Quantifying airborne fraction trends and the destination of anthropogenic CO ₂ by tracking carbon flows in a simple climate model

Journal Article · 20 April 2023 · Environmental Research Letters

DOI: https://doi.org/10.1088/1748-9326/acca35 · OSTI ID:1971152

Pressburger, Leeya ; Dorheim, Kalyn ; Keenan, Trevor F. ; McJeon, Haewon ; Smith, Steven J. ; Bond-Lamberty, Ben

Abstract Atmospheric carbon dioxide (CO ₂ ) concentrations have increased as a direct result of human activity and are at their highest level over the last 2 million years, with profound impacts on the Earth system. However, the magnitude and future dynamics of land and ocean carbon sinks are not well understood; therefore, the amount of anthropogenic fossil fuel emissions that remain in the atmosphere (the airborne fraction) is poorly constrained. This work aims to quantify the sources and controls of atmospheric CO ₂ , the fate of anthropogenic CO ₂ over time, and the likelihood of a trend in the airborne fraction. We use Hector v3.0, a coupled simple climate and carbon cycle model with the novel ability to explicitly track carbon as it flows through the Earth system. We use key model parameters in a Monte Carlo analysis of 15 000 model runs from 1750 to 2300. Results are filtered for physical realism against historical observations and CMIP6 projection data, and we calculate the relative importance of parameters controlling how much anthropogenic carbon ends up in the atmosphere. Modeled airborne fraction was roughly 52%, consistent with observational studies. The overwhelming majority of model runs exhibited a negative trend in the airborne fraction from 1960–2020, implying that current-day land and ocean sinks are proportionally taking up more carbon than the atmosphere. However, the percentage of atmospheric CO ₂ derived from anthropogenic origins can be much higher because of Earth system feedbacks. We find it peaks at over 90% between 2010–2050. Moreover, when looking at the destination of anthropogenic fossil fuel emissions, only a quarter ends up in the atmosphere while more than half of emissions are taken up by the land sink on centennial timescales. This study evaluates the likelihood of airborne fraction trends and provides insights into the dynamics of anthropogenic CO ₂ in the Earth system.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE; USDOE Office of Science (SC)
• Grant/Contract Number: AC05-76RL01830
• OSTI ID: 1971152
• Report Number(s): PNNL-SA-178364
• Journal Information: Environmental Research Letters, Journal Name: Environmental Research Letters Journal Issue: 5 Vol. 18; ISSN 1748-9326
• Publisher: IOP PublishingCopyright Statement
• Country of Publication: United Kingdom
• Language: English