| The radiocarbon content of atmospheric CO2 has undergone dramatic changes over the past
century. Radiocarbon in CO2 has been diluted by the combustion of 14C-free fossil fuels
since the industrial revolution, causing a slow decline in the relative abundance of 14C to total
carbon (Δ14C) in the early 1900s. This decline was interrupted by nuclear weapons
testing in the 1950s and 60s, which nearly doubled the atmospheric inventory of 14C.
Following the extraordinary rise in radiocarbon content, a quasi-exponential decrease
was observed as excess radiocarbon was assimilated by carbon reservoirs in the
ocean and on land. Recently, fossil fuel emissions have once again become the
dominant influence on the long-term trend in Δ14C of CO2. This presentation will
investigate the trajectory of atmospheric Δ14C to 2100 in relation to the Representative
Concentration Pathways (RCPs) using a simple carbon cycle model. In all scenarios,
Δ14C of CO2 is projected to drop below the zero per mil level in the next decade.
Simulated atmospheric Δ14C is lower than -200 per mil in 2100 in the scenario
with the largest growth in fossil fuel emissions, while the most ambitious emission
reductions are projected to sustain Δ14C near zero per mil. The presentation will
discuss the implications of these changes in atmospheric composition on isotopic
disequilibria and net fluxes of radiocarbon between different reservoirs, including the
sensitivity of atmospheric Δ14C to fossil fuel emissions on global and regional scales. |