I do not think building a city on Mars is going to solve anything at all! First of all, it’s unlikely a city on Mars will be built due to the incredible problem of transporting parts and materials to that planet. We all would like this to happen, since we screwed this planet, to go to a new planet. However, Mars has such an inhospitable climate that you wouldn’t be able to go outside without exposure risk of harmful UV light and cosmic radiation. These things give you cancer. There’s neither an organized magnetic field not an atmosphere (well, technically there’s an atmosphere 100 times weaker than our own. What’s left is poisonous to us.) Without an organized magnetic field there’s no aurora borealis to protect you by drawing off the sun’s harmful rays.