming. Major upward revision of damage estimates.
  • Kahn-Mohaddes-Ng-Pesaran-Raissi-Yang 2021 (Energy Economics) IMF working paper — climate-vulnerable countries lose more GDP, 7.2 % global GDP loss in RCP 8.5 by 2100.
  • Carleton-Jina-Delgado-Greenstone-Houser-Hsiang-Hultgren-Kopp-McCusker-Nath-Rising-Rode-Seo-Viaene-Yuan-Zhang 2022 QJE — health-mortality damages.
  • Howard-Sterner 2017 (“Few and Not So Far Between: A Meta-analysis of Climate Damage Estimates”, Env Resource Econ): meta-analysis raises damages above DICE.

5. 1.5 °C and 2 °C pathways

5.1 IPCC SR1.5 (Special Report on 1.5 °C, Oct 2018)

Masson-Delmotte-Zhai-Pörtner-Roberts-Skea-Shukla-Pirani-Moufouma-Okia-Péan-Pidcock-Connors-Matthews-Chen-Zhou-Gomis-Lonnoy-Maycock-Tignor-Waterfield 2018. Headline: limiting warming to 1.5 °C requires “rapid, far-reaching and unprecedented changes in all aspects of society”. 4 illustrative pathways:

  • P1: low energy demand, low BECCS, no DACS.
  • P2: sustainability + reforestation focus.
  • P3: middle-of-the-road with moderate CDR (~150 GtCO2 by 2100).
  • P4: resource + energy intensive, large BECCS (~1200 GtCO2 by 2100), high overshoot.

Median pathway: -45 % global net CO2 emissions by 2030 (vs 2010), net zero by 2050.

5.2 Overshoot pathways

Allow temporary exceedance of 1.5 °C followed by CDR-driven decline. AR6 WG3 categorised:

  • C1: no/limited overshoot, peak warming <1.5 °C (50 % prob).
  • C2: high overshoot, return to 1.5 °C by 2100.
  • C3: 2 °C limit (67 % prob).
  • C4-C8: warmer outcomes.

Overshoot risks: irreversible coral loss, Arctic sea-ice loss, Greenland MISI triggering (Schleussner 2016).

5.3 CDR requirements

AR6 1.5 °C pathways need cumulative CDR 100–1 000 GtCO2 by 2100. BECCS dominates in most IAM pathways; afforestation + DAC supplemental. Babiker-Bertoldi-Buckeridge-Cartwright-deConinck-Riahi-Schweikert-Smith-Tavoni-Wilson 2022 AR6 WG3 Ch 12 chapter on cross-sectoral perspectives. CDR scale-up concerns: land + water + biodiversity for BECCS (Smith-Davis-Creutzig-Fuss-Minx-Gabrielle-Kato-Jackson-Cowie-Kriegler-vanVuuren-Rogelj-Ciais-Milne-Canadell-McCollum-Peters-Andrew-Krey-Shrestha-Friedlingstein-Gasser-Grübler-Heidug-Jonas-Jones-Kraxner-Littleton-Lowe-Moreira-Nakicenovic-Obersteiner-Patwardhan-Rogner-Rubin-Sharifi-Torvanger-Yamagata-Edmonds-Yongsung 2016 Nature Climate Change).

5.4 Carbon-budget framing

TCRE-based remaining budget for 1.5 °C (50 % prob): ~250 GtCO2 from start of 2024 (Forster 2024 Indicators of Global Climate Change update). 67 % prob budget ~150 GtCO2. Current emissions ~40 GtCO2 yr-1 → 5–6 years at current pace.

6. UNFCCC and the policy interface

6.1 UNFCCC

UN Framework Convention on Climate Change opened for signature Rio Earth Summit 1992, entered force Mar 1994. 198 Parties. Annual COPs (Conference of the Parties): COP1 Berlin 1995 → COP28 Dubai 2023 (UAE Presidency, “UAE Consensus” — first text mentioning “transitioning away from fossil fuels in energy systems”) → COP29 Baku 2024 → COP30 Belém Brazil 2025.

6.2 Kyoto Protocol (1997, entered force 2005)

Annex I (developed) countries binding 5.2 % below 1990 emissions in 2008–12 commitment period. Flexibility mechanisms: Clean Development Mechanism (CDM), Joint Implementation (JI), Emissions Trading (ET). Doha Amendment (2012) for 2013–20 second commitment period; US never ratified Kyoto; Canada withdrew 2011.

