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

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