Well, humans have caused the vast majority of the spike in CO2 and it coincides with a global increase in temperature. I don't think there's much doubt we're to blame for the bulk of global warming.
You won't wean people off of air transport, and my own view is that aviation emissions are going to be far harder to curtail than emissions in other spheres of industry. In terms of reducing global CO2 emissions, it would make sense to concentrate our efforts on other sectors first.
That said, I took the train from Canberra to Sydney recently and it made British trains look modern which is saying something, though it was a lot roomier than British trains and the food was better. There probably is a lot of scope for improving rail links such as this one though I grant you a flight from Sydney to Darwin is always going to beat the train.