Carbon Budgets
Only 235 Gt CO₂ remaining for 1.5°C (50% probability)—6 years at current 37.9 Gt/year. 83% probability budget exhausted in 2025. For 2°C: 1,110 Gt left (27 years at current rate). Already emitted 2,550 Gt since 1850. Paris NDCs pathway: 2.4-2.7°C by 2100. Overshoot likely—need CDR (carbon dioxide removal) to return below 1.5°C post-2050.
Key Carbon Budget Insights
Budget Nearly Exhausted
1.5°C budget 235 Gt remaining (50% probability), 6 years at 37.9 Gt/year. 83% probability budget already exhausted—means likely overshoot 1.5°C then return via CDR. Total 1.5°C budget ~3,760 Gt (from 1750), used 2,550 Gt (68%). Every 0.1°C warming costs ~420 Gt budget. Uncertainty ±250 Gt due to climate sensitivity (how much warming per CO₂), non-CO₂ gases (methane, N₂O), carbon cycle feedbacks (permafrost, forests). Budget shrinking 38 Gt/year—delay makes 1.5°C mathematically impossible.
2°C More Feasible, Still Hard
2°C budget 1,110 Gt remaining (50% probability), 27 years at current rate. 67% probability budget: 900 Gt (22 years). More breathing room than 1.5°C but requires -43% emissions by 2030 vs current +2.5% (2020-2026). Paris Agreement goal: "well below 2°C, pursue 1.5°C." Current NDCs (national pledges): 2.4-2.7°C warming by 2100. Emissions gap: 15 Gt/year between Paris pledges and 2°C pathway. Even 2°C means severe impacts: 99% coral loss, 10m sea rise (centuries), extreme heat frequent.
Net Zero by 2050 Critical
IPCC 1.5°C pathways require net zero CO₂ by 2050, net zero GHG by 2070. "Net zero" = emissions balanced by removals (forests, DACCS, BECCS). Peak warming 1.6-1.8°C (mid-century overshoot), return to 1.5°C by 2100 via 5-10 Gt/year CDR. Current trajectory: net zero 2080 (EU), 2050 (USA target), 2060 (China), 2070 (India). Gap: need -7%/year emissions cuts vs current +0.5%/year. Every 5 years delay adds 0.1°C warming. Already committed to 1.5°C from emissions-to-date plus thermal inertia.
Budget Uncertainties Large
1.5°C budget uncertainty ±250 Gt (±50%). Climate sensitivity range: 2.5-4°C per CO₂ doubling (IPCC AR6). If high (4°C), budget 100 Gt not 235. Non-CO₂ warming: methane 30% of warming—if cuts fail, CO₂ budget shrinks. Carbon cycle feedbacks: permafrost thaw (+200 Gt CO₂e by 2100), Amazon dieback (carbon source not sink), ocean acidification (reduces uptake). Tipping points: AMOC collapse, ice sheet disintegration—low probability but irreversible. Precautionary principle: aim for 67% or 83% probability (stricter budgets) not 50%.
Remaining Carbon Budgets by Temperature Target
Gigatonnes CO₂ remaining from 2026 for 50% and 67% probabilities
Key Finding: 1.5°C (50%): 235 Gt, 1.5°C (83%): exhausted (negative), 1.7°C (50%): 585 Gt, 2°C (50%): 1,110 Gt, 2°C (67%): 900 Gt. Already used 2,550 Gt (1850-2026). Higher probability = stricter budget—83% confidence means earlier net zero. Every 0.1°C costs ~420 Gt. Uncertainty bars ±250 Gt (climate sensitivity, non-CO₂, feedbacks). Context: 1.5°C budget 6 years, 2°C budget 27 years at 37.9 Gt/year current rate. Paris Agreement ambiguity: "well below 2°C, pursue 1.5°C"—budgets differ by 3×.
