Preprints
https://doi.org/10.5194/bg-2020-254
https://doi.org/10.5194/bg-2020-254

  16 Jul 2020

16 Jul 2020

Review status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Ocean Carbon Uptake Under Aggressive Emission Mitigation

Sean Ridge and Galen McKinley Sean Ridge and Galen McKinley
  • Lamont-Doherty Earth Observatory of Columbia University, P.O. Box 1000, 61 Route 9W, Palisades, NY

Abstract. Nearly every nation has signed the UNFCC Paris Agreement, committing to mitigate global anthropogenic carbon (Cant) emissions and limit global mean temperature increase to 1.5 °C. A consequence of emission mitigation is reduced efficiency of ocean Cant uptake, which is driven by mechanisms that have not been studied in detail. The historical pattern of continual increase in atmospheric CO2 has resulted in a proportional increase in Cant uptake. Here, we explore how this proportionality will weaken and find significant effects related to changes in the vertical transfer of Cant from the surface to the deep ocean, and also ocean chemistry. We define ocean uptake growth consistent with an exact proportionality to the atmospheric growth rate, i.e. the historical scaling, to be 100 % efficient. Using a model hierarchy consisting of a commonly used one-dimensional ocean carbon cycle model and a complex Earth System Model (ESM), we find that declines in the efficiency of ocean uptake are greatest under aggressive emission mitigation. To understand the drivers of efficiency declines, we use the ESM to compare scenarios with aggressive emission mitigation (1.5 °C), intermediate emission mitigation (RCP4.5), and no emission mitigation (RCP8.5). Using the one-dimensional ocean carbon cycle model, we demonstrate how growth of ocean Cant uptake is a balance between enhancement due to a positive atmospheric CO2 growth rate, and decreases due to the positive growth rate of dissolved CO2 in the surface ocean. Without emission mitigation (RCP8.5), changes in efficiency are almost entirely the result of changes in the buffer capacity of the ocean, which accelerates the growth rate of dissolved CO2 in the surface ocean. Under the declining CO2 regime of the 1.5 °C scenario, the dominant driver of efficiency decline is the carbon gradient effect, wherein Cant in the ocean interior slows the removal of Cant from the surface. Although the carbon gradient effect is an unavoidable consequence of emission mitigation, it can be reduced by hastily pursuing emission mitigation.

Sean Ridge and Galen McKinley

 
Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement

Sean Ridge and Galen McKinley

Sean Ridge and Galen McKinley

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Short summary
Approximately 40 % of the CO2 emissions from fossil fuel combustion and cement production have been absorbed by the ocean. The goal of the UNFCCC Paris Agreement is to reduce CO2 emissions in an attempt to limit global warming to less than 1.5 °C. In the future, we find that reduced CO2 emissions reduces the efficiency of ocean CO2 absorption. Reduced efficiency is mostly due to reduced downward mixing of CO2. This efficiency reduction can be avoided if we don't delay reducing CO2 emissions.
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