6.3 Paris Agreement (2015, entered force Nov 2016)

Adopted COP21 Paris. 195 Parties. Goals:

  • Hold global temperature rise “well below 2 °C above pre-industrial” and pursue efforts to limit to 1.5 °C.
  • Increase adaptive capacity, foster climate resilience.
  • Make finance flows consistent with low-GHG, climate-resilient development.

Mechanism: Nationally Determined Contributions (NDCs), updated every 5 years with progressively ambition (no retreat). First NDCs 2015–16, updates 2020–21, due again 2025 (NDC 3.0) and 2030.

Current NDCs (as of CAT/UNFCCC NDC Synthesis Report 2023): if fully implemented imply 2.5–2.9 °C warming by 2100. With current policies (CAT Climate Action Tracker Nov 2024): 2.7 °C. “Pledges + Targets” (full net-zero implementation): 2.1 °C. Implementation gap of ~50 % between policies and pledges.

6.4 Global Stocktake (GST)

Paris Agreement Article 14. Every 5 years from 2023. GST1 concluded COP28 Dec 2023 — first formal assessment: “Parties are not on track” to meet 1.5 °C; called for tripling renewables and doubling energy efficiency by 2030, transitioning away from fossil fuels in energy systems. Second GST due 2028, informed by AR7.

6.5 Loss and Damage Fund

COP27 Sharm el-Sheikh Nov 2022 created loss and damage fund (LDF) for climate-vulnerable developing countries. Operationalised COP28 Nov-Dec 2023: World Bank as interim host for 4 yr; initial pledges ~100 M, Germany 17.5 M — criticised as inadequate). Board met 2024; first disbursements anticipated 2025. Standalone treaty (vs UNFCCC subsidiary body) status under negotiation.

6.6 Article 6

Paris Agreement Article 6 establishes voluntary cooperation mechanisms:

  • 6.2 Internationally Transferred Mitigation Outcomes (ITMOs): bilateral/multilateral transfers; first ITMOs Switzerland-Ghana 2024.
  • 6.4 Sustainable Development Mechanism: centralised, replacing CDM. Rulebook finalised COP29 Baku Nov 2024.
  • 6.8 Non-market approaches.

6.7 Carbon markets

EU ETS (cross-ref carbon-markets-and-compliance); California Cap-and-Trade; RGGI; UK ETS; China national ETS (2021+, world’s largest by emissions, power sector); Korea ETS; New Zealand ETS; CORSIA aviation offsetting.

CBAM: EU Carbon Border Adjustment Mechanism (Regulation 2023/956) — cement, iron+steel, aluminium, fertilisers, electricity, hydrogen. Transitional phase Oct 2023, full obligations 2026.

7. Climate Action Tracker (CAT) and accountability

Independent scientific analysis (Climate Analytics + NewClimate Institute) tracking governmental climate action. Ratings: 1.5 °C compatible / Almost sufficient / Insufficient / Highly insufficient / Critically insufficient.

Nov 2024 ratings (selected): India “Highly insufficient”, China “Highly insufficient”, US “Insufficient”, EU “Insufficient”, UK “Almost sufficient”, Australia “Insufficient”, Saudi Arabia “Critically insufficient”, Türkiye “Critically insufficient”. No major emitter “1.5 °C compatible”.

8. Net-zero pledges

By Apr 2024 (Net Zero Tracker, ECIU + Oxford):

  • 150 countries with net-zero targets (mostly 2050, China 2060, India 2070).
  • 1 000+ subnational jurisdictions.
  • 1 700+ corporates (Microsoft -2030, Apple -2030, Amazon -2040, Shell -2050).

Concerns:

  • Only ~30 % of country targets are in law.
  • Many rely heavily on offsets (Bezos Earth Fund + Verra credibility crises 2023 Guardian investigation; ICVCM/VCMI 2024 reforms).
  • “Carbon-neutral” vs “net-zero” definitional ambiguity.
  • Race-to-Zero campaign criteria tightened 2022.

SBTi (Science Based Targets initiative) provides corporate target validation; SBTi 1.5 °C pathway requires ~4.2 % yr-1 emission reduction. SBTi Corporate Net-Zero Standard (2021, updated 2024).

9. Recent integrative work

  • IPCC AR6 SYR Mar 2023.
  • UNEP Emissions Gap Report (annual, Oct/Nov). 2024 edition “No more hot air … please!”: current policies → 3.1 °C; full implementation of NDCs → 2.6–2.8 °C; full implementation + NZ pledges → 1.9 °C.
  • IEA World Energy Outlook (annual Nov) + Net Zero by 2050 roadmap (2021, updated 2023).
  • IRENA World Energy Transitions Outlook.
  • Global Carbon Project annual budget (Friedlingstein 2024).

