I hypothesize increasing the CO2 concentration from 280 to 400 ppm over a period of 140 years will increase global mean temperature by 1.2C
Now all we need is 140 years and a spare Earth to test this real quick...
That's already been done and it failed miserably.
The extra 120 ppm of CO2 added over the last 160 years has only managed to raise the temperature of Earth about 0.1ºC.
The rest of the 0.76ºC rise in global temperature was caused by a very large variety of things that are rarely considered.
Natural causes:
Solar Activity, Melting Permafrost, ENSO, Earth’s tilt, ozone hole and ozone feedback, solar variability ** (NASA says 25%), water vapour, the most significant GHG, methane emissions from Arctic seabeds, bugs, rotting vegetation and other sources.
Coal seam fires, forest fires cause (temporary) deforestation, natural climate cycles, volcanic activity above and beneath the oceans, continental drift, far-infrared surface emissivity, etc.
Human causes:
CO2 From burning fossil fuels, methane emissions from animals and agriculture, such as rice paddies, deforestation, “especially tropical forests for wood, pulp and conversion to farmland,” fertilizer use, cement production, chlorofluorocarbons and their reduction, population Increase, etc.