scholarly journals Marine carbon cycle response to a warmer Southern Ocean: the case of the Last Interglacial

2021 ◽  
Author(s):  
Dipayan Choudhury ◽  
Laurie Menviel ◽  
Katrin J. Meissner ◽  
Nicholas K. H. Yeung ◽  
Matthew Chamberlain ◽  
...  
Keyword(s):  
Carbon Cycle ◽  
Sea Ice ◽  
Bottom Water ◽  
Dissolved Inorganic Carbon ◽  
Carbon Sink ◽  
Deep Ocean ◽  
Oceanic Circulation ◽  
Last Interglacial ◽  
Sea Ice Extent ◽  
Carbon Dioxide Co2

Abstract. Recent studies investigating future warming scenarios have shown that the ocean carbon sink will weaken over the coming century due to ocean warming and changes in oceanic circulation. However, significant uncertainties remain regarding the magnitude of the oceanic carbon cycle response to warming. Here, we investigate the Southern Ocean’s (SO, south of 40° S) carbon cycle response to warmer conditions, as simulated under Last Interglacial boundary conditions (LIG, 129–115 thousand years ago, ka). We find a ∼150 % increase in carbon dioxide (CO2) outgassing over the SO at the LIG compared to pre-industrial (PI), due to a 0.5 °C increase in SO sea surface temperatures. This is partly compensated by an equatorward shift of the Southern Hemisphere westerlies and weaker Antarctic Bottom Water formation, which lead to an increase in dissolved inorganic carbon (DIC) in the deep ocean at the LIG compared to PI. These deep ocean DIC changes arise from increased deep and bottom water residence times, and higher remineralization rates due to higher temperatures. While our LIG simulation features a large reduction in SO sea-ice compared to PI, we find that changes in sea ice extent exert a minor control on the marine carbon cycle. Our results thus suggest that the projected poleward intensification of the SH westerlies, coupled with warmer conditions at the surface of the SO would weaken the SO carbon uptake over the coming century.

scholarly journals Diagnosis of CO2 dynamics and fluxes in global coastal oceans

2019 ◽  
Vol 7 (4) ◽  
pp. 786-797 ◽  
Author(s):  
Zhimian Cao ◽  
Wei Yang ◽  
Yangyang Zhao ◽  
Xianghui Guo ◽  
Zhiqiang Yin ◽  
...  
Keyword(s):  
Carbon Cycle ◽  
Dissolved Inorganic Carbon ◽  
Net Primary Production ◽  
Carbon Sink ◽  
Analytical Framework ◽  
Coastal System ◽  
Coastal Oceans ◽  
Co2 Dynamics ◽  
Earth System Models

Abstract Global coastal oceans as a whole represent an important carbon sink but, due to high spatial–temporal variability, a mechanistic conceptualization of the coastal carbon cycle is still under development, hindering the modelling and inclusion of coastal carbon in Earth System Models. Although temperature is considered an important control of sea surface pCO2, we show that the latitudinal distribution of global coastal surface pCO2 does not match that of temperature, and its inter-seasonal changes are substantially regulated by non-thermal factors such as water mass mixing and net primary production. These processes operate in both ocean-dominated and river-dominated margins, with carbon and nutrients sourced from the open ocean and land, respectively. These can be conceptualized by a semi-analytical framework that assesses the consumption of dissolved inorganic carbon relative to nutrients, to determine how a coastal system is a CO2 source or sink. The framework also finds utility in accounting for additional nutrients in organic forms and testing hypotheses such as using Redfield stoichiometry, and is therefore an essential step toward comprehensively understanding and modelling the role of the coastal ocean in the global carbon cycle.

scholarly journals Comment on "Carbon farming in hot, dry coastal areas: an option for climate change mitigation" by Becker et al. (2013)

2013 ◽  
Vol 4 (2) ◽  
pp. 869-873
Author(s):  
M. Heimann
Keyword(s):  
Carbon Cycle ◽  
Carbon Sink ◽  
Co2 Concentration ◽  
Global Carbon Cycle ◽  
Atmospheric Co2 ◽  
Reference Scenario ◽  
Carbon Cycle Model ◽  
Terrestrial Carbon Sink ◽  
Carbon Dioxide Co2 ◽  
Global Carbon

