Land use effects on atmospheric ¹³C imply a sizable terrestrial CO₂ sink in tropical latitudes

Records of atmospheric CO 2 and 13 CO 2 can be used to distinguish terrestrial vs. oceanic exchanges of CO 2 with the atmosphere. However, this approach has proven difficult in the tropics, partly due to extensive land conversion from C 3 to C 4 vegetation. We estimated the effects of such conversion on biosphere‐atmosphere 13 C exchange for 1991–2000, and then explored how this “land‐use disequilibrium” altered the partitioning of net atmospheric CO 2 exchanges between ocean and land using NOAA‐CMDL data and a 2D, zonally averaged atmospheric transport model. Our results suggest sizable CO 2 uptake in C 3 ‐dominated tropical regions in 8 of the 10 years; 1997 and 1998, which included a strong ENSO event, are near neutral. Since these fluxes include any deforestation source, our findings imply either that such sources are smaller than previously estimated, and/or the existence of a large equatorial terrestrial CO 2 sink.