Predicted net efflux of radiocarbon from the ocean and increase in atmospheric radiocarbon content

Prior to changes introduced by man, production of radiocarbon (C-14) in the stratosphere nearly balanced the flux of C-14 from the atmosphere to the ocean and land biosphere, which in turn nearly balanced radioactive decay in these C-14 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 C-14 content by about 80% by the mid-1960's. Since that time, the C-14 content of the atmosphere has been diminishing as this bomb radiocarbon has been entering the oceans and terrestrial biosphere. However, we predict that atmospheric C-14 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 C-14. This will happen because fossil-fuel carbon diminishes the net flux of C-14 from the atmosphere to the oceans and land biosphere, forcing C-14 to accumulate in the atmosphere. Furthermore, the net flux of both bomb and natural C-14 into the ocean are predicted to continue to slow and then, in the middle of the next century, to reverse, so that there will be a net flux of C-14 from the ocean to the atmosphere. The predicted reversal of net C-14 fluxes into the ocean is a further example of human impacts on the global carbon cycle.