Coal, oil and natural gas have been found all over the world.
Geologists work out how much of these fossil fuels there are in each place.
So far, only a small amount of the discoveries have been burned.
Between the years 2000 and 2011, the burning of fossil fuels has produced about 320 gigatonnes of carbon dioxide.
Scientists have worked out that adding around another 565 gigatonnes will increase global temperatures by 2 degrees C above the pre-industrial level.
Burning all the known remaining fossil fuels would produce over 2,500 gigatonnes of carbon dioxide.
So if all the fossil fuels already discovered were burned, temperatures would soar to very high levels.
This is why many experts say that most fossil fuels must stay in the ground, and that looking for more is a waste of money.