Orbital Baseline as Measurement Instrument: CO₂ Forcing Classification, ECS Determination,  and 2025–2100 Projection     

Using the orbital transfer function calibrated in Gray (2026c) as a fixed measurement instrument, this paper determines the role of CO₂ forcing in the 1850-2024 instrumental temperature record and projects its impact to 2100. The orbital mechanics contribution over 1850-2024 is computed from the Laskar (2004) solution as −0.010°C-negligible relative to the observed HadCRUT5 warming of +1.182°C. The industrial residual RT = +1.192°C is regressed against log₂(CO₂/CO₂baseline) across the full 1850-2024 record (n = 175 annual data points) yielding a bestfit Equilibrium Climate Sensitivity (ECS) of 2.13°C per doubling of CO₂ (r² = 0.9136, RMSE = 0.1008°C). This value is derived solely from ordinary least-squares regression on raw HadCRUT5 temperature data (Morice et al. 2021) and raw Mauna Loa / Law Dome CO₂ data (NOAA GML; MacFarling Meure 2006); no GCM output, no IPCC framework value, and no assumed forcing parameter is used. Two independent satellite-era instrument systems confirm the result: HadCRUT5 restricted to 1979-2024 yields ECS = 1.87°C; UAH TLT v6.1 yields ECS = 2.15°C. Three-instrument cross-validation bracket: 1.87-2.15°C, with the full-record estimate of 2.13°C falling within this range. Three CO₂ verdict zones are evaluated: Zone 1 (nominal factor, ECS < 0.5°C) is falsified; Zone 3 (primary driver, ECS ≥ 2.5°C) is falsified jointly by the instrumental regression and the MIS 5e proxy constraint (Jouzel 2007 EDC3); Zone 2 (secondary amplifier, ECS 1.0-2.5°C) is confirmed. Scenario projections to 2100 use T(t) = 2.13°C × log₂(CO₂(t)/285.2) without GCM parameterisation: business-as-usual (~627 ppmv) yields +2.42°C; a CO₂ plateau (~486 ppmv) yields +1.64°C; a net-zero trajectory (~400 ppmv) yields +1.04°C. All projections carry an uncertainty band from ECS range 1.5-2.6°C. On millennial timescales, CO₂ forcing decays as the carbon cycle equilibrates and orbital mechanics reassert dominance; glacial inception remains on track per Gray (2026c). Supplementary Material File (gray2026d_v3_final.pdf) - Download - 1.22 MB Information & Authors Information Version history Copyright This work is licensed under a Non Exclusive No Reuse License.

Authors Metrics & Citations Metrics Article Usage 173views 110downloads Citations Download citation Jack H. Gray. Orbital Baseline as Measurement Instrument: CO₂ Forcing Classification, ECS Determination, and 2025–2100 Projection . Authorea. 23 March 2026. DOI: https://doi.org/10.22541/au.177429791.13589699/v1 DOI: https://doi.org/10.22541/au.177429791.13589699/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu.