IPCC SSP Scenario Tracker — Which Climate Scenario Is the World Following?

Key Metrics

Closest Scenario

SSP4-3.4

GCAM4 · Moderate mitigation pathway

Forecast Cumul. Range

1,672–1,786

GtCO₂ · 2000–2050 · 4 methods

Most Likely Scenarios

SSP Scenario Probabilities by Forecast Method

How probabilities are calculated

Each forecast method generates a distribution of cumulative CO₂ totals (2000–2050) via Monte Carlo sampling of its 95% CI. The fraction of samples falling between each pair of adjacent SSP cumulative benchmarks determines the scenario probability. The ensemble sits in the SSP4-3.4 to SSP2-4.5 corridor (65%/35%), though the Growth Rate method — with its higher cumulative mean — leans mostly SSP2-4.5 (83%), while RW-Quad 25yr places 100% of samples in SSP4-3.4.

Method	Cumul. Mean	SSP1-2.6	SSP4-3.4 ★	SSP2-4.5	SSP4-6.0
Growth Rate Extrap.	1,786	0%	16%	83%	1%
Gaussian Process	1,684	2%	76%	22%	0%
RW-Quad 25yr	1,672	0%	100%	0%	0%
RW-Quad 20yr	1,726	0%	66%	34%	0%
Ensemble Average	1,717	0%	65%	35%	0%

Annual Emissions Pathways (GtCO₂/yr)

Forecast Methods vs RCMIP SSP Scenarios

Dotted lines show fixed RCMIP IAM scenario pathways. Solid lines are observationally-constrained statistical forecasts from 2025.

Running Cumulative Emissions from 2000

Cumulative Fossil CO₂ (GtCO₂) — Methods vs SSP Pathways

Cumulative totals determine which SSP corridor current trajectories align with. All forecast methods converge between SSP4-3.4 (1,582) and SSP2-4.5 (1,877) by 2050.

Projection Uncertainty vs SSP Envelope

Cumulative CO₂ Budget Comparison (2000–2050) — Error Bars Show 95% CI

Diamonds mark RCMIP SSP fixed pathways. Circles with error bars show forecast method central estimates and uncertainty ranges. The shaded band highlights the forecast ensemble corridor.

Sources & Methodology

Scenario data: RCMIP Phase 3 Emissions v1.0.0 (Emissions|CO2|Energy and Industrial Processes) · Observed: Global Carbon Budget 2025 (Friedlingstein et al.) for 2000–2024; 2025 = GCB preliminary estimate (38.1 GtCO₂) · globalcarbonbudget.org
SSP Models: IMAGE (SSP1-1.9, SSP1-2.6) · GCAM4 (SSP4-3.4, SSP4-6.0) · MESSAGE-GLOBIOM (SSP2-4.5) · AIM/CGE (SSP3-7.0) · REMIND-MAGPIE (SSP5-3.4-OS, SSP5-8.5)
Probability method: Monte Carlo sampling (50,000 draws) of each forecast method's CI band, approximated as normal about the central estimate. The fraction of cumulative samples in each SSP inter-scenario interval is reported as the scenario probability.
Key finding: Methods project cumulative fossil CO₂ totals of 1,672–1,786 GtCO₂ (2000–2050). The ensemble average (~1,717 GtCO₂) sits in the SSP4-3.4/SSP2-4.5 corridor — 65% SSP4-3.4, 35% SSP2-4.5. The Growth Rate method (highest cumulative at 1,786 GtCO₂) leans 83% SSP2-4.5; shorter-window methods sit predominantly in SSP4-3.4 territory, implying roughly 1.7–2.9°C warming by 2100 under typical climate sensitivity.

Which IPCC Scenario Is the World Currently Tracking?

How probabilities are calculated