Electrolysis powered by wind, solar, geothermal (Iceland already plans to be a big exporter of hydrogen). Reformation of methane now vented to the atmosphere from garbage dumps and oil wells. That's for now. Next, direct production from coal with the CO2 sequestered and then biological production of hydrogen by algae. At worst, in the immediate, reforming of methane or methanol at the service station or in the car itself constitutes a huge reduction in overall emissions.
Shane Mage
"Thunderbolt steers all things."
Herakleitos of Ephesos, fr. 64