Carbon Budget Depletion Timeline
Years until budget exhaustion at different emission reduction rates
Key Finding: No cuts (37.9 Gt/year flat): 1.5°C budget 6 years (2032), 2°C budget 27 years (2053). Slow cuts -2%/year: 1.5°C 8 years, 2°C 35 years. Moderate -5%/year: 1.5°C 10 years, 2°C 50 years. Aggressive -7%/year (IPCC 1.5°C pathway): 1.5°C 12 years, 2°C 65 years. Current trend +0.5%/year (2020-2026): budgets exhausted faster. Delays costly: every 5 years inaction costs 0.1°C warming. 2010 vs 2026: budget shrunk 600 Gt (16 years × 37 Gt/year average). Lesson: the sooner cuts start, the longer runway to net zero.
Temperature vs Cumulative Emissions
Relationship between total CO₂ emitted and global warming (TCRE)
Key Finding: Near-linear relationship: +0.45°C per 1,000 Gt CO₂ (TCRE—transient climate response to emissions). Emitted 2,550 Gt → +1.15°C from CO₂. Add non-CO₂ (methane +0.5°C, N₂O +0.1°C, aerosols cooling -0.5°C) → total +1.36°C (2026). For 1.5°C: total budget 3,760 Gt (from 1750). For 2°C: 5,200 Gt. Already used 68% of 1.5°C budget. Uncertainty band ±0.15°C (climate sensitivity range 2.5-4°C per CO₂ doubling). Key insight: warming proportional to cumulative emissions—annual rate doesn't matter, only total. Can't "pause" warming by pausing emissions growth—need net zero to stabilize temperature.
Emissions Pathways for Temperature Targets (SSPs)
IPCC scenarios: SSP1-1.9 (1.5°C), SSP1-2.6 (2°C), SSP2-4.5 (2.7°C), SSP5-8.5 (4°C+)
Key Finding: SSP1-1.9 (1.5°C, 50% overshoot): emissions peak 2025, -43% by 2030, net zero 2050, negative 2060-2100 (-5 Gt/year CDR). SSP1-2.6 (2°C, no overshoot): peak 2030, -25% by 2030, net zero 2070. SSP2-4.5 (Paris NDCs current): peak 2040, slow decline, 2.7°C by 2100. SSP5-8.5 (no policy): +1%/year growth, 80 Gt by 2100, 4-5°C warming. Current trajectory: between SSP2-4.5 and SSP3-7.0 (2.4-2.7°C). Gap: need SSP1-1.9 pathway (-43% by 2030) vs current +2.5% (2020-2026). Paris pledges insufficient—emissions gap 15 Gt/year in 2030.
Annual Emissions Needed for 1.5°C & 2°C
Required emissions trajectory 2026-2050 vs current path
Key Finding: 1.5°C pathway: 37.9 Gt (2026) → 22 Gt (2030, -43%) → 10 Gt (2040) → net zero (2050) → negative (2060-2100, -5 Gt/year). 2°C pathway: 37.9 Gt (2026) → 28 Gt (2030, -25%) → 18 Gt (2040) → 5 Gt (2050) → net zero (2070). Current trajectory (Paris NDCs): 37.9 Gt (2026) → 40 Gt (2030, +5%) → 38 Gt (2050) → 25 Gt (2100), 2.5°C warming. Gap enormous: need -7%/year cuts (1.5°C) vs current +0.5%/year. Requires 3× faster renewable deployment, fossil fuel phaseout by 2040, massive CDR post-2050. No credible pathway without overshoot—1.6-1.8°C peak, return to 1.5°C via CDR by 2100.
Probability Distribution of Peak Warming
Likelihood of different temperature outcomes under current policies vs Paris targets
Key Finding: Current policies: <5% chance 1.5°C, 10% chance 2°C, 50% chance 2.5°C, 25% chance 3°C+. Paris NDCs (if met): 10% chance 1.5°C, 30% chance 2°C, 50% chance 2.4-2.7°C. Net zero 2050 (1.5°C pathways): 50% chance 1.5°C (with overshoot), 83% chance below 2°C. Uncertainty large: climate sensitivity (2.5-4°C per CO₂ doubling), carbon cycle feedbacks, tipping points (AMOC, ice sheets). High-end risk: 5% chance 4°C+ even with moderate policy. Low-end optimism: 17% chance stay below 1.5°C if aggressive action starts immediately. Precautionary principle: plan for 67% or 83% confidence, not 50%.
