While public transport is a good way to fight global warming, this is by no means the only reason to use it. Firstly, even if global warming isn’t a problem, the pollution causing it certainly is. PT uses less fuel than cars, so it also deals with peak oil.
Public transport can also move more people than cars, so they can reduce congestion and parking problems. By extension of their efficiency, PT takes up less space and requires less subsidy/is more profitable than cars, while still being cheaper, providing mobility to those who can’t or don’t drive. Many people just find it more convenient and less stressful than driving. (ABS Public Transport Use for Work and Study)
In conclusion, to say that global warming isn’t true is no reason to drive everywhere and ignore public transport.