It is important to say that the main argument in all this discussion of the zero fare in the public transport is related with the fact the we need cities more equitable socially and economically speaking. Beyond global warming and environmental problems, the concrete change related with our subject it's the displacement of people in the cities.
Being direct: a town only exists for those who can move inside of it. For those who can live all the urban equipments that the modern city provide to us. The right to the transport is only a right that leads us to other rights (health, education, leisure, work etc.)
That said, I would say we need to focus, internationally speaking, our work in more social arguments and not only the environmental ones. We know that both are important, but when it comes to Zero Fare, in practice we are talking about income redistribution, which directly affects social changes.