Growth rates, seasonal cycles, and meridional gradients of atmospheric CO2 and δ13C using an atmospheric transport model for the period 1948–2021

Understanding the processes of carbon dioxide (CO2) sources and sinks at various scales is required for climate mitigation action, and carbon isotopes (13/14C) are ideal markers of exchange processes between reservoirs. However, the lack of joint simulation of CO2 and δ13C using a 3-dimensional atmospheric chemistry-transport model (ACTM) has limited our understanding of the long-term trends and drivers of the inter-decadal and inter-annual variations of the global carbon cycle covering the period of rapid industrialization and land-use change, i.e., the 1940s to present. We simulated atmospheric CO2 and δ13C for the period 1948 − 2021, using fossil-fuel emissions (GridFED inventory), oceanic fluxes (LENS model), and two cases of land-biosphere fluxes (VISIT and LENS models) in a newly developed MIROC4-ACTM framework. The combined-flux MIROC4-ACTM simulations are analysed by comparing with (1) a merged precise ice core and firn-air reconstructions (1948–1980; deseasonalised) and (2) direct measurements by flask air sampling from the Scripps Institution of Oceanography (SIO) network (1958 − 2021 for CO2 and 1977 − 2021 for δ13C). Simulated Seasonal Cycle Amplitudes (SCAs) of both CO2 and δ13C (using VISIT and LENS land flux combinations—GVL and GLL, respectively) have increased over 1950s − 2010s at Northern Hemisphere (NH) mid-to-high latitudes and remained relatively unchanged in the Southern Hemisphere (SH). These patterns are supported by atmospheric observations of CO2, caused by an intensified terrestrial carbon exchange in the NH. At the NH sites, observed and modeled δ13C SCA shows only weak changes since the 1980s (i.e., in recent decades), in agreement with a recent study. The model inter-site gradients in both CO2 and δ13C have increased since the 1960s, mainly due to increased fossil fuel emissions. The overall consistencies of GVL and GLL simulations with observed CO2 and δ13C highlights recent advances in our understanding of global carbon cycle processes, but differences persist at inter-decadal and inter-annual timescales, which must be better understood for future projection of carbon-climate feedback.