Is it possible to estimate the most likely values for climate (temperature) sensitivity to CO2 concentrations by studying the atmosphere of the planet Mars?
If it is, the results suggest even a +1C/doubling of CO2 concentration may be too large a value.
First the constants in the calculations (values from Wikipedia unless linked elsewhere):
CO2 molar mass: 44.0095
For Earth’s atmosphere
molar mass: 28.97
mass: 5.148 * 10^18 kg
CO2 concentration: 385ppmv
pre-industrial CO2 concentration: 278ppmv
Greenhouse effect: +33C
Contribution by CO2 to Greenhouse effect: 9% (+2.97C)
For Mars’s atmosphere
molar mass: 4.33
mass: 2.5 * 10^16 kg
CO2 concentration: 953,200ppmv
Greenhouse effect: +5C (only due to CO2)
What is then the mass ratio of CO2 between Mars and Earth?
In Earth’s atmosphere: 385ppm volume –> 585ppm mass –> 3.01 * 10^15 kg
In Mars’s atmosphere: 953,200ppm volume –> 967,925ppm mass –> 2.42 * 10^16 kg
CO2 (Mars) = 8.04 * CO2 (Earth) (by mass)
Let’s imagine now to increase CO2 (Earth) by 8.04 times by mass, equating what’s in the Martian atmosphere.
In that case, quick computations show terrestrial atmospheric CO2 concentration would increase to 4,680ppm mass, that is 3,081ppm volume: 8 times the current values, and 11 times the estimated pre-industrial concentration.
What would be the effect on Earth of such an increase? If we follow RealClimate’s and the IPCC’s estimation of a 3C/doubling sensitivity, the result is a whopping +12C compared to today’s, i.e. +14.97C in total.
If, instead, we stick to the estimation that, on its own, a doubling of CO2 should bring around +1C of temperature, the result is +4C compared to today’s. And that would provide a grand total for the CO2-related greenhouse effect of +6.97C
Compare now the figures of +14.97C and +6.97C with what is considered to be the same effect on Mars: +5C.
The above appears to indicate that a plausible value for climate sensitivity is in the region of +0.5C/doubling CO2 concentration.