Abstract. Becker et al. (2013) argue that an afforestation of 0.73 109 ha with Jatropha curcas plants would generate an additional terrestrial carbon sink of 4.3 PgC yr−1, enough to stabilise the atmospheric mixing ratio of carbon dioxide (CO2) at current levels. However, this is not consistent with the dynamics of the global carbon cycle. Using a well established global carbon cycle model, the effect of adding such a hypothetical sink leads to a reduction of atmospheric CO2 levels in the year 2030 by 25 ppm compared to a reference scenario. However, the stabilisation of the atmospheric CO2 concentration requires a much larger additional sink or corresponding reduction of anthropogenic emissions.

scholarly journals Timing and magnitude of Southern Ocean sea ice/carbon cycle feedbacks

2020 ◽  
Vol 117 (9) ◽  
pp. 4498-4504 ◽  
Author(s):  
Karl Stein ◽  
Axel Timmermann ◽  
Eun Young Kwon ◽  
Tobias Friedrich
Keyword(s):  
Carbon Cycle ◽  
Southern Ocean ◽  
Sea Ice ◽  
General Circulation ◽  
Deep Ocean ◽  
General Circulation Models ◽  
Ice Dynamics ◽  
Carbon Cycle Model ◽  
Ocean Stratification ◽  
Ocean Ventilation

The Southern Ocean (SO) played a prominent role in the exchange of carbon between ocean and atmosphere on glacial timescales through its regulation of deep ocean ventilation. Previous studies indicated that SO sea ice could dynamically link several processes of carbon sequestration, but these studies relied on models with simplified ocean and sea ice dynamics or snapshot simulations with general circulation models. Here, we use a transient run of an intermediate complexity climate model, covering the past eight glacial cycles, to investigate the orbital-scale dynamics of deep ocean ventilation changes due to SO sea ice. Cold climates increase sea ice cover, sea ice export, and Antarctic Bottom Water formation, which are accompanied by increased SO upwelling, stronger poleward export of Circumpolar Deep Water, and a reduction of the atmospheric exposure time of surface waters by a factor of 10. Moreover, increased brine formation around Antarctica enhances deep ocean stratification, which could act to decrease vertical mixing by a factor of four compared with the current climate. Sensitivity tests with a steady-state carbon cycle model indicate that the two mechanisms combined can reduce atmospheric carbon by 40 ppm, with ocean stratification acting early within a glacial cycle to amplify the carbon cycle response.

scholarly journals A synthesis of marine sediment core δ<sup>13</sup>C data over the last 150 000 years

2010 ◽  
Vol 6 (5) ◽  
pp. 645-673 ◽  
Author(s):  
K. I. C. Oliver ◽  
B. A. A. Hoogakker ◽  
S. Crowhurst ◽  
G. M. Henderson ◽  
R. E. M. Rickaby ◽  
...  
Keyword(s):  
Gas Exchange ◽  
Carbon Storage ◽  
Ocean Circulation ◽  
Marine Sediment ◽  
Large Scale ◽  
Dissolved Inorganic Carbon ◽  
Deep Ocean ◽  
Global Ocean ◽  
Last Interglacial ◽  
Basin Scale

Abstract. The isotopic composition of carbon, δ13C, in seawater is used in reconstructions of ocean circulation, marine productivity, air-sea gas exchange, and biosphere carbon storage. Here, a synthesis of δ13C measurements taken from foraminifera in marine sediment cores over the last 150 000 years is presented. The dataset comprises previously published and unpublished data from benthic and planktonic records throughout the global ocean. Data are placed on a common δ18O age scale suitable for examining orbital timescale variability but not millennial events, which are removed by a 10 ka filter. Error estimates account for the resolution and scatter of the original data, and uncertainty in the relationship between δ13C of calcite and of dissolved inorganic carbon (DIC) in seawater. This will assist comparison with δ13C of DIC output from models, which can be further improved using model outputs such as temperature, DIC concentration, and alkalinity to improve estimates of fractionation during calcite formation. High global deep ocean δ13C, indicating isotopically heavy carbon, is obtained during Marine Isotope Stages (MIS) 1, 3, 5a, c and e, and low δ13C during MIS 2, 4 and 6, which are temperature minima, with larger amplitude variability in the Atlantic Ocean than the Pacific Ocean. This is likely to result from changes in biosphere carbon storage, modulated by changes in ocean circulation, productivity, and air-sea gas exchange. The North Atlantic vertical δ13C gradient is greater during temperature minima than temperature maxima, attributed to changes in the spatial extent of Atlantic source waters. There are insufficient data from shallower than 2500 m to obtain a coherent pattern in other ocean basins. The data synthesis indicates that basin-scale δ13C during the last interglacial (MIS 5e) is not clearly distinguishable from the Holocene (MIS 1) or from MIS 5a and 5c, despite significant differences in ice volume and atmospheric CO2 concentration during these intervals. Similarly, MIS 6 is only distinguishable from MIS 2 or 4 due to globally lower δ13C values both in benthic and planktonic data. This result is obtained despite individual records showing differences between these intervals, indicating that care must be used in interpreting large scale signals from a small number of records.

