Department of Earth System Science

During the mid-1960s, large seasonal amplitudes were observed in surface measurements of Δ¹⁴C in the Northern Hemisphere. These seasonal oscillations were initially caused by stratosphere-troposphere exchange, with the injection of bomb ¹⁴C into the troposphere during winter and spring mixing. Here we show how fossil, ocean, and terrestrial biosphere fluxes modified the stratospheric signal during the 1960s, and the evolution of each of these components in the post bomb era. In our analysis, we used the Goddard Institute for Space Studies (GISS) atmospheric tracer model, gross ocean CO₂ fluxes from the Lawrence Livermore National Laboratory (LLNL) ocean model, and terrestrial CO₂ fluxes from a biosphere-atmosphere model driven by normalized difference vegetation index and surface air temperatures. We found that ¹⁴C-depeleted respiration from the terrestrial biosphere partially canceled the ¹⁴C-enriched stratosphere flux in the Northern Hemisphere in the mid and late 1960s. In more recent decades, our analysis suggested that the terrestrial biosphere contribution to the Δ¹⁴C seasonal cycle reversed phase, with the terrestrial biosphere currently releasing relatively ¹⁴C-enriched CO₂ that mixes with relatively depleted troposphere CO₂. The timing of this reversal depended on the residence times of carbon within the footprint of the observation station. Measurements of Δ¹⁴C in respiration from tundra and boreal ecosystems in Alaska provide evidence that some boreal forests have undergone this transition, while some tundra ecosystems have not. We predict that over the next century, several features of the latitudinal profile of Δ¹⁴C will substantially change becaus