Just to clarify some things. 14C has a half life of about 5750 years. Since fossil formed many millions of years ago and they are almost entirely depleted in 14C. 14C is formed naturally in the upper atmosphere. This is why atmospheric 14C remains relatively stable. And since 14C is absorbed by the ocean it remains stable there as well. However, from about 1850 to 1950 the amount of 14C-to-12C/13C in both the atmosphere and biosphere began declining. Then around 1950 14C spiked up quickly. This is known as the bomb spike because 14C is byproduct of nuclear explosions. So the anthroprogenic signal in regards to 14C is apparent only in the early stages of the industrial revolution. After WWII nuclear bomb testing swamped the signal. The rate at which the 14C-to-12C/13C ratio changed prior to the bomb spike is a near perfect match to the accelerating rate of fossil fuel burning. Photosynthesis is preferential to 12C vs 13C especially in fossil fuel remains because those were the result of the high CO2 species of C3 carbon fixation than even today's C3 or C4 species. So not only are fossil fuels depleted in 14C, but they are partially depleted in 13C as well. So when fossil fuels are burned they release lower 13C-to-12C molecules into the atmosphere. This lowers the 13C-to-12C ratio. The rate at which the 13C-to-12C ratio changes is a near perfect match to the accelerating rate of fossil fuel burning. When fossil fuels are burned they release C molecules. These are then chemically bonded with O2 molecules during the burning process. Since O2 is decreasing in the atmosphere that means the increase in CO2 is due to burning of a C rich source. And the rate at which O2 is declining is a near perfect match to the rate at which fossil fuels have burned. The other thing to keep in mind is that about 2000 Gt of carbon has been added to the carbon cycle. 1000 Gt went into the atmosphere and 1000 Gt went into the ocean. Atmospheric concentrations and oceanic pH confirm this rate of uptake. Not only is 2000 Gt nearly the exact amount of carbon released from burning fossil fuels over the last 200 years, but the accumulation curves in both the atmosphere and ocean are a near perfect match to the burn rate of the fossil fuel. We know without a shadow of doubt that the 130 ppm that has been added to the atmosphere is caused by humans. Most AGW skeptics even acknowledge this.