A possible new role for atmospheric 13CO2 in global models

The promise of utilizing large-scale atmospheric δ13CO2 measurements to understand terrestrial processes has not been fully realized. Here, we will present recent progress in the use of measurements and simulations of atmospheric δ13C to better understand the signals of the biosphere contained within atmospheric data. The motivation for measuring atmospheric δ13C over the past 20 years has been to help partition global surface carbon fluxes into terrestrial and oceanic components. While this is still possible, it is probably not the most effective use of δ13C. First, isotopic disequilibria and their interannual variability introduce great uncertainty into land/ocean partitioning estimates. Second, the presence of a greatly increased density of terrestrial CO2 observations in the Northern Hemisphere obviate this application of δ13C to a large extent. What this suggests, however, is that in a system in which land/ocean uptake is already well constrained, δ13C can constrain terrestrial biosphere processes. For example, analysis of atmospheric CO2 and δ13C correlations at regional spatial scales (105 - 106 km22) suggests a seasonality in photosynthetic discrimination and stomatal conductance. Other analyses of CO2 and δ13C correlations show changes in C4 and C3 productivity. This suggests that within a carbon data assimilation system, such as NOAA/ESRL's CarbonTracker, atmospheric δ13C data could be used to optimize large-scale land surface characteristics like C3/C4 mixtures or stomatal conductance. We will present forward modeling simulations of atmospheric δ13C with comparisons to observations as well as model tests quantifying the sensitivity of atmospheric δ13C to C3 and C4 carbon flux variations and stomatal conductance parameterizations