Beyond that, I personally believe that NASA and the Soviet Space Program already saved the world once by allowing the US and the USSR to have a public competition in the midst of extreme nationalism. Without the space race, who knows if we would have been launching ICBMs instead of Apollos?
But let's look a little deeper why pollution is happening in the first place. Don't you think that people's greed and desires of having more and more things contributes to this at least a little? Overconsumption? Short-term profits? But what if we can fix _that_ at least a bit? Now that's a question I'm much more interested, since I believe it can have real effect and stop the runaway consumption culture.
Our consumer consumption culture won't change until it has to, whether that means folks going into space and living in an incredibly demanding environment or it becomes untenable on this planet. Wish it were different, but that's just how this stuff works.
Colonies on Mars is kind of a grey area, only because Mars isn't the final destination, there's plenty more in this solar system/galaxy/universe to explore. There are some people who probably see it as just another place for humans to take over, but I'd suggest those aren't the pioneers, they tend to be the ones that come in later once the risks are reduced.
The reason we have a solar power industry at all is because satellites needed robust long term power sources, not because someone cared about the environment. The first solar cells were invented in the 1880s, and were so inefficient -- less than .5%! -- they were little more than a scientific novelty. They stayed this way for 70 long years, until Bell Labs began researching them as a source of power for satellites in the 1950s.
Space is an extremely demanding environment, and it's precisely those demands that actually drive the development of technologies that enable us to radically reduce our environment impact on the Earth.
Human beings living on the Moon or on Mars will need to reuse and recycle everything imaginable, for instance, creating closed-loop systems where there is little to no waste. They will have to grow food in ways that recycle nutrients instead of destroying topsoil. They will have to be able to make extremely reliable products, and they'll have to be able to reuse and repair these items in ways we will never have the incentive to do in a disposable consumer economy.
It's simply not economically efficient to develop those kind of technologies terrestrially. It's cheaper to dump obsolete electronics and ship them to some impoverished part of the world, like rural China, where they contaminate the ground water and poison the air and the residents.( http://tinyurl.com/toxic-e-waste )
The planet is big and human labor and human life is cheap here. Recycling is optional here. Up there, it's not. It's only the extreme demands of manned space that will drive these sorts of advances.
It ain't just about shootin' rockets up into the sky.