1. Classic IPCC (Myhre et al. 1998 – AR1 to AR5)
ΔF = 5.35 × ln(C/C₀) W/m² → 3.0 K per doubling (includes all feedbacks)
CO₂: 420 ppm | Forcing: 2.07 W/m²
ΔT = +1.68 K | +0.145 meV
Myhre et al. 1998 / IPCC AR5 best estimate ECS = 3.0 K
2. CMIP6 Multi-Model Mean (2020–present)
CMIP6 ensemble-mean ≈4.0 W/m² → ≈3.9 K per doubling
CO₂: 420 ppm | ERF (approx): 2.24 W/m²
ΔT = +2.18 K | +0.188 meV
CMIP6 unweighted mean effective ECS ≈3.9 K (range 1.8–5.6 K)
3. Happer–van Wijngaarden (2020–2023) – No water-vapor feedback
HITRAN line-by-line, fixed absolute humidity → ≈1.4–1.5 K on doubling
CO₂: 420 ppm | Forcing (clear-sky): 1.74 W/m²
ΔT = +0.81 K | +0.070 meV
Happer & van Wijngaarden (arXiv) – Planck response only
With standard water-vapor feedback ≈2.2–2.3 K