Timing and magnitude of Southern Ocean sea ice/carbon cycle feedbacks over the last eight glacial cycles

2020 ◽  
Author(s):  
Karl Stein ◽  
Axel Timmermann ◽  
Eun Young Kwon ◽  
Tobias Friedrich
Keyword(s):  
Carbon Sequestration ◽  
Carbon Cycle ◽  
Southern Ocean ◽  
Sea Ice ◽  
Deep Ocean ◽  
Glacial Cycles ◽  
Ice Dynamics ◽  
Ocean Stratification ◽  
Ocean Ventilation ◽  
The Impact

&lt;p class=&quot;p1&quot;&gt;&lt;span class=&quot;s1&quot;&gt;The Southern Ocean (SO) played a prominent role in the exchange of carbon between ocean and atmosphere on glacial timescales through its regulation of deep ocean ventilation. Previous studies indicated that SO sea ice could dynamically link several processes of carbon sequestration, but these studies relied on models with simplified ocean and sea ice dynamics or snapshot simulations with general circulation models. Here we use a transient run of the LOVECLIM intermediate complexity climate model, covering the past eight glacial cycles, to investigate the orbital-scale dynamics of deep ocean ventilation changes due to SO sea ice. Cold climates increase sea ice cover, sea-ice export, and Antarctic Bottom Water formation, which are accompanied by increased SO upwelling, stronger poleward export of Circumpolar Deep Water, and a reduction of the atmospheric exposure time of surface waters by a factor of ten. Moreover, increased brine formation around Antarctica enhances deep ocean stratification, which could act to decrease vertical mixing by a factor of four compared to the current climate. The impact of the two mechanisms on carbon sequestration was then tested within a steady-state carbon cycle. The two mechanisms combined can reduce atmospheric carbon by 40 ppm, of which approximately 30 ppm is due to ocean stratification. Moreover, ocean stratification from increased SO sea ice production acts early within glacial cycles to amplify the carbon cycle response.&lt;/span&gt;&lt;/p&gt;

scholarly journals Comment on "Carbon farming in hot, dry coastal areas: an option for climate change mitigation" by Becker et al. (2013)

2014 ◽  
Vol 5 (1) ◽  
pp. 41-42 ◽  
Author(s):  
M. Heimann
Keyword(s):  
Carbon Cycle ◽  
Carbon Sink ◽  
Co2 Concentration ◽  
Global Carbon Cycle ◽  
Atmospheric Co2 ◽  
Reference Scenario ◽  
Carbon Cycle Model ◽  
Terrestrial Carbon Sink ◽  
Carbon Dioxide Co2 ◽  
Global Carbon

Abstract. Becker et al. (2013) argue that an afforestation of 0.73 × 109 ha with Jatropha curcas plants would generate an additional terrestrial carbon sink of 4.3 PgC yr−1, enough to stabilise the atmospheric mixing ratio of carbon dioxide (CO2) at current levels. However, this is not consistent with the dynamics of the global carbon cycle. Using a well-established global carbon cycle model, the effect of adding such a hypothetical sink leads to a reduction of atmospheric CO2 levels in the year 2030 by 25 ppm compared to a reference scenario. However, the stabilisation of the atmospheric CO2 concentration requires a much larger additional sink or corresponding reduction of anthropogenic emissions.

scholarly journals Including an ocean carbon cycle model into <i>i</i>LOVECLIM (v1.0)

2015 ◽  
Vol 8 (5) ◽  
pp. 1563-1576 ◽  
Author(s):  
N. Bouttes ◽  
D. M. Roche ◽  
V. Mariotti ◽  
L. Bopp
Keyword(s):  
Carbon Cycle ◽  
Atmospheric Carbon Dioxide ◽  
Climate Model ◽  
Dissolved Inorganic Carbon ◽  
Carbon Dioxide Concentration ◽  
Deep Ocean ◽  
Cycle Model ◽  
Carbon Cycle Model ◽  
Radiative Balance ◽  
Ocean Carbon

Abstract. The atmospheric carbon dioxide concentration plays a crucial role in the radiative balance and as such has a strong influence on the evolution of climate. Because of the numerous interactions between climate and the carbon cycle, it is necessary to include a model of the carbon cycle within a climate model to understand and simulate past and future changes of the carbon cycle. In particular, natural variations of atmospheric CO2 have happened in the past, while anthropogenic carbon emissions are likely to continue in the future. To study changes of the carbon cycle and climate on timescales of a few hundred to a few thousand years, we have included a simple carbon cycle model into the iLOVECLIM Earth System Model. In this study, we describe the ocean and terrestrial biosphere carbon cycle models and their performance relative to observational data. We focus on the main carbon cycle variables including the carbon isotope ratios δ13C and the Δ14C. We show that the model results are in good agreement with modern observations both at the surface and in the deep ocean for the main variables, in particular phosphates, dissolved inorganic carbon and the carbon isotopes.

scholarly journals A synthesis of marine sediment core δ<sup>13</sup>C data over the last 150 000 years

2009 ◽  
Vol 5 (6) ◽  
pp. 2497-2554 ◽  
Author(s):  
K. I. C. Oliver ◽  
B. A. A. Hoogakker ◽  
S. Crowhurst ◽  
G. M. Henderson ◽  
R. E. M. Rickaby ◽  
...  
Keyword(s):  
Gas Exchange ◽  
Carbon Storage ◽  
Ocean Circulation ◽  
Marine Sediment ◽  
Large Scale ◽  
Dissolved Inorganic Carbon ◽  
Deep Ocean ◽  
Global Ocean ◽  
Last Interglacial ◽  
Basin Scale

Abstract. The isotopic composition of carbon, δ13C, in seawater is used in reconstructions of ocean circulation, marine productivity, air-sea gas exchange, and biosphere carbon storage. Here, a synthesis of δ13C measurements taken from foraminifera in marine sediment cores over the last 150 000 years is presented. The dataset comprises previously published and unpublished data from benthic and planktonic records throughout the global ocean. Data are placed on a common δ18O age scale and filtered to remove timescales shorter than 6 kyr. Error estimates account for the resolution and scatter of the original data, and uncertainty in the relationship between δ13C of calcite and of dissolved inorganic carbon (DIC) in seawater. This will assist comparison with δ13C of DIC output from models, which can be further improved using model outputs such as temperature, DIC concentration, and alkalinity to improve estimates of fractionation during calcite formation. High global deep ocean δ13C, indicating isotopically heavy carbon, is obtained during Marine Isotope Stages (MIS) 1, 3, 5a, 5c and 5e, and low δ13C during MIS 2, 4 and 6, which are temperature minima, with larger amplitude variability in the Atlantic Ocean than the Pacific Ocean. This is likely to result from changes in biosphere carbon storage, modulated by changes in ocean circulation, productivity, and air-sea gas exchange. The North Atlantic vertical δ13C gradient is greater during temperature minima than temperature maxima, attributed to changes in the spatial extent of Atlantic source waters. There are insufficient data from shallower than 2500 m to obtain a coherent pattern in other ocean basins. The data synthesis indicates that basin-scale δ13C during the last interglacial (MIS 5e) is not clearly distinguishable from the Holocene (MIS 1) or from MIS 5a and 5c, despite significant differences in ice volume and atmospheric CO2 concentration during these intervals. Similarly, MIS 6 is only distinguishable from MIS 2 or 4 due to globally lower δ13C values both in benthic and planktonic data. This result is obtained despite individual records showing differences between these intervals, indicating that care must be used in interpreting large scale signals from a small number of records.

scholarly journals The devil's in the disequilibrium: sensitivity of ocean carbon storage to climate state and iron fertilization in a general circulation model

2017 ◽  
Author(s):  
Sarah Eggleston ◽  
Eric D. Galbraith
Keyword(s):  
Sea Ice ◽  
Carbon Storage ◽  
General Circulation ◽  
Dissolved Inorganic Carbon ◽  
Circulation Model ◽  
Deep Ocean ◽  
Inverse Correlation ◽  
Biogeochemical Model ◽  
Iron Fertilization ◽  
Ocean Carbon

Abstract. Ocean dissolved inorganic carbon (DIC) storage can be conceptualized as the sum of four components: saturation (DICsat), disequilibrium (DICdis), carbonate (DICcarb) and soft tissue (DICsoft). Among these, DICdis and DICsoft have the potential for large changes that are relatively difficult to predict. Here we explore changes in DICsoft and DICdis in a large suite of simulations with a complex coupled climate-biogeochemical model, driven by changes in orbital forcing, ice sheets and the radiative effect of CO2. Both DICdis and DICsoft vary over a range of 40 μmol kg−1 in response to the climate forcing, equivalent to changes in atmospheric CO2 on the order of 50 ppm for each. We find that, despite the broad range of climate states represented, changes in global DICsoft can be well-approximated by the product of deep ocean ideal age and the global export production flux, while global DICdis is dominantly controlled by the fraction of the ocean filled by Antarctic Bottom Water (AABW). Because the AABW fraction and ideal age are inversely correlated between the simulations, DICdis and DICsoft are also inversely correlated. This inverse correlation could be decoupled if changes in deep ocean mixing were to alter ideal age independently of AABW fraction, or if independent ecosystem changes were to alter export and remineralization, thereby modifying DICsoft. As an example of the latter, iron fertilization causes DICsoft to increase, and causes DICdis to also increase by a similar or greater amount, to a degree that depends on climate state. We propose a simple framework to consider the global contribution of DICsoft + DICdis to ocean carbon storage as a function of the surface preformed nitrate and DICdis of dense water formation regions, the global volume fractions ventilated by these regions, and the global nitrate inventory. More extensive sea ice increases DICdis, and when sea ice becomes very extensive it also causes significant O2 disequilibrium, which may have contributed to reconstructions of low O2 in the Southern Ocean during the glacial. Global DICdis reaches a minimum near modern CO2 because the AABW fraction reaches a minimum, which may have contributed to preventing further CO2 rise during interglacial periods.

scholarly journals Evaluating the physical and biogeochemical state of the global ocean component of UKESM1 in CMIP6 Historical simulations

2020 ◽  
Author(s):  
Andrew Yool ◽  
Julien Palmiéri ◽  
Colin G. Jones ◽  
Lee de Mora ◽  
Till Kuhlbrodt ◽  
...  
Keyword(s):  
Sea Ice ◽  
Climate Model ◽  
Dissolved Inorganic Carbon ◽  
Lower Surface ◽  
Global Ocean ◽  
Earth System ◽  
The Earth ◽  
Model Improvement ◽  
Marine Productivity ◽  
Carbon Dioxide Co2

Abstract. The ocean plays a key role in modulating the climate of the Earth system (ES). At the present time it is also a major sink both for the carbon dioxide (CO2) released by human activities as well as for the excess heat driven by the resulting atmospheric greenhouse effect. Understanding the ocean's role in these processes is critical for model projections of future change and its potential impacts on human societies. A necessary first step in assessing the credibility of such future projections is an evaluation of their performance against the present state of the ocean. Here we use a range of observational properties to validate the physical and biogeochemical performance of the ocean component of UKESM1, a new Earth system (ESM) for CMIP6 built upon the HadGEM3 physical climate model. Analysis focuses on the realism of the ocean's physical state and circulation, its key elemental cycles, and its marine productivity. UKESM1 generally performs well across a broad spectrum of properties, but it exhibits a number of notable biases. Physically, these include a global warm bias inherited from model spin-up, excess northern sea-ice but insufficient southern sea-ice, and sluggish interior circulation. Biogeochemical biases found include shallow remineralisation of sinking organic matter, excessive iron stress in regions such as the Equatorial Pacific, and generally lower surface alkalinity that results in decreased surface and interior dissolved inorganic carbon (DIC) concentrations. The mechanisms driving these biases are explored to identify consequences for the behaviour of UKESM1 under future climate scenarios, and avenues for model improvement. Finally, across key biogeochemical properties, UKESM1 improves in performance relative to its CMIP5 precursor, and compares favourably to fellow members of the CMIP6 ensemble.

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