Observations of radiocarbon in CO 2 at seven global sampling sites in the Scripps flask network: Analysis of spatial gradients and seasonal cycles

High precision measurements of Δ 14 C were conducted for monthly samples of CO 2 from seven global stations over 2‐ to 16‐year periods ending in 2007. Mean Δ 14 C over 2005–07 in the Northern Hemisphere was 5 ‰ lower than Δ 14 C in the Southern Hemisphere, similar to recent observations from I. Levin. This is a significant shift from 1988–89 when Δ 14 C in the Northern Hemisphere was slightly higher than the South. The influence of fossil fuel CO 2 emission and transport was simulated for each of the observation sites by the TM3 atmospheric transport model and compared to other models that participated in the Transcom 3 Experiment. The simulated interhemispheric gradient caused by fossil fuel CO 2 emissions was nearly the same in both 1988–89 and 2005–07, due to compensating effects from rising emissions and decreasing sensitivity of Δ 14 C to fossil fuel CO 2 . The observed 5 ‰ shift must therefore have been caused by non‐fossil influences, most likely due to changes in the air‐sea 14 C flux in the Southern Ocean. Seasonal cycles with higher Δ 14 C in summer or fall were evident at most stations, with largest amplitudes observed at Point Barrow (71°N) and La Jolla (32°N). Fossil fuel emissions do not account for the seasonal cycles of Δ 14 C in either hemisphere, indicating strong contributions from non‐fossil influences, most likely from stratosphere‐troposphere exchange.