Observation‐based global biospheric excess radiocarbon inventory 1963–2005

For the very first time, we present an observation‐based estimate of the temporal development of the biospheric excess radiocarbon ( 14 C) inventory I B 14, E , i.e., the change in the biospheric 14 C inventory relative to prebomb times (1940s). I B 14, E was calculated for the period 1963–2005 with a simple budget approach as the difference between the accumulated excess 14 C production by atmospheric nuclear bomb tests and the nuclear industry and observation‐based reconstructions of the excess 14 C inventories in the atmosphere and the ocean. I B 14, E increased from the late 1950s onward to maximum values between 126 and 177 × 10 26 atoms 14 C between 1981 and 1985. In the early 1980s, the biosphere turned from a sink to a source of excess 14 C. Consequently, I B 14, E decreased to values of 108–167 × 10 26 atoms 14 C in 2005. The uncertainty of I B 14, E is dominated by uncertainties in the total bomb 14 C production and the oceanic excess 14 C inventory. Unfortunately, atmospheric Δ 14 CO 2 from the early 1980s lack the necessary precision to reveal the expected small change in the amplitude and phase of atmospheric Δ 14 C seasonal cycle due to the sign flip in the biospheric net 14 C flux during that time.