CLIM-15 / SPM.B / Research primer
How much does a remaining carbon budget depend on its assumptions?
How do published remaining-budget estimates vary with temperature limit, likelihood and assumptions about non-CO2 warming?
NASA/Apollo 17 crew. Earth with Africa and Antarctica visible, photographed by the Apollo 17 crew on December 7, 1972. Contextual image; not used to estimate a trend. Image credits.
The question
A remaining carbon budget is conditional accounting, not a deadline on a clock. The proposed comparison separates emissions since an assessment from revisions to its underlying assumptions.
Why it matters
A single budget number can conceal materially different risk tolerances and non-CO2 pathways, changing the interpretation of near-term mitigation choices.
The AR6 anchor
AR6 assesses cumulative CO2 emissions and this decade's greenhouse gas reductions as central to limiting warming; overshoot brings additional risks (high confidence).
AR6 Synthesis Report (2023) / B.5; B.7.
Evidence and comparison
- Unit of analysis
- Global budget-assessment-case and year of emissions accounting.
- Baseline and denominator
- Keep each budget's assessment start date, temperature reference and probability definition. Align emitted CO2 only after reconciling land-use and cement-carbonation boundaries.
- First descriptive test
- Compare published budget values across temperature limits and likelihoods; distinguish elapsed emissions from method revisions rather than attributing every revision to new emissions.
- Deeper analysis
- Reproduce only published sensitivity cases for non-CO2 assumptions and zero-emissions commitment. Any constant-emissions arithmetic is a labeled illustration, not an exhaustion forecast.
- Attribution status
- projection/scenario analysis
- Uncertainty
- State which uncertainties each likelihood includes; a range conditional on climate response alone is not total uncertainty about a temperature outcome.
Principal sources
- Indicators of Global Climate Change
Remaining carbon budget (GtCO2); Temperature limit and likelihood; Published sensitivity cases.
Target IGCC 2025 budgets from start-2026; table and sensitivity export checks pending. Candidate source; import and publication checks are still required. - Global Carbon Budget
Annual fossil and land-use CO2; Published accounting adjustments and uncertainty.
Target 2019-2024 plus separately flagged 2025 projection; release checks pending. Candidate source; import and publication checks are still required.
Alternative explanations
- Non-CO2 pathways
- Earth-system feedbacks
- Zero-emissions commitment
- Inventory revisions
The visual argument
Provisional: budget interval comparison with likelihood selection; annotate assessment date, included uncertainties and accounting revisions, avoiding countdown or deadline imagery.
- Temperature-limit comparison
- Published sensitivity-case table
- Elapsed-emissions accounting
- Assessment-boundary timeline
What these data do not show
- A budget does not assign national entitlements or legal obligations.
- Budget exhaustion and crossing a warming level are different events.
Reproducibility and next release
Plan src/build.mjs and data/processed/budget-cases.csv from versioned data/raw tables; check units, likelihood labels, start dates and accounting boundaries, preserving source dependence, hashes and rights in provenance.
- After approval: carbon-budget study
- Article and conditional-accounting methods
- Responsive figures and exact cases
- 16:9 hero and LinkedIn launch
- Optional PDF carousel
- Chart exports and case CSV