Hmm, to be pedantic, oil isn't really a fossil fuel and the carbon in oil never was in the atmosphere. Oil is made from geological methane by archaia bacteria deep under ground. Coal, yes, that was in the atmosphere, since it is plant (fern) residue. It probably took much longer than 10,000 years for the archaia to make 1 year's worth of consumable oil. On the other hand, oil production is still going on, but being so effing slow, it doesn't help.