Worst case Coal which costs far less than Oil. The really good news is the US has a lot of Coal and Coal has a drasticly lower carbon footprint because of reduced extration, refining, and transportation costs per BTU.

The worst case may be coal, but the most likely case (for America) is that the additional electricity needs are met by Natural Gas. More specifically, natural gas derived from the oil shale deposits.

Here's a link with some interesting info on this: http://blogs.forbes.com/jeffmcmahon/2011/06/30/cheap-natural...

While this isn't the ideal solution for the environment (frankly, I then that solar towers with molten salt storage reservoirs are the best all around energy solution right now), natural gas is altogether far less damaging to the environment than coal.

Take a look at this study: http://media.kentucky.com/smedia/2009/10/19/10/HiddenCosts.s...

In a nutshell, coal is vastly more damaging to the environment than any other form of generating electricity.