The ocean is the largest sink for anthropogenic carbon dioxide (CO
), having absorbed roughly 40 per cent of CO
emissions since the beginning of the industrial era. Recent data show that oceanic CO
...uptake rates have been growing over the past decade, reversing a trend of stagnant or declining carbon uptake during the 1990s. Here we show that ocean circulation variability is the primary driver of these changes in oceanic CO
uptake over the past several decades. We use a global inverse model to quantify the mean ocean circulation during the 1980s, 1990s and 2000s, and then estimate the impact of decadal circulation changes on the oceanic CO
sink using a carbon cycling model. We find that during the 1990s an enhanced upper-ocean overturning circulation drove increased outgassing of natural CO
, thus weakening the global CO
sink. This trend reversed during the 2000s as the overturning circulation weakened. Continued weakening of the upper-ocean overturning is likely to strengthen the CO
sink in the near future by trapping natural CO
in the deep ocean, but ultimately may limit oceanic uptake of anthropogenic CO
.
Abstract
Mid-depth North Pacific waters are rich in nutrients and respired carbon accumulated over centuries. The rates and pathways with which these waters exchange with the surface ocean are ...uncertain, with divergent paradigms of the Pacific overturning: one envisions bottom waters upwelling to 1.5 km depth; the other confines overturning beneath a mid-depth Pacific shadow zone (PSZ) shielded from mean advection. Here global inverse modelling reveals a PSZ where mean ages exceed 1400 years with overturning beneath. The PSZ is supplied primarily by Antarctic and North-Atlantic ventilated waters diffusing from below and from the south. Half of PSZ waters re-surface in the Southern Ocean, a quarter in the subarctic Pacific. The abyssal North Pacific, despite strong overturning, has mean re-surfacing times also exceeding 1400 years because of diffusion into the overlying PSZ. These results imply that diffusive transports – distinct from overturning transports – are a leading control on Pacific nutrient and carbon storage.
The ocean's organic carbon export is a key control on atmospheric pCO2 and stimulating this export could potentially mitigate climate change. We use a data‐constrained model to calculate the ...sensitivity of atmospheric pCO2 to local changes in export using an adjoint approach. A perpetual enhancement of the biological pump's export by 0.1 PgC/yr could achieve a roughly 1% reduction in pCO2 at average sensitivity. The sensitivity varies roughly 5‐fold across different ocean regions and is proportional to the difference between the mean sequestration time τseq of regenerated carbon and the response time τpre of performed carbon, which is the reduction in the preformed carbon inventory per unit increase in local export production. Air‐sea CO2 disequilibrium modulates the geographic pattern of τpre, causing particularly high sensitivities (2–3 times the global mean) in the Antarctic Divergence region of the Southern Ocean.
Plain Language Summary
Atmospheric CO2 levels could be reduced by stimulating plankton in the ocean to produce more organic carbon that then sinks to depth (is exported) at a higher rate. The resulting carbon deficit in surface waters drives atmospheric carbon into the ocean. The efficacy of this process depends on how long the exported carbon stays isolated from the surface and how quickly the surface deficit can be filled. Here we investigate how sensitive atmospheric CO2 levels are to a given enhancement in carbon export and where such enhancements would be most effective. We show that the sensitivity is determined by the difference between the time for which exported carbon stays sequestered at depth and the time with which the rest of the carbon in the ocean responds to the additional export rate. High sequestration times are found where the organic carbon is exported into old deep waters, while fast response times are found where it is easy for gas exchange to inject carbon into the ocean and where the additionally exported carbon is less likely to resurface and escape into the atmosphere. These factors result in the Southern Ocean and the tropics having greatest sensitivity.
Key Points
pCO2 sensitivity to carbon export perturbations is governed by sequestration time, preformed response time, and atmospheric turnover time
High sensitivity occurs in the tropics and Southern Ocean where sequestration time is long and preformed response time is short
The preformed response time is modulated by air‐sea CO2 disequilibrium and shortest in the Atlantic and Southern Ocean
Surface westerly winds in the Southern Hemisphere have intensified over the past few decades, primarily in response to the formation of the Antarctic ozone hole, and there is intense debate on the ...impact of this on the ocean's circulation and uptake and redistribution of atmospheric gases. We used measurements of chlorofluorocarbon-12 (CFC-12) made in the southern oceans in the early 1990s and mid- to late 2000s to examine changes in ocean ventilation. Our analysis of the CFC-12 data reveals a decrease in the age of subtropical subantarctic mode waters and an increase in the age of circumpolar deep waters, suggesting that the formation of the Antarctic ozone hole has caused large-scale coherent changes in the ventilation of the southern oceans.
