As odd as it may seem, the human race is fully capable of accomplishing both space-expansion efforts and cleaning things up here on Earth. More than that, space opens the door to a wealth of possibilities for ameliorating or even solving many of the major problems we face today. That includes environmental efforts and energy production.

Even if we were to pursue a massive space race, it'd only consume a relatively small fraction of humanity's industrial capability and resources as they exist today. In the context of a long-term terraforming effort, after a significant space-borne industry has been created, these sorts of binary comparisons become all the more laughable.