Have a look at past and projected US energy use here:

http://www.eia.doe.gov/oiaf/aeo/execsummary.html

Click thru to the table under Figure 1.

1. For 2008, calculate US CO2 emissions from these different sources.

2. If all the US coal-fired plants were converted to natural gas, what would be the fractional drop in US CO2 emissions?