I'm a believer in the potential of hydrogen fuel cells, but it'll be a long time and a load of money before they become viable for use.
Fuel cell busses are already in the works, being developed and researched by several companies. If their technology makes it to cars and becomes affordable, it could mean another significant reduction in global warming, but only, and I stress this, if other countries cooperate. Countries like china really don't like changing their ways, only recently did they start to switch to nuclear energy plants. As of now they're predominantly using coal for power, but with the construction of 24 new nuclear power plants that should change in the next 10-15 years. Hopefully that's soon enough.
If the US gets a president who actually gives a crap about global warming, they can get other countries to cooperate.