CLIM-17 / SPM.B / Research primer
How does time above a warming level differ across scenarios?
What do compact global scenario trajectories show about the magnitude and duration of warming-level exceedance within their published time horizons?
NASA/STS-44 crew. Sunlight revealing internal-wave patterns in the western Indian Ocean, photographed from Atlantis in 1991. Contextual image; not used to estimate a trend. Image credits.
The question
Two trajectories can share an endpoint while following different histories. This primer examines that distinction without assuming that returning to a temperature level reverses earlier impacts.
Why it matters
Endpoint-only comparisons can omit exposure accumulated during overshoot and the additional carbon-removal requirements assessed by the IPCC.
The AR6 anchor
AR6 assesses additional removal requirements and greater feasibility concerns under overshoot; its impacts and risks grow with exceedance magnitude and duration (high confidence).
AR6 Synthesis Report (2023) / B.7; B.3.
Evidence and comparison
- Unit of analysis
- Model-scenario-year or documented multiyear window; observations retain their own status.
- Baseline and denominator
- A 1.5 or 2 degC comparison requires documented 1850-1900 references and consistent warming-level smoothing. Do not bridge incompatible anomalies with an assumed offset.
- First descriptive test
- Compare published trajectories, peak warming and time above the selected level only where temporal resolution supports calculation; mark exceedances unfinished at the horizon.
- Deeper analysis
- Compare model-specific crossing and return dates where available. If only period summaries export, report bracketed exceedance windows rather than interpolating fictional annual paths.
- Attribution status
- projection/scenario analysis
- Uncertainty
- Retain model ranges and right-censoring at 2100. A threshold crossing in an annual observation is not equivalent to scenario overshoot of sustained warming.
Principal sources
- Copernicus Interactive Climate Atlas
Global mean temperature trajectory; Scenario and model; Published smoothing and uncertainty.
Target 2021-2100 compact global trajectories; temporal-resolution and release checks pending. Candidate source; import and publication checks are still required. - Indicators of Global Climate Change
Observed global temperature anomaly; Published decadal warming estimates.
Target 2006-2025 IGCC observations for context; export verification pending. Candidate source; import and publication checks are still required.
Alternative explanations
- Internal variability
- Smoothing conventions
- Model selection
- Non-CO2 pathway assumptions
The visual argument
Provisional: aligned scenario trajectories with threshold selection; annotate peak, exceedance window and censored return, leaving historical observations visually separate.
- Exceedance-duration intervals
- Peak-versus-endpoint comparison
- Historical decade context
- Irreversibility assessment notes
What these data do not show
- Exceedance duration is not a transferable biological damage function.
- Returning below a level does not establish recovery of ice sheets or ecosystems.
Reproducibility and next release
Plan src/build.mjs and data/processed/exceedance-windows.csv from versioned data/raw exports; check smoothing, baselines, time resolution, crossings and censoring, retaining queries, hashes and rights in provenance.
- After approval: overshoot-window study
- Article and threshold methods
- Responsive trajectories and exact values
- 16:9 hero and LinkedIn launch
- Optional PDF carousel
- Chart exports and scenario CSV