Why can carpooling and using mass transit systems minimize some problems associated with the use of fossil fuel?

Carpooling and using mass-transit reduce the numbers of vehicles on the roads, and so the amount of fossil fuel is also reduced.

Problems associated with the use of fossil fuels (coal, oil and natural gas) are largely pollution, particularly the greenhouse gas carbon dioxide (CO2), which is causing global warming.

So car-pooling and mass transit systems help us reduce the rate of global warming.