scholarly journals Ocean carbon uptake under aggressive emission mitigation

2021 ◽  
Vol 18 (8) ◽  
pp. 2711-2725
Author(s):  
Sean M. Ridge ◽  
Galen A. McKinley
Keyword(s):  
Carbon Sink ◽  
Reduced Form ◽  
Carbon Uptake ◽  
Cycle Model ◽  
Emission Mitigation ◽  
Ocean Carbon Cycle ◽  
Carbon Cycle Model ◽  
Anthropogenic Carbon ◽  
Ocean Carbon ◽  
Global Mean

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.

Natural carbon release over-rides anthropogenic carbon uptake when Southern Hemispheric westerlies strengthen

2020 ◽  
Author(s):  
Paul Spence ◽  
Laurie Menviel ◽  
Darryn Waugh
Keyword(s):  
Total Carbon ◽  
Carbon Uptake ◽  
Cycle Model ◽  
Carbon Loss ◽  
Carbon Cycle Model ◽  
Anthropogenic Carbon ◽  
Carbon Release ◽  
Natural Carbon ◽  
Ocean Carbon ◽  
The Impact

<p>The Southern Ocean is one of today's largest sink of carbon, having absorbed about 10\% of the anthropogenic carbon emissions. Southern Ocean's dynamics are principally modulated by the strength of the Southern Hemispheric westerlies,  which are projected to increase over the coming century. Here, using a high-resolution ocean-sea-ice-carbon cycle model, we explore the impact of idealized changes in Southern Hemispheric westerlies on the ocean carbon storage . We find that a 20\% strengthening of the Southern Hemispheric westerlies leads to a $\sim$25 Gt loss of natural carbon, while an additional 13 Gt of anthropogenic carbon is absorbed compared to the control run, thus resulting in a net loss of $\sim$12 GtC from the ocean over a period of 42 years. This tendency is enhanced if the westerlies are also shifted polewards, with a total natural carbon loss of almost 37 GtC, and an additional anthropogenic carbon uptake of 18 GtC. While both experiments display a large natural carbon loss south of 10$^\circ$S, the amplitude is three times greater in the poleward strengthening case, which is  not fully compensated by the increase in anthropogenic carbon content. However, the poleward wind shift leads to significant differences in the pattern of DIC change due to a weakening of the upper overturning cell,  which leads to an increase in natural and total carbon north of 35$^\circ$S in the upper 2000 m.</p>

scholarly journals Ocean Carbon Uptake Under Aggressive Emission Mitigation

2020 ◽  
Author(s):  
Sean Ridge ◽  
Galen McKinley
Keyword(s):  
Growth Rate ◽  
Carbon Cycle ◽  
Cycle Model ◽  
Emission Mitigation ◽  
Ocean Carbon Cycle ◽  
Carbon Cycle Model ◽  
One Dimensional ◽  
Surface Ocean ◽  
Gradient Effect ◽  
Ocean Carbon

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.

scholarly journals TREE RING RECONSTRUCTION OF MODERN RADIOCARBON DIOXIDE VARIABILITY OVER THE SOUTHERN OCEAN

2021 ◽  
Author(s):  
Rachel Corran
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
Tree Ring ◽  
Deep Water ◽  
Latitudinal Gradient ◽  
Carbon Sink ◽  
Carbon Uptake ◽  
Co2 Uptake ◽  
Anthropogenic Carbon ◽  
Ocean Carbon

<p><b>The Southern Ocean is the largest ocean carbon sink region. However, its trend of increasing carbon uptake has shown variability over recent decades. It is important to understand the underlying mechanisms of anthropogenic carbon uptake such that the future response of the Southern Ocean carbon sink under climate forcing can be predicted. </b></p><p>The carbon uptake of the Southern Ocean is characterised by the balance of outgassing of CO2 from carbon-rich deep water and sequestration of anthropogenic carbon into surface waters. Atmospheric radiocarbon dioxide (Del14CO2) in the Southern Hemisphere is sensitive to the release of CO2 from the upwelling of ‘old’ 14C-depleted carbon-rich deep water at high southern latitudes, but is insensitive to CO2 uptake into the ocean. Thus Del14CO2 has the potential to be used as a tracer of the upwelling observed, thereby isolating the outgassing carbon component. </p><p>The Southern Ocean Region has limited atmospheric Del14CO2 measurements, with sparse long-term sampling sites and few shipboard flask measurements. Therefore in this PhD project I exploit annual growth tree rings, which record the Del14C content of atmospheric CO2, to reconstruct Del14CO2 back in time. Within tree ring sample pretreatment for 14C measurement I automate the organic solvent wash method at the Rafter Radiocarbon Laboratory. I present new annual-resolution reconstructions of atmospheric Del14CO2 from tree rings, from coastal sites in New Zealand and Chile, spanning a latitudinal range of 44 S to 55 S, for the period of interest, 1985 – 2015. Data quality analysis using a range of replicate 14C measurements conducted within this project leads to assignment of apx 1.9 ‰ uncertainties for all results, in line with atmospheric measurements. </p><p>In this project I also develop a harmonised dataset of atmospheric Del14CO2 measurements in the Southern Hemisphere for this period from different research groups, including the new tree ring Del14CO2 records alongside existing data. The harmonised atmospheric Del14CO2 dataset has a wide range of applications, but specifically here allows investigation of temporal and spatial variability of atmospheric Del14CO2 over the Southern Ocean over recent decades, thereby also considering the role of upwelling in recent Southern Ocean carbon sink variability. Backward trajectories are produced for the tree ring sites from an atmospheric transport model, to help inform interpretation of results. </p><p>Over recent decades a latitudinal gradient of 3.7 ‰ is observed between 41 S and 53 S in the New Zealand sector, with a smaller gradient of 1.6 ‰ between 48 S and 55S in the Chile sector. This is consistent with other studies, with the spatial variability of atmospheric Del14CO2 attributed to air-sea 14C disequilibrium associated with carbon outgassing from 14C-depleted carbon-rich deep water upwelling at around 60 S, driving a latitudinal gradient of atmospheric Del14CO2 in the Southern Hemisphere, with longitudinal variability also observed. A stronger atmospheric Del14CO2 latitudinal gradient is observed in the 1980s/1990s relative to later 1990s/2000s. Stronger atmospheric Del14CO2 latitudinal gradients observed in 1980s/1990s suggest stronger deep water upwelling thereby greater associated outgassing of 14C-depleted CO2. These Del14CO2-based observations are consistent with modelling studies that predict changes in deep-water upwelling have controlled decadal variability in CO2 uptake, and are consistent with observation-based studies of decadal changes in rate of CO2 uptake of the Southern Ocean. The results presented in this thesis present the first observation-based confirmation that decadal changes in the strength of deep-water upwelling can explain decadal changes in the rate of CO2 uptake. </p>

Quantifying the ocean carbon sink for 1994-2007: Combined evidence from multiple methods

2021 ◽  
Author(s):  
Galen A. McKinley ◽  
Jessica Cross ◽  
Timothy DeVries ◽  
Judith Hauck ◽  
Amanda Fay ◽  
...  
Keyword(s):  
Steady State ◽  
Working Group ◽  
Carbon Sink ◽  
Carbon Fluxes ◽  
Carbon Uptake ◽  
Best Estimate ◽  
Anthropogenic Carbon ◽  
Natural Carbon ◽  
Ocean Models ◽  
Ocean Carbon

&lt;p&gt;By means of a variety of international observing and modeling efforts, the ocean carbon community has developed numerous estimates for ocean carbon uptake. In this presentation, we report on the synthesis effort we are undertaking under the auspices of an Ocean Carbon and Biogeochemistry Working Group. &amp;#160;Our initial goal for this working group is to determine the best estimate for the net and anthropogenic carbon sink from 1994-2007 based on three approaches that independently use interior data, surface data or hindcast ocean models. Combining two approaches that use interior ocean data to estimate anthropogenic carbon, F&lt;sub&gt;ant&lt;/sub&gt; = -2.40+-0.21 PgC/yr (2 sigma uncertainty). Estimates for the net, or contemporary, ocean carbon uptake come from 6 products that interpolate surface ocean pCO&lt;sub&gt;2&lt;/sub&gt; data to global coverage: F&lt;sub&gt;net&lt;/sub&gt; = -1.58+-0.19 &amp;#160;PgC/yr for 1994-2007. Uncertain closure terms for naturally-outgassed river-derived carbon and non-steady state natural carbon fluxes in the open ocean are then added to derive F&lt;sub&gt;ant&lt;/sub&gt; from surface observation-based Fnet. Ocean models do not include river-derived carbon, but do include non-steady state natural carbon fluxes, and thus a third estimate for Fant is derived. The combined best-estimate is F&lt;sub&gt;ant&lt;/sub&gt; = -2.35+-0.53 PgC/yr.&amp;#160; We detail the uncertainties and assumptions made in deriving these estimates, and suggest paths forward to further reduce uncertainties.&lt;/p&gt;

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