The burning of fossil fuels for energy in the modern era of life on earth is resulting in a significant increase in total co2 concentration in earth's atmosphere. however, the ratio of 14c to 12c is decreasing. why is this?

The fossil fuel which basically contains high carbon sources when burnt will produce a lot of carbon dioxide increasing its concentration in the atmosphere. But the ratio of 14C to 12C is decreasing because the age of these fossil fuel is so high that the carbon content is ruined.