10. IAM technology assumptions

10.1 Energy supply

IAM technology databases include: solar PV (utility-scale, distributed), CSP, onshore + offshore wind, hydro (run-of-river, reservoir), nuclear (Gen-III LWR, SMR, Gen-IV), CCS-paired fossil (coal-IGCC-CCS, gas-CCS), bioenergy (with + without CCS), geothermal (hydrothermal + enhanced/EGS), tidal/wave.

Learning rates (cost reductions per doubling of cumulative deployment):

  • Solar PV modules: 23 % (Wright 1936; Way-Ives-Mealy-Farmer 2022 Joule).
  • Onshore wind: 8–12 %.
  • Lithium-ion batteries: 20 % (Ziegler-Trancik 2021 Energy Env Sci).
  • Electrolysers (PEM/alkaline): 15–18 % projected.
  • Nuclear: negative learning rate observed historically (Grubler 2010 Energy Policy).

10.2 End-use sectors

  • Industry: steel (BF-BOF, EAF, DRI-H2), cement (clinker substitution, CCS), chemicals (electrification, e-fuels), aluminium (inert anodes).
  • Transport: BEV, FCV, biofuels, SAF, e-fuels; modal shift.
  • Buildings: heat pumps, deep retrofit, district heating, electrification of cooking.
  • Agriculture + land: dietary shift, food-system efficiency, afforestation.

10.3 Negative emissions in IAMs

Median IPCC SR1.5 1.5 °C-with-no/limited-overshoot pathway uses ~150 GtCO2 cumulative CDR by 2100 (BECCS dominant). Higher-overshoot uses 500–1 200 GtCO2. C1 category (AR6) median ~340 GtCO2 cumulative CDR.

10.4 Energy demand

IAM “demand-side” mitigation studies (Grubler-Wilson-Bento-Boza-Kiss-Krey-McCollum-Rao-Riahi-Rogelj-DeStercke-Cullen-Frank-Fricko-Guo-Gidden-Havlík-Huppmann-Kiesewetter-Rafaj-Schoepp-Valin 2018 LED low-energy-demand pathway). End-use efficiency, sufficiency.

11. Macroeconomic and welfare frameworks

11.1 Production functions

Cobb-Douglas Y = A · K^alpha · L^(1-alpha) standard in DICE. CES (constant elasticity of substitution) in REMIND. Sector-level CGE in MIT EPPA + MERGE.

11.2 Discount-rate frameworks

  • Prescriptive (ethical): low pure-time preference, focus on intergenerational equity (Stern, Cline 1992, Broome 2012).
  • Descriptive (revealed preference): market interest rates, ~4–5 %.
  • Ramsey rule r = delta + eta · g.
  • Declining discount rate (DDR): Weitzman 2001 + Newell-Pizer 2003; UK Treasury Green Book (2003 + updated 2018) uses 3.5 % for first 30 yr → 3.0 % 31–75 → 2.5 % 76–125 → 2.0 % 126–200 → 1.5 % 201–300.

11.3 Equity-weighted SCC

If aggregating across countries with different income, equity weights raise SCC ~5×; Anthoff-Hepburn-Tol 2009 + Adler-Anthoff-Bosetti-Garner-Keller-Treich 2017.

11.4 Damage function shape

  • Quadratic (DICE).
  • Sigmoidal (Weitzman 2010 tipping points).
  • Multiplicative across sectors (Howard-Sterner 2017).
  • Climate-induced productivity damage (Burke-Hsiang-Miguel 2015).

12. Sectoral mitigation pathways (IPCC AR6 WG3)

12.1 Electricity

Decarbonisation requires >90 % low-carbon by 2050 in 1.5 °C pathways. Renewables (solar + wind + storage) dominate; backup via dispatchable low-carbon (nuclear, hydro, gas-CCS, bioenergy-CCS).

12.2 Industry

Heavy industry hardest to abate. Steel: H2-DRI route emerging (HYBRIT Sweden 2026 first commercial, Stegra/H2 Green Steel 2026 5 Mt/yr). Cement: Heliogen + Sublime Systems + Brimstone + Fortera alternative chemistries 2024 pilots.

12.3 Transport

  • LDV: passenger BEVs widely deployed; 18 % global new car sales 2023 (IEA EV Outlook 2024).
  • HDV: BEVs + FCVs + sustainable diesel for long-haul.
  • Aviation: SAF + e-fuels; ICAO CORSIA + EU SAF mandate ReFuelEU 2 % 2025 → 70 % 2050; battery-electric short-haul.
  • Shipping: ammonia + methanol e-fuels; IMO 2023 strategy 20 % reduction 2030, net-zero 2050.

