"Some think earthlings have no right to mess with the climate of another planet."
WTF?
That is one of the most idiotic statements that I have EVER heard. Humanity is expanding more rapidly than the Earth can contain, and will VERY soon reach a point where the Earth alone can't support it.
We need some room to grow, and since we are still a ways off from interstellar travel, terraforming Mars is a VERY logical choice.