Improvements in public transit in urban areas should lead to a reduction in global warming.
The reason for this is that such improvements should lead to more people using public transit. This would mean that they were not driving their own cars. Public transit would emit less greenhouse gasses per rider than would be emitted if everyone drove their own car. If less greenhouse gasses are emitted, the rate of global warming should decrease.
This is why so many people who are concerned with climate change want to increase the use of mass transit. They believe that it would reduce the amount of greenhouse gasses being emitted by commuters.