12.4 Buildings

Heat pumps replacing gas furnaces (UK 600 k 2028 target; US IRA Sec 25C 4.5 B Home Electrification rebates).

12.5 Agriculture, Forestry, Other Land Use (AFOLU)

CDR via afforestation; methane reduction (rice management, livestock); N2O reduction (precision fertilisation); dietary shift.

13. CDR in IAMs

13.1 BECCS

Most prominent IAM CDR. Limits: 1–7 GtCO2 yr-1 by 2100 (Fuss-Lamb-Callaghan-Hilaire-Creutzig-Amann-Beringer-deOliveira-Garcia-Hartmann-Khanna-Luderer-Nemet-Rogelj-Smith-Vicente-Wilcox-delMar-Zamora-Dominguez-Minx 2018 Env Res Lett review of NETs). Land + water + biodiversity constraints binding above ~3 GtCO2 yr-1.

13.2 Afforestation in IAMs

Bottom-up estimates 0.5–3.6 GtCO2 yr-1 (IPCC SRCCL 2019). MAgPIE land-use model coupled with REMIND. GLOBIOM coupled with MESSAGE.

13.3 DAC in IAMs

DAC + DACCS rarely modelled at >5 GtCO2 yr-1 due to energy + cost. AR6 Ch 12 included DACCS more prominently than AR5.

13.4 Critique

van Vuuren-Stehfest-Gernaat-vandenBerg-Bijl-deBoer-Daioglou-Doelman-Edelenbosch-Harmsen-Hof-vanSluisveld 2018 (Nature Climate Change) “Alternative pathways to the 1.5 °C target reduce the need for negative emission technologies” — showed feasibility of low-NET pathways via radical demand-side change.

14. National + corporate accountability

14.1 NDC quality

Climate Watch + WRI track NDCs. Most NDCs lack 2030 sectoral pathways; only ~30 % include sector-specific targets.

14.2 Long-Term Strategies (LTS)

Paris Article 4.19 invites Parties to submit mid-century LTS. ~70 LTS submitted as of 2024 — US LTS Nov 2021, EU 2050 strategy 2018, China 2030 + 2060 commitments.

14.3 EU Fit for 55

European Climate Law (Regulation 2021/1119) — 55 % reduction by 2030 vs 1990, climate neutrality 2050. Fit-for-55 package: ETS reform (Phase 4 cap reduction, MSR), ETS2 for road + buildings (2027+), CBAM, RED III (renewables target 42.5 % 2030), Energy Efficiency Directive recast, AFIR (Alternative Fuels Infrastructure), Effort Sharing Regulation, LULUCF.

14.4 US IRA

Inflation Reduction Act Aug 2022: 0.60–7 500), 25C/25D home efficiency. Bipartisan Infrastructure Law (2021) 73 B power grid, $7.5 B EV chargers.

14.5 China

14th Five-Year Plan (2021–25) climate provisions. National ETS launched July 2021 — power sector only, ~4 500 facilities, ~4 GtCO2 coverage (largest ETS). Cement + aluminium expected 2025. “1+N” policy framework (Working Guidance on Carbon Peaking + Carbon Neutrality Goals, Oct 2021). Recent: Apr 2024 announcement to bring methane to NDC by NDC 3.0.

14.6 Corporate

SBTi: 4 000+ companies committed by 2024; ~2 000 have validated 1.5 °C-aligned targets. CDP disclosure: 24 000+ companies disclosed 2024.

ISSB (International Sustainability Standards Board) IFRS S1 + S2 issued Jun 2023; voluntary global standards; EU CSRD (Corporate Sustainability Reporting Directive 2023) mandatory; SEC climate disclosure rule Mar 2024 (then voluntarily stayed pending litigation).

TCFD (Task Force on Climate-related Financial Disclosures 2017 + 2021) framework; now folded into ISSB.

15. Climate finance

15.1 $100 B promise + NCQG

Copenhagen 2009 pledged developed-country 300 B yr-1 by 2035 (developing country expectation $1 T+; politically contested).

15.2 GCF

Green Climate Fund (operational 2015): $13 B replenished by 2024. Projects in 130+ countries.

15.3 Multilateral development banks

World Bank + ADB + AfDB + IDB + EBRD + AIIB committed 50–80 % climate alignment by 2025–30. Just Energy Transition Partnerships (JETPs): SA $8.5 B (2021), Indonesia $20 B (2022), Vietnam $15.5 B (2022), Senegal €2.5 B (2023).

15.4 Adaptation finance

~215–387 B yr-1 needed by 2030 (UNEP Adaptation Gap Report 2024).

Adjacent

Graph View