Simulating Long-Term Trends and Seasonal Dynamics of Carbon Isotopes in Atmospheric CO2 Using a 3D Transport Model

Carbon Dioxide (CO₂), the primary anthropogenic greenhouse gas (GHG), plays a significant role in global warming. Earth’s global surface air temperate was higher by 1.09 °C in 2011–2020 than in 1850–1900. This rise is overwhelmed by 47% in atmospheric CO₂ during the period (IPCC AR6, 2021). Analysis of carbon isotopes (¹³C and ¹⁴C) of CO₂ plays a pivotal role in separating the anthropogenic and natural carbon release and uptake across land, ocean and atmosphere carbon pools. Despite their utility to understand carbon cycle dynamics, simulating the seasonal variations and long-term trends of ¹³C and ¹⁴C remains challenging. Bridging of the budget gaps requires robust modeling approaches to simulate the isotopic exchange fluxes since the rapid increase in fossil fuel emissions began in the 1950s.

This study has quantified the monthly exchange fluxes of ¹³C and ¹⁴C between the atmosphere and terrestrial biosphere, and between the atmosphere and ocean, as well as ¹³C and ¹⁴C emissions from fossil fuel, nuclear bomb tests, and nuclear power plants, for the period from 1940 to 2020. We have used fossil fuel emissions from GridFED (Jones et al., 2023), land biosphere fluxes are taken from LENS, LPJ and VISIT (NCAR ref., Scholze et al., 2008, Ito et al., 2007), and ocean exchange fluxes are taken from CESM2, LENS (NCAR ref., Danabasoglu et al., 2020). The Model for Interdisciplinary Research on Climate version 4 (MIROC4) atmospheric general circulation model (AGCM)-based chemistry-transport model (referred to as MIROC4-ACTM) has been used for the simulation of the prepared fluxes of ¹³C and ¹⁴C.

Our model simulated the observed concentrations of Δ¹⁴C at Jungfraujoch (JFJ; ICOS ref., Levin et al., 2021) and Baring Head (BHD; NIWA ref., Turnbull et al., 2007); e.g., the rise from -24.3 ‰ to 272.4 ‰ during 1950−1960, followed by a slow (near exponential) decay during 1965 to 2020. The two model cases using LENS and LPJ land model fluxes showed noticeable differences during 1970s. The model simulations of δ¹³C were compared with nine sites of SIO (Keeling et al., 2001); they successfully reproduced the long-term declining trend driven by the Suess Effect, which is the isotopic depletion of atmospheric CO₂ caused by the combustion of δ¹³C-depleted fossil fuels. Seasonal variations were well captured, with enriched δ¹³C during photosynthetic periods (summer) and depleted δ¹³C during respiration periods (winter). In our simulations, the interhemispheric gradient in δ¹³C was evident, with stronger seasonal cycles and steeper declines in the Northern Hemisphere (e.g., Barrow, Mauna Loa) due to proximity to major anthropogenic CO₂ sources, while Southern Hemisphere sites (e.g., Baring Head, South Pole) showed weaker seasonal variations, reflecting the dominance of ocean uptake and isotopic mixing. Discrepancies in Δ¹⁴C during 1955–1965 and 1980–2000 due to uncertainties in bomb-test emissions and biospheric uptake fluxes remain a challenge in accurately reproducing the observations.