Radiative forcing ΔF from a change in CO2 concentration:

   ΔF = ⍺·ln(C/C₀) W/m²
      = ERF·log2(C/C₀) W/m²

where:
   ERF = ⍺·ln(2)
       = “Effective Radiative Forcing” per doubling of CO2

Estimates of ⍺ used in CMIP5 GCMs range from 3.7 to 6.3 W/m²
which corresponds to ERF of 2.6 to 4.3 W/m² per doubling of CO2.