ANewLeif: "I do not believe humans can turn Earth into a gas giant."
Gaiagirl specifically stated that she was speaking of a worst case scenario (which no one is suggesting is in any way likely, or that humans would have any way of knowing about, should it occur), of a critical climatic spiral in part triggered by human activity.
ANewLeif: "CO2 is not a pollutant just because it is not conducive to long-term perpetuation of current human enterprise."
While your considerations are fairly thoughtful, they seem to rely on overly rigid and literal definitions. Earth is a term that can be construed in numerous scalable, but still meaningful ways, as is the word pollutant. That tens of thousands of climate scientists (and the U.S. Supreme Court) are able to parse these ideas enough to move forward without becoming trapped in fruitless quibbling over philosophical semantics suggests that a consensus on the meaning intended is clear.
That some creatures would adapt and thrive as a result of new energy sources and limited competition from any unprecedented physical change to the earth's environment is not only theoretically true, but borne out by the history of the planet, yet it is a trivial and needlessly fatalistic argument in the face of a profound reduction in complex species as a result of controllable, elective human activity.
Whether or not humanity deserves to survive, having over-populated and built up beachfront properties, is just as irrelevant a question. The desire to survive is hardly species-specific. The ability to control carbon emissions enough to attempt to evade a foreseeable trouble, in this instance, is.