Understanding Carbon Budgets
Key Concepts
What is a Carbon Budget?: Total amount of CO₂ that can be emitted while staying below a temperature target (e.g., 1.5°C). Cumulative, not annual—sums all emissions from 1750 to net zero date. Based on near-linear relationship: warming proportional to cumulative CO₂ (TCRE—transient climate response to emissions ≈ 0.45°C per 1,000 Gt). Total 1.5°C budget ~3,760 Gt CO₂ (from 1750). Already emitted 2,550 Gt (1850-2026), leaving 235 Gt. Budget decreases every year—shrinking 37.9 Gt/year at current rate.
50% vs 67% vs 83% Probability: Budgets stated with probability of staying below target. 50% = coin flip (1.5°C budget 235 Gt). 67% = "likely" (IPCC terminology, budget ~100 Gt or exhausted). 83% = "very likely" (budget negative—already overshot). Higher confidence = stricter budget. Paris Agreement doesn't specify probability—"well below 2°C, pursue 1.5°C" ambiguous. Scientists recommend 67% for risk management. 50% budgets often cited because less alarming, but 50% means equal chance of exceeding target.
Remaining vs Total Budget: Total = cumulative emissions from 1750 to net zero that keep warming below target. Remaining = total minus already emitted. 1.5°C total ~3,760 Gt, emitted 2,550 Gt, remaining 235 Gt (some estimates 200-250 Gt range, updated annually as science improves). 2°C total ~5,200 Gt, emitted 2,550 Gt, remaining 1,110 Gt. Budgets revised by IPCC every 5-7 years—AR5 (2013), AR6 (2021), next AR7 (2028). Generally shrinking due to better understanding of climate sensitivity and non-CO₂ warming.
Overshoot Scenarios: 1.5°C budget so small, most pathways assume overshoot—peak 1.6-1.8°C mid-century, return to 1.5°C by 2100 via CDR (carbon dioxide removal). Requires net negative emissions -5 to -10 Gt/year (2060-2100). CDR methods: DACCS (direct air capture $200-600/tonne), BECCS (bioenergy + CCS $100-200/tonne), afforestation (cheap but land-limited), soil carbon, ocean alkalinity. Overshoot risks: tipping points crossed (coral die-off, permafrost thaw), irreversible losses. "Temporary" overshoot may be permanent if CDR fails to scale.
Uncertainties & Challenges
- Climate Sensitivity: How much warming per CO₂ doubling (280 → 560 ppm). IPCC AR6 range: 2.5-4°C (66% confidence). If low (2.5°C), 1.5°C budget 400 Gt. If high (4°C), budget 100 Gt or negative. Affects carbon budgets ±250 Gt. Paleoclimate data (ice cores, corals) suggest middle (3°C) most likely. Clouds, water vapor feedbacks poorly constrained—biggest uncertainty source.
- Non-CO₂ Gases: Methane (CH₄) causes 30% of warming, nitrous oxide (N₂O) 6%, F-gases 2%. Warming from these reduces CO₂ budget available. If methane cuts fail (livestock, oil/gas leaks), CO₂ budget shrinks. Methane 28× CO₂ over 100 years but short-lived (12-year lifetime)—fast cuts reduce warming quickly. N₂O long-lived (120 years), mainly agriculture (fertilizer)—hard to cut. Budgets usually state "CO₂ only"—actual GHG budget lower.
- Carbon Cycle Feedbacks: Warming weakens natural sinks. Forests stressed (drought, fire)—Amazon could flip from sink to source (releases 10 Gt/year not absorbs 5 Gt). Permafrost thaw releases 200 Gt CO₂e by 2100 (methane, CO₂). Ocean warming reduces CO₂ uptake—cold water absorbs more. Airborne fraction rising: 45% (1960s) → 52% (2020s). Feedback loop: more warming → weaker sinks → faster warming. Not fully included in simple carbon budgets—real budget likely smaller.
