If you assume that sending a person to live on Mars requires about as much fuel as sending three people to stay the Moon for a day or so did, then each person we send to live on Mars requires about 770 cubic metres of RP-11. Burning this fuel produces about 1.9
million kg of CO2. If we assume that the production and other costs associated with a launch double this2 then each person we send to live on Mars results in about 3.8 million kg of CO2 being emitted.
This is entirely negligible if we want to send a small number of people to live on Mars. But colonizing Mars means sending, perhaps, a million people to live there: now it's not negligible: now it's about 40% of the entire annual emissions of CO2 caused by all humans.
We're not going to colonize Mars while we rely on fossil fuels, unless we want cause catastrophic effects for the vast majority of people who remain.