Source‐Labeled Anthropogenic Carbon Reveals a Large Shift of Preindustrial Carbon From the Ocean to the Atmosphere. Issue 10 (4th October 2022)

• Record Type: Journal Article
• Authors: Holzer, Mark; DeVries, Tim
• Is Part Of: Global biogeochemical cycles. Volume 36:Issue 10(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-10-04
• Subjects: carbon cycle -- anthropogenic carbon -- atmospheric CO2 change -- oceanic carbon storage -- ocean circulation inverse model -- tracer methods; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2022GB007405
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Two centuries of anthropogenic CO2 emissions have increased the CO2 concentration of the atmosphere and the dissolved inorganic carbon (DIC) concentration of the ocean compared to preindustrial times. These anthropogenic carbon perturbations are often equated to the amount of anthropogenically emitted carbon in the atmosphere or ocean, which ignores the possibility of a shift of natural carbon between the oceanic and atmospheric carbon reservoirs. Here we use a data‐assimilated ocean circulation model and numerical tracers akin to ideal isotopes to label carbon when it is emitted by anthropogenic sources. We find that emitted carbon accounts for only about 45% of the atmospheric CO2 increase since preindustrial times, the remaining 55% being natural CO2 that outgassed from the ocean in response to anthropogenically emitted carbon invading the ocean. This outgassing is driven by the order‐10 seawater carbonate buffer factor which causes increased leakage of natural CO2 as DIC concentrations increase. By 2020, the ocean had outgassed ∼159 Pg of natural carbon, which is counteracted by the ocean absorbing ∼347 Pg of emitted carbon, about 1.8 times more than the net increase in oceanic carbon storage of ∼188 PgC. These results do not challenge existing estimates of anthropogenically driven changes in atmospheric or oceanic carbon inventories, but they shed new light on the composition of these changes and the fate of anthropogenically emitted carbon in the Earth system. Plain Language Summary: The observed increase of atmospheric CO2 concentrations since preindustrial times is often considered to consist entirely of CO2 emitted through human activities such as fossil‐fuel burning, changes in agriculture, and deforestation. Here we show that this picture misses a large shift of natural carbon from the ocean to the atmosphere, with the ocean compensating by taking up nearly twice as much emitted carbon than commonly estimated. We use an ocean circulation model to track emitted carbon through the atmosphere‐ocean system and find that only about 45% of the atmospheric CO2 increase is emitted carbon, with the remaining 55% being natural carbon that has outgassed from the ocean. As the ocean absorbs emitted carbon, the surface ocean becomes more "leaky" for carbon as a result of seawater carbon chemistry. Thus, as the ocean absorbs ever more emitted carbon, it increasingly outgasses natural carbon that was dissolved in the ocean before industrial carbon sources played a significant role. Key Points: Anthropogenically emitted carbon accounts for about half of the atmospheric CO2 increase since preindustrial times The remaining half of the atmospheric CO2 increase is due to outgassing of preindustrial carbon from the ocean By 2020, the ocean had lost 1 preindustrial CO2 molecule for every 2.2 anthropogenically emitted CO2 molecules gained