On Climate Sensitivity on Earth and Mars

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.

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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

Therefore:
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.

Positive Feedback…or Oscillation?

A very interesting remark by Roy Spencer in a guest blog at Roger Pielke Sr’s:

[…] Oscillation, or any other mode of internal variability will ALWAYS look like positive feedback – even if there is no feedback present.  The question of how the neglect of this effect has contaminated observational estimates of feedback has never even been addressed, let alone answered.

I repeat: to the extent that any non-feedback radiative fluctuations occur, their signature in climate data LOOKS LIKE positive feedback.  And when modelers use those relationships to help formulate cloud parameterizations, it can lead to models that are too sensitive. […]

Obviously the above does not mean there is _no_ positive feedback. It “just” means that this “fake feedback” effect has to be removed from the data BEFORE announcing to the world how sensitive climate is eg to CO2 concentrations.