Predicted net efflux of radiocarbon from the ocean and increase in atmospheric radiocarbon content
K. Caldeira, G.H. Rau, & P.B. Duffy
Since pre Industrial times, there should have been a large decrease in radiocarbon (14C) concentration in the atmosphere due to rises in CO2 concentration. However, its 2-3% drop is much less than would be expected. Radiocarbon's decrease has been counteracted by bombs and fossil fuel burning. Despite the fact that fossil fuels have no radiocarbon, they still have an effect on its concentration. The way that fossil fuel use has indirectly affected radiocarbon flux is outlined in this paper.
Prior to changes introduced by man, production of radiocarbon (14C) in the stratosphere nearly balanced the flux of 14C from the atmosphere to the ocean and land biosphere, which in turn nearly balance radioactive decay in these 14C reservoirs. This balance has been altered by land-use changes, fossil-fuel burning, and atmospheric nuclear detonations. Here, we use a model of the global carbon cycle to quantify these radiocarbon fluxes and make predictions about their magnitude in the future. Atmospheric nuclear detonations increased atmospheric 14C content by about 80% by the mid-1960's. Since that time, the 14C content of the atmosphere has been diminishing as this bomb radiocarbon has been entering the oceans and terrestrial biosphere. however, we predict that atmospheric 14C content will reach a minimum and start to increase within the next few years if fossil-fuel burning continues according to a "business-as-usual" scenario, even though fossil fuels are devoid of 14C. This will happen because fossil-fuel carbon diminishes the net flux of 14C from the atmosphere to the oceans and land biosphere, forcing 14C to accumulate in the atmosphere. Furthermore, the net flux of both bomb and natural 14C into the ocean are predicted to continue and slow and then, in the middle of the next century, to reverse, so that there will be a net flux of 14C from the ocean to the atmosphere. The predicted reversal of net 14C fluxes into the ocean is a further example of human impacts on the carbon cycle.