[tsimionescu]

Energy on Earth isn't conserved, as it's not an isolated system - the Sun is constantly adding energy to the Earth, and we are constantly radiating energy out into space as well. Coal and oil were basically batteries filled with solar energy from millions of years ago. We are now discharging those batteries, adding back all of the solar energy captured over millions of years to the Earth today, in addition to all of the energy the Sun is still adding at more or less the same rate.

It's the same principle as what makes a battery fire so destructive - all of the energy stored in the battery in a plant somewhere else suddenly gets discharged all at once in your car.