- Tipping Points: Low probability but high impact. AMOC (Atlantic circulation) collapse—Northern Europe cools, monsoons shift. West Antarctic ice sheet collapse—3m sea rise over centuries, irreversible. Amazon dieback—rainforest → savanna, releases 100 Gt CO₂. Arctic sea ice loss (already occurring)—albedo feedback accelerates warming. Tipping cascades possible. Budgets assume linear climate response—tipping points break that. Precautionary approach: stay well below 2°C to avoid risks.
Paris Agreement & National Pledges
Paris targets: "Well below 2°C, pursue 1.5°C" above pre-industrial. Submitted NDCs (nationally determined contributions)—national pledges. Current NDCs (2025 update): lead to 2.4-2.7°C warming by 2100 (Climate Action Tracker). Emissions gap: 15 Gt/year difference between Paris pathway and NDC pathway in 2030. Only 6 G20 countries on track: EU, UK, USA (barely), Japan (barely), Russia (collapse not policy), Argentina. Major emitters exceeding pledges: China +5%, India +45%, Indonesia +60% vs 2019 baselines. Net zero pledges: 90+ countries pledged (88% of emissions), but few with credible policies. EU 2050 (on track), USA 2050 (off track), China 2060 (vague), India 2070 (aspirational). Net zero requires -100% fossil CO₂ + CDR to offset residual (aviation, agriculture). Current trajectory: net zero 2080-2090 not 2050.
Emissions Reduction Pathways
IPCC 1.5°C pathways (AR6 WG3): 97 scenarios limiting warming to 1.5°C (50% or 67%). Key features: emissions peak by 2025 (already missed for many), -43% by 2030 (vs 2019), -84% by 2050, net zero 2050-2055, net negative 2060-2100. Requires: renewables 60-80% electricity by 2030, coal phaseout 2040, EVs 60% sales by 2030, energy efficiency +3%/year, CCS 5-10 Gt/year by 2050, CDR 5-10 Gt/year by 2070. Wedges approach (Pacala & Socolow): Need 10-15 concurrent strategies (each ~2 Gt/year reduction)—renewables, nuclear, efficiency, CCS, forestry, behavior change. No silver bullet. Current progress: Renewables on track, coal declining, but transport, industry, agriculture lagging. Emissions still rising (+0.5%/year) not falling (-7%/year needed).
Carbon Dioxide Removal (CDR)
All 1.5°C pathways require CDR 5-10 Gt/year by 2070 to compensate overshoot. Methods: (1) DACCS (direct air capture + storage): $200-600/tonne, 0.01 Gt/year currently (Climeworks, Carbon Engineering), scalability uncertain. (2) BECCS (bioenergy + CCS): grow crops, burn for power, capture CO₂, store underground. $100-200/tonne, competes with food for land. 0.5 Gt/year potential without land conflict, 5 Gt if dedicate cropland. (3) Afforestation/reforestation: plant trees, sequester 1 Gt/year at scale. Cheap ($10-50/tonne), co-benefits (biodiversity), but land-limited, slow (decades to mature), reversible (fires). (4) Soil carbon: regenerative agriculture, biochar—0.5-1 Gt/year, cheap, co-benefits. (5) Ocean alkalinity: add lime to ocean, increases CO₂ uptake—experimental, ecological risks. Challenges: CDR competes with mitigation for funding. Moral hazard—relying on future CDR delays cuts today. Technology unproven at scale—DACCS only 0.01 Gt/year vs 10 Gt needed.
Data Sources & Updates
Primary: IPCC AR6 WG1 (carbon budgets, climate sensitivity), IPCC AR6 WG3 (mitigation pathways), Global Carbon Budget (annual emissions update—adjusts remaining budget). Tracking: Climate Action Tracker (NDC progress, temperature projections), UNEP Emissions Gap Report (annual assessment policy vs targets). Real-time: Carbon Brief (science communication), CarbonBrief budget tracker (interactive). Budgets revised annually as emissions accumulate and science improves. IPCC updates every 5-7 years (next AR7 in 2028)—expect budgets to shrink further as understanding of feedbacks, tipping points improves.