Wind and solar are good ways of 'greening up' but they won't work for autos. Hydrogen cars aren't viable because of technological constraints; hydrogen eats away at metal so you'd have to have something else to contain the fuel, and this would mean a very big fuel tank given that hydrogen takes up more volume than petrol. Also, hydrogen is explosive. A car crash in this case could explode and cause a chain reaction in other hydrogen cars that could take out a city block. Biofuels aren't going to work either not only because they economically aren't workable but because they aren't as effecient as petroleum. You need to burn 10 - 20% more ethanol in order to match petrol's power. Also, you'd be devoting a lot of farmland to grow corn or whatever to make ethanol, and ethanol still pollutes the air, but not as much. If you wanted ethanol to power your cities, it likely won't be viable since you'd need to use most of the farmland in the USA just to grow crops to power the cities let alone make fuel for your cars (and how would you eat, unless we import all of our food from China/India).
As for coal power, coal power plants can be built to be more cleaner and efficient than before. In reality, the largest producer of pollution in the USA is our military and our cars, not electrical generation. Industry pollutes quite a bit but not as much as in the past. You can make synthetic gasoline and jet fuel from coal (Nazis did this in WW2) and you might be able to buy some time but you're stuck with the problem of high pollution and smog. We have around 200 - 300 years of coal supply left in the USA at current consumption levels; if we start making fuel from coal we maybe have 100 years left at the most given what we need to keep the cars driving. Natural gas is going to run out even sooner than oil, and natural gas cars require a large amount of energy investment for a return that won't be even close to what you invest in it. Also, I can't see us burning wood or whiskey or anything else in our cars since you run into a deforestation problem and we don't have enough booze for everyone even in this country.
The only power sources I can think of that could save the USA would be nuclear and fusion power. Fusion power theoretically is basically unlimited power but is still an experimental technology. Fissile material supplies (U238 and thorium) are at 70 years at current consumption levels worldwide. There is a way to extract uranium and thorium from seawater (some form of electrolysis used here) that could produce several MILLION years of fissile material supply but economically it won't be feasible unless U238 goes to more than $600-700 per oz. YOu can use nuclear power to power the cities and also to help develop fusion power (this is what powers the sun, BTW). As for cars, you can still have them, but they will likely be much smaller than today's cars. Cars over 2000 lbs (900 kg) won't be viable in the future, and if you are in the market for a car today, consider getting something below 3000 lbs (1300 kg) as your SUVs and Hummer and such won't be useful in a decade.
That's what I think anyways.
