The US uses roughly 4.0 million Gigawatt-hours per year in electricity. Currently 50% of out electricity comes from coal. If every 1.0 kilowatt-hour of coal burned releases 2 lbs of carbon dioxide, how much carbon dioxide is released from burning coal for electricity each year in the US?

4 trillion pounds.

1 gigawatt-hour is equal to 1,000,000 kilowatt-hours.

Out of our 4,000,000 gigawatt-hours, 50% of it comes from coal; this is 2,000,000 gigawatt-hours.

2,000,000 (1,000,000) = 2,000,000,000,000 (2 trillion) kilowatt-hours.

Since each kilowatt-hour is 2 lbs of carbon dioxide, 2 trillion * 2 = 4 trillion lbs of carbon dioxide.