First, we will of course not boil up because of an increase of a few degress in average temperatures, but the ensuing disruption of some ecosystems will cause massive damage.
Second, it's colder on Mars because its distance to the sun is larger.
Third, you claim that plants are able to "eat" extra amounts of CO2, convert it into oxygen (and flourish because of the positive effects CO2 seems to have upon them which I don't deny, you know more about biology than I do). This is wrong as CO2 levels in the atmosphere have constantly risen during the last decades. I am too lazy to search for decent data, so just take a look here: http://en.wikipedia.org/wiki/Carbon_...9;s_atmosphere