Let's just assume for the sake of argument that there's GW, & that people cause it.There is no climate change to stop, so maybe we can use that useless rhetoric to power our interplanetary spaceship.
This has no bearing on space travel & colonization.
Colonies on Mars with this century's technology is not a "Plan B" to preserve our species.
Such colonies would be entirely dependent upon Earth for support, & would have only a
tiny tiny unsustainable population. The enormous resources consumed by such a project
would be better used here.
If we really fear humanity's demise, global warming won't be the cause. There'd be some
upheaval as populations move, but we'll survive. The real humanity killer would be a
Chicxalub type of event. To survive that, we need to think in those terms. The Svalbard
seed bank is one useful component of survival. We'd need communities designed to
survive exactly that kind of catastrophe.