Mars has a much thinner atmosphere than Earth but it is 95% composed of CO2. One might therefore expect a larger greenhouse effect, since there are 20 times more CO2 molecules in the Martian atmosphere than there are on Earth. To quantify this I have run my greenhouse model (described here) for Mars, using the following parameters:

- lapse rate = 10.5 K/km
- g=3.7 m/s2
- P0 = 600 Pa
- Ts = 210 deg.C.
- Following Nasa we take the top of the troposphere as 7km.

The result is shown in Figure 1.

Fig 1: The up-going radiation flux within the 13-17 micron CO2 band for Martian atmosphere. The zero level is the flux for temperature 210K with zero CO2.

The reduction in outgoing flux at the ground from CO2 is 8 watts/m2. Using the formula DS/DT = 4.sigmaT^3 results in a net CO2 induced warming effect of just 2 degrees.C ! In fact measurements show that the greenhouse effect on Mars is in fact only between 1-5 degrees C. So how come the total warming is so small on Mars ? There are a lot of hand waving arguments about the low density being responsible, but the real difference between Mars and Earth is that the surface temperature is much cooler to start with. Total IR radiation to space goes as T^4 so there is far nearly 4 times less flux to be absorbed by CO2 molecules on Mars than on the Earth. So although there are 20 times more CO2 molecules above each square meter on Mars than on Earth, they absorb less net energy and consequently the greenhouse warming is less.

What about Venus ? I’ll look into that next.

*Related*

Clive-retired earth scientist/mathematician here. Couple queries. Ok the Mars example. I just found it that it has 95% co2, then why is it heating up aka via the theory presented by the global warming alarmists? Ok so they may answer oh it’s too far away. But when I try to explain Venus to them & compare Venus with Earth, point being we have a couple things Venus does not have & thats ocean basins & vegetation. Venus essentially banded in clouds & now geologically dead. They don’t get that the Earth has 2 major sinks & that helps counteract their nonsense. But also too when they propose we send X gigatons of co2 into the air & of course the only part they’re concerned of is the fossil fuel stuff & not basic exhaled breath, I keep saying well thenn why does co2 remain at 0.038%. How would you answer that?

Although the atmosphere is 95% CO2 the air is very thin so the total amount of CO2. The pressure is only 0.006 that on earth. So the total amount of CO2 is about the same as that on earth. (0.004). It seems that Mars lost its atmosphere because it doesn’t have a protective magnetic field to shield it from the solar wind.

It is true that the earth has oceans and life which helps stabilise the climate.