Changes in Southern Ocean ventilation imprint on dissolved gases, nutrients, radiocarbon, temperature, and salinity. We deconvolve tracer measurements for the distribution,
G, of times and locations ...of last ventilation using a maximum‐entropy approach. Decadal changes of
G are quantified by deconvolving hydrographies measured during the early 1990s and again some 15 years later. Our analysis reveals coherent changes across the five meridional sections analyzed: The fraction of water younger than 30 years decreased by ∼20% per decade south of 40°S in circumpolar deep water (CDW) and increased by ∼10% per decade north of ∼40°S in subantarctic mode water (SAMW). Ventilation locations shifted, with more water south of 40°S being ventilated north of the subantarctic front. These ventilation changes impacted CFC uptake, with concentrations south of 40°S less than (and north of 40°S higher than) expected for steady flow. The inferred changes imply increased SAMW formation and CDW upwelling consistent with strengthened westerly winds.
Key Points
Maximum‐entropy deconvolutions of hydrographies from the 1990s and 2000s reveal coherent decadal changes in Southern Ocean ventilation
The fraction of water younger than 30 years decreased south of 40°S and increased north of 40°S with a northward shift in ventilation
Changes in ventilation cause a reduction of CFCs in CDW and an increase of CFCs in SAMW compared to steady flow
We constrain tropospheric transport from Northern Hemisphere midlatitudes to the Southern Hemisphere (SH) surface using measurements of SF6, CFCs, and CFC replacement gases and a novel ...maximum‐entropy‐based inversion approach. We provide the first estimate of the width Δ of the tropospheric interhemispheric transit time distribution (TTD). We find that Δ has a value of ∼1.3 years that varies little with SH latitude, compared to the mean transit time Γ that increases from ∼1.1 years in the SH tropics to ∼1.4 years at the South Pole. The TTD shape parameter Δ/Γ is thus larger in the SH tropics than at middle and high SH latitudes. Our analysis introduces a simple path‐dependent lifetime that parameterizes chemical losses. The path‐dependent lifetimes are estimated for CFC replacements, and systematic differences between path‐dependent and global lifetimes are interpreted. The path‐dependent lifetimes have the potential to provide new observational constraints on tropospheric and stratospheric loss processes.
Key Points
NH‐to‐SH mean transit time increases from 1.1 years at Samoa to 1.4 years at the pole
The width of the TTD varies little with SH latitude and is about 1.3 years
Path‐dependent and global lifetimes differ based on reactivity and transit time
Two centuries of anthropogenic CO2 emissions have increased the CO2 concentration of the atmosphere and the dissolved inorganic carbon (DIC) concentration of the ocean compared to preindustrial ...times. These anthropogenic carbon perturbations are often equated to the amount of anthropogenically emitted carbon in the atmosphere or ocean, which ignores the possibility of a shift of natural carbon between the oceanic and atmospheric carbon reservoirs. Here we use a data‐assimilated ocean circulation model and numerical tracers akin to ideal isotopes to label carbon when it is emitted by anthropogenic sources. We find that emitted carbon accounts for only about 45% of the atmospheric CO2 increase since preindustrial times, the remaining 55% being natural CO2 that outgassed from the ocean in response to anthropogenically emitted carbon invading the ocean. This outgassing is driven by the order‐10 seawater carbonate buffer factor which causes increased leakage of natural CO2 as DIC concentrations increase. By 2020, the ocean had outgassed ∼159 Pg of natural carbon, which is counteracted by the ocean absorbing ∼347 Pg of emitted carbon, about 1.8 times more than the net increase in oceanic carbon storage of ∼188 PgC. These results do not challenge existing estimates of anthropogenically driven changes in atmospheric or oceanic carbon inventories, but they shed new light on the composition of these changes and the fate of anthropogenically emitted carbon in the Earth system.
Plain Language Summary
The observed increase of atmospheric CO2 concentrations since preindustrial times is often considered to consist entirely of CO2 emitted through human activities such as fossil‐fuel burning, changes in agriculture