The seasonal cycle of δ13C of atmospheric carbon dioxide: Influences of
land and ocean carbon fluxes and drivers.
Abstract
In situ measurements of the seasonal cycle of δ13C(CO2) provide
complementary information on the seasonality of the global carbon cycle,
but are currently not exploited in the context of process-based carbon
cycle models. We use isotope-enabled simulations of the Bern3D-LPX Earth
System Model of Intermediate Complexity and fossil fuel emission
estimates together with a model of atmospheric transport to simulate
local atmospheric δ13C(CO2). We find good agreement between the measured
and simulated seasonal cycle of atmospheric δ13C(CO2) (mean seasonal
amplitude mismatch of 0.02 ‰ across 19 sites), particularly at high
northern latitude sites. Factorial simulations reveal that the seasonal
cycle of δ13C(CO2) is primarily driven by land biosphere carbon
exchange. Spatial and temporal fluxes of CO2 and their signatures are
analyzed to quantify the terrestrial drivers. The influence of external
forcings (climate and land use change) on seasonal amplitude is found to
be small. Unlike the growth of seasonal amplitude of CO2, no consistent
change in seasonal amplitude of δ13C(CO2) is simulated over the
historical period, nor evident in the available observations. We
conclude that the seasonal cycle of δ13C(CO2) is influenced by different
carbon cycle processes, and its potential as a novel atmospheric
constraint should be further explored.