An analysis of growth rates, seasonal cycles, and meridional gradients of atmospheric CO2 and δ13 C using an atmospheric transport model (1948−2021)

Understanding the processes of carbon dioxide (CO ₂ ) sources and sinks at various scales is required for climate mitigation action, and carbon isotopes are an ideal marker of exchange processes between reservoirs. We have developed a global transport model (MIROC4-ACTM) to simulate CO ₂ and δ ¹³ C for the period 1948 − 2021, using fossil-fuel emissions (GridFED), oceanic fluxes (LENS), and two cases of land-biosphere fluxes (VISIT and LENS). Model outputs were evaluated by using (1) a merged precise ice core and firn-air reconstructions (1948–1980; deseasonalised) and (2) direct measurements by flask air sampling from the Scrips Institution of Oceanography (SIO) network (1958 − 2021 for CO ₂ and 1977 − 2021 for δ ¹³ C). The model simulations are further used to analyze the drivers of observed long-term trends, seasonal cycle and evolving seasonal cycle amplitude (SCA), as well as growth rates, and inter-site gradients for both atmospheric CO ₂ and δ ¹³ C. The model simulations closely follow the observed CO ₂ and δ ¹³ C tendencies, SCAs, and interannual variabilities that are linked to large-scale climate variability during 1958/1977 − 2021. The model simulated SCAs of both CO ₂ and δ ¹³ C 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 CO ₂ during 1960s-2010s, indicating intensified terrestrial carbon exchange in the NH; however, the changes in observed and model δ ¹³ C SCA remain small during recent decades (1980s − 2010s). The model inter-site gradients in both CO ₂ and δ ¹³ C have increased since the 1960s, due to the increase in fossil fuel emissions and enhanced biospheric activities. The overall consistencies between simulations (using the VISIT and LENS fluxes) and multi-decadal CO ₂ and δ ¹³ C measurements highlights recent advances in our understanding of the global carbon cycle processes, but differences persist at inter-decadal time scales; those must be better understood for future projection of carbon-climate feedback.