Why is Mars not warming? Its CO2 levels are exponentially greater than earth's.

[verite...@gmail.com]

If Mars has >95% abundance of CO2 in its atmosphere (albeit much thinner atmosphere than earth, which has 0.04% abundance of atmospheric CO2), why does it not experience a steady increase in temperature? 

[Peter L. Ward]

Good question. The atmosphere of Mars has 96% CO₂ while the atmosphere of Venus has 96.5% CO₂. Atmospheric pressure on Mars is less than 1% Earth’s atmospheric pressure, while for Venus, it is 92 times that of Earth. Mars is 1.52 times as far from the Sun as Earth, while Venus is 0.7 times as far. Temperatures on Mars range from −143 °C at the winter polar caps to highs of up to 35 °C in equatorial summer. Mean surface temperatures on Venus are 462 °C.

It is the stratosphere of Earth, heated primarily by oxygen being dissociated by absorbing solar ultraviolet-C radiation, that keeps Earth 33 ^oC warmer than expected. I suspect that Venus is warmed by CO₂ being dissociated by absorbing ultraviolet-C and that the higher density means more molecules of CO₂ absorb solar ultraviolet-C providing much greater warming. By this logic, the very thin atmosphere on Mars would mean very few molecules of CO₂ absorb solar ultraviolet-C, providing very limited warming.