Shame on you, TR, thinking something is true because a *web site* tells you so. You should know better by now.
"Composition of the Martian air is 95% Carbon Dioxide, 3% nitrogen and trace amounts of oxygen and water. The density of the Martian atmosphere is only 1% of Earth."
- I'll accept that much, because a *whole raft* of websites say the same thing, and it's numerical information which people can check up on and complain about if it's wrong.
On the other hand
"Mars density is so low that the carbon dioxide creates only a minor greenhouse effect"
is handwavy nonsense.
Even with all the CO2 we've been pouring into the atmosphere lately, CO2 is only about 370 parts per million of the Earth atmosphere. In terms of its contribution to atmospheric pressure, that's about 0.37 millibars. The pressure of CO2 on Mars is 95% of 10 millibars i.e. about 9.5 millibars. That's about 26 times as much as on Earth - pretty close to ES's figure of 30 times. The difference can be accounted for by him using some slightly different figures either for Mars atmospheric pressure or Earth CO2 levels.
On the face of it, there should be 26 times as much greenhouse effect due to CO2 on Mars as there is on Earth. And I am not aware of any non-handwavy argument which would suggest otherwise. The greenhouse effect is due to light hitting molecules of the greenhouse gas and being absorbed by them. The molecules then radiate heat in random directions, which means that something like 50% will be radiated downwards, increasing temperatures at lower heights. I do not see how this process could require a dense atmosphere, or how increasing the density of the Martian atmosphere by introducing only non-greenhouse gases could have a significant effect on the process.
The situation is complicated because the greenhouse effect is caused not only by CO2. Water vapour is probably the biggest contributor to the greenhouse effect on Earth, though it's difficult to quantify because the stuff gathers into clouds instead of spreading out evenly. Estimates of the proportion of the greenhouse effect due to water seem to vary wildly, from 30% to 88% (according to http://encyclopedia.laborlawtalk.com/Greenhouse_effect ) and it tends to get ignored because it isn't under human control to the same extent that CO2 emissions are. And there's methane and other greenhouse gases too that we could try to take into account, but none of this alters the fact that there should be a significant greenhouse effect on Mars.