Ken Caldeira: Heating from CO2 is 100,000 times greater than the heat from burning the fuel
http://www.researchgate.net/publication/267381482_Warming_from_fossil_fuels
The Earth is heated both when reduced carbon is oxidized to carbon dioxide and when outgoing longwave radiation is trapped by carbon dioxide in the atmosphere (i.e., the CO2 “greenhouse effect”). The purpose of this work is to compare the relative magnitude of these two effects.
(Snip the math)
...we can see that over time, the burning of carbon heats the Earth about 100,000 times more through the trapping of outgoing longwave radiation than it does by direct heating through the release of chemical energy. In other words, when we burn carbon and release CO2 to the atmosphere, only 0.001 % of the total warming comes directly from the release of chemical energy during burning. The remaining 99.999 % of the warming is associated with the trapping of outgoing longwave radiation by that CO2 in the atmosphere.
Kinda puts things in perspective, doesn't it? That's where calculations like "4 Hiroshima bombs per second" comes from.