Changes in Global Ocean Bottom Properties and Volume Transports in CMIP5 Models under Climate Change Scenarios

Heuzé, Céline; Heywood, Karen J.; Stevens, David P.; Ridley, Jeff

doi:10.1175/jcli-d-14-00381.1

Cited by 88 publications

(107 citation statements)

References 59 publications

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“…On a centennial timescale, the weakening of AMOC and AABW formation in our simulation is consistent with the results of CMIP5 models (Weaver et al, 2012;Heuzé et al, 2015). Changes in oceanic carbon uptake and reductions in export production due to global warming are in close agreement with current simulations using ESMs, as mentioned in Sect.…”

Section: Discussionsupporting

confidence: 90%

Long-term response of oceanic carbon uptake to global warming via physical and biological pumps

2018

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Abstract. Global warming is expected to significantly decrease oceanic carbon uptake and therefore increase atmospheric CO 2 and global warming. The primary reasons given in previous studies for such changes in the oceanic carbon uptake are the solubility reduction due to seawater warming and changes in the ocean circulation and biological pump. However, the quantitative contributions of different processes to the overall reduction in ocean uptake are still unclear. In this study, we investigated multi-millennium responses of oceanic carbon uptake to global warming and quantified the contributions of the physical and biological pumps to these responses using an atmosphere-ocean general circulation model and a biogeochemical model. We found that global warming reduced oceanic CO 2 uptake by 13 % (30 %) in the first 140 years (after 2000 model years), consistent with previous studies. Our sensitivity experiments showed that this reduction is primarily driven by changes in the organic matter cycle via ocean circulation change and solubility change due to seawater warming. These results differ from most previous studies, in which circulation changes and solubility change from seawater warming are the dominant processes. The weakening of biological production and carbon export induced by circulation change and lower nutrient supply, diminishes the vertical DIC gradient and substantially reduces the CO 2 uptake. The weaker deep-ocean circulation decreases the downward transport of CO 2 from the surface to the deep ocean, leading to a drop in CO 2 uptake in high-latitude regions. Conversely, weaker equatorial upwelling reduces the upward transport of natural CO 2 and therefore enhances the CO 2 uptake in low-latitude regions. Because these effects cancel each other out, circulation change plays only a small direct role in the reduction of CO 2 uptake due to global warming but a large indirect role through nutrient transport and biological processes.

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Section: Discussionsupporting

confidence: 90%

Long-term response of oceanic carbon uptake to global warming via physical and biological pumps

2018

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“…Predictions of AMOC have received more attention so far, and AMOC slowdown and partial or full recovery emerges in other multimillennial simulations (Zickfeld et al, 2013;Li et al, 2013;Weaver et al, 2012). AMOC and Southern Ocean overturning in CMIP5 Earth system models were analyzed by Heuzé et al (2015). They found AMOC and Southern Ocean overturning is positively correlated in most CMIP5 models by the end of the 21st century.…”

Section: Uncertainties In O 2 Projectionsmentioning

confidence: 99%

Hazards of decreasing marine oxygen: the near-term and millennial-scale benefits of meeting the Paris climate targets

Battaglia

Joos

2018

Earth Syst. Dynam.

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Abstract. Ocean deoxygenation is recognized as key ecosystem stressor of the future ocean and associated climate-related ocean risks are relevant for current policy decisions. In particular, benefits of reaching the ambitious 1.5 • C warming target mentioned by the Paris Agreement compared to higher temperature targets are of high interest. Here, we model oceanic oxygen, warming and their compound hazard in terms of metabolic conditions on multi-millennial timescales for a range of equilibrium temperature targets. Scenarios where radiative forcing is stabilized by 2300 are used in ensemble simulations with the Bern3D Earth System Model of Intermediate Complexity. Transiently, the global mean ocean oxygen concentration decreases by a few percent under low forcing and by 40 % under high forcing. Deoxygenation peaks about a thousand years after stabilization of radiative forcing and new steady-state conditions are established after AD 8000 in our model. Hypoxic waters expand over the next millennium and recovery is slow and remains incomplete under high forcing. Largest transient decreases in oxygen are projected for the deep sea. Distinct and near-linear relationships between the equilibrium temperature response and marine O 2 loss emerge. These point to the effectiveness of the Paris climate target in reducing marine hazards and risks. Mitigation measures are projected to reduce peak decreases in oceanic oxygen inventory by 4.4 % • C −1 of avoided equilibrium warming. In the upper ocean, the decline of a metabolic index, quantified by the ratio of O 2 supply to an organism's O 2 demand, is reduced by 6.2 % • C −1 of avoided equilibrium warming. Definitions of peak hypoxia demonstrate strong sensitivity to additional warming. Volumes of water with less than 50 mmol O 2 m −3 , for instance, increase between 36 % and 76 % • C −1 of equilibrium temperature response. Our results show that millennial-scale responses should be considered in assessments of ocean deoxygenation and associated climate-related ocean risks. Peak hazards occur long after stabilization of radiative forcing and new steady-state conditions establish after AD 8000.

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“…Inclusion of this mechanism requires shelf-sourced AABW to be correctly represented. Many climate models, and indeed those of the CMIP5 generation, rely primarily on open ocean convection for AABW formation (Heuzé et al 2013(Heuzé et al , 2015. The different responses attainable by including shelf processes indicate a limitation of many climate models and emphasize the importance of incorporating shelf-sourced AABW in the bottom water formation process.…”

Section: B Shelf and Deep Ocean Responsementioning

confidence: 99%

Sensitivity of Antarctic Bottom Water to Changes in Surface Buoyancy Fluxes

et al. 2015

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The influence of freshwater and heat flux changes on Antarctic Bottom Water (AABW) properties are investigated within a realistic bathymetry coupled ocean-ice sector model of the Atlantic Ocean. The model simulations are conducted at eddy-permitting resolution where dense shelf water production dominates over open ocean convection in forming AABW. Freshwater and heat flux perturbations are applied independently and have contradictory surface responses, with increased upper-ocean temperature and reduced ice formation under heating and the opposite under increased freshwater fluxes. AABW transport into the abyssal ocean reduces under both flux changes, with the reduction in transport being proportional to the net buoyancy flux anomaly south of 608S.Through inclusion of shelf-sourced AABW, a process absent from most current generation climate models, cooling and freshening of dense source water is facilitated via reduced on-shelf/off-shelf exchange flow. Such cooling is propagated to the abyssal ocean, while compensating warming in the deep ocean under heating introduces a decadal-scale variability of the abyssal water masses. This study emphasizes the fundamental role buoyancy plays in controlling AABW, as well as the importance of the inclusion of shelf-sourced AABW within climate models in order to attain the complete spectrum of possible climate change responses.

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Changes in Global Ocean Bottom Properties and Volume Transports in CMIP5 Models under Climate Change Scenarios

Cited by 88 publications

References 59 publications

Long-term response of oceanic carbon uptake to global warming via physical and biological pumps

Long-term response of oceanic carbon uptake to global warming via physical and biological pumps

Hazards of decreasing marine oxygen: the near-term and millennial-scale benefits of meeting the Paris climate targets

Sensitivity of Antarctic Bottom Water to Changes in Surface Buoyancy Fluxes

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