Noted physicist Lowell Wood recently outlined a bold plan to terraform Mars. He defines humanity as a terraforming species, pointing to the alterations of our own home planet over the last 10,000 years. He makes some really good points, but the real key phrase he uses when predicting our terraforming future is: "if-and-as" humanity becomes a truly space faring civilization.
A lot depends on whether humanity gets a foothold in space before we push Earth's biosphere past the point of being able to support human civilization. Building a space program, whether private or public, requires a complex infrastructure. If environmental change advances faster than our ability to adapt to the changes, we may no longer be able to mount a space program over and above our struggle to survive. Ironically, we still seem to lack an effective motivation. Even the promise of space tourism on the horizon isn't exactly taking off like a rocket. Maybe the real secret is to tell big business they can emit all the greenhouse gases they want on Mars.