Forty to 50 years ago there was “general agreement” in estimates of the resulting radiative forcing (1 to 2 W/m²) and surface temperature change (0.5°C ±0.3°C) when directly doubling CO2 concentrations from 280 to 560 ppm. By the late 1980s the “consensus” estimates doubled to 3.7 W/m² forcing and 1.2°C warming instead. Apparently “basic physics” changed.
It is today considered IPCC-endorsed “settled science” that doubling CO2 concentrations from their preindustrial value (280 ppm to 560 ppm) directly leads to a temperature change of 1.2°C without the alleged positive feedbacks with water vapor and clouds to amplify this warming further.
Image Source: SkepticalScience and IPCC (2001)
As recently as the early 1980s, however, the “general agreement amongst different modelers” put the global surface temperature change resulting from doubling CO2 “between 0.2 and 0.4K.”
Image Source: Schuurman, 1983
In 1984 Andrew Lacis – the scientist who later claimed CO2 is the Earth’s temperature control knob – co-authored a paper that put the radiative forcing response to doubling CO2 at “only 1 – 2 W/m²” , which is the temperature equivalent of between about 0.2 and 0.8°C.
Image Source: Fung et al., 1984
Again, 1 to 2 W/m² and 0.2 to 0.8°C were the norm for the no-feedback climate sensitivity estimates throughout the 1970s and first half of the 1980s, or before the “consensus” opinion doubled these values in the late-1980s.
Here are some examples.
Newell and Dopplick, 1979 (2x CO2 = 0.8 to 1.5 W/m² or <0.25°C )
Ramanathan, 1981 (2x CO2 = 1.2 W/m² or 0.4°C)
Idso, 1980 (2x CO2 = 2.28 W/m² or ≤0.26°C )
Zdunkowski et al., 1975 (2x CO2 = 0.3 to 0.4°C, 7x CO2 = ~1°C)
Gates et al., 1981 (2x CO2 = 0.3°C, 4x CO2 = 0.48°C)
In the early 1970s, it was also the “consensus” that the spectral band where CO2 exerts its radiative effect is saturated or nearly so, which means diminished warming the more CO2 rises. Thus, multiplying CO2 by a factor of 6 or 8 will still produce less than 2°C warming.
Weare and Snell, 1974 (2x CO2 = 0.7°C, 6x CO2 = 1.7°C)
Rasool and Schneider, 1971 2XCO2 = 0.8°C, 8xCO2 = <2°C
Low climate sensitivity estimates became a problem for those wishing to portray CO2 as much more influential variable. By the mid-1980s it became more and more acceptable to say doubling CO2 produces a forcing of 3.7 W/m² and a warming of 1.2°C. And to this day these values are assumed to be “basic physics.”