Coal was once the largest source of energy in the world, and it still holds second place behind oil. Moreover, in terms of the energy reserves tied up in coal, it is far and away the number one potential source in the world. Consequently, coal will likely play a major role in the future energy supply. But coal suffers from the deficit that it produces twice as much carbon dioxide as natural gas per unit of energy generated. The solution to these increased carbon emissions is clean coal technologies that capture all the undesirable waste products and sequester them underground, so that the carbon materials do not pollute the atmosphere and lead to global warming. Coal is also a reliable source of energy that can be cranked up or turned down as the demand changes.