These are my off-the-cuff thoughts. Subject to revision, of course. :-)

I think we import a lot of natural gas from Canada and elsewhere. CNG is a good near-term transportation fuel as it pollutes less than gasoline and it's relatively easy to convert gas engines to use it.

Wind has a problem that the power density is pretty low. You need big farms of windmills to get a significant amount of power out of them. Similarly with solar cells. Neither are very good for powering transportation.

Fuel cells are still years away, partially because they often use expensive noble metals as catalysts.

Hydroelectric power is probably the friendliest renewable power source except for the tiny problem that it usually requires daming a river. :-( If there were more sites like the Bay of Fundy it might be easier to get hydro power without the environmental problems. It's also not very good for powering transportation.

There are other ways to get power without burning stuff. For instance, years ago one of the big US defense contractors (Lockheed?) had plans for making power plants that would be in the deep ocean. There would be a very long heat pipe in the plant that would stretch from the warm surface waters to the cold, deep ocean waters. A metal that has a temperature difference will have a voltage difference generated and thus you can get electric power from it. (The thermoelectric generators that Boxley likes work this principle with the heat supplied by radioactive decay.) An obvious problem with such a plant is getting the power from the plant to the power grid...

For powering transportation, burning hydrocarbons is going to be important for a while. Hybrids are going to push up fuel efficiency. I would think that the natural progression would be a turbo diesel hybrid - if the pollution issues can be addressed for a reasonable cost.

It's an important problem. It's a shame that the two Skull and Bones Yalies won't address it seriously.

Cheers,
Scott.