:::There is also the fact that the greater distance of Mars from the sun greatly reduces its insolation relative to that of the Earth (it is less than half that of Earth: http://www.nasa.gov/mission_pages/station/research/experiments/1188.html). Earth at the orbital distance of Mars would very probably be a snowball even with current Earth CO2 levels. The abundance of water on Earth would increase the albedo, decreasing temperatures, in this instance.