Ocean carbon uptake under aggressive emission mitigation

Sean M Ridge,Galen A Mckinley

doi:10.5194/bg-18-2711-2021

Sean M Ridge, Galen A Mckinley

Open Access

https://doi.org/10.5194/bg-18-2711-2021

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Abstract

Abstract. Nearly every nation has signed the UNFCC Paris Agreement, committing to mitigate anthropogenic carbon emissions so as to limit the global mean temperature increase above pre-industrial levels to well below 2 ∘C, and ideally to no more than 1.5 ∘C. A consequence of emission mitigation that has received limited attention is a reduced efficiency of the ocean carbon sink. Historically, the roughly exponential increase in atmospheric CO2 has resulted in a proportional increase in anthropogenic carbon uptake by the ocean. We define growth of the ocean carbon sink exactly proportional to the atmospheric growth rate to be 100 % efficient. Using a model hierarchy consisting of a common reduced-form ocean carbon cycle model and the Community Earth System Model (CESM), we assess the mechanisms of future change in the efficiency of the ocean carbon sink under three emission scenarios: aggressive mitigation (1.5 ∘C), intermediate mitigation (RCP4.5), and high emissions (RCP8.5). The reduced-form ocean carbon cycle model is tuned to emulate the global-mean behavior of the CESM and then allows for mechanistic decomposition. With intermediate or no mitigation (RCP4.5, RCP8.5), changes in efficiency through 2080 are almost entirely the result of future reductions in the carbonate buffer capacity of the ocean. Under the 1.5 ∘C scenario, the dominant driver of efficiency decline is the ocean's reduced ability to transport anthropogenic carbon from surface to depth. As the global-mean upper-ocean gradient of anthropogenic carbon reverses sign, carbon can be re-entrained in surface waters where it slows further removal from the atmosphere. Reducing uncertainty in ocean circulation is critical to better understanding the transport of anthropogenic carbon from surface to depth and to improving quantification of its role in the future ocean carbon sink.

Highlights

The ocean has absorbed excess carbon equivalent to 39 % of the CO2 from industrial-era fossil fuel combustion and cement production (Friedlingstein et al, 2019)
The difference in partial pressures has grown over time; ocean uptake of atmospheric CO2 has increased over the industrial era (Khatiwala et al, 2009; DeVries, 2014)
By parameterizing the ocean’s global-mean removal of carbon to depth as a constant process, models based on an impulse response function (IRF) can replicate ocean anthropogenic carbon uptake that is quantitatively consistent with the uptake of complex models and observations (Oeschger et al, 1975; Joos et al, 1996)

Summary

Introduction

The ocean has absorbed excess carbon equivalent to 39 % of the CO2 from industrial-era fossil fuel combustion and cement production (Friedlingstein et al, 2019). The rate of ocean anthropogenic carbon uptake is further controlled by carbon chemistry in seawater and physical removal of anthropogenic carbon from the surface ocean into the ocean interior (Graven et al, 2012). Various processes set the rate of transport from surface to depth of anthropogenic carbon (Bopp et al, 2015; Gnanadesikan et al, 2015; Iudicone et al, 2016). By parameterizing the ocean’s global-mean removal of carbon to depth as a constant process, models based on an impulse response function (IRF) can replicate ocean anthropogenic carbon uptake that is quantitatively consistent with the uptake of complex models and observations (Oeschger et al, 1975; Joos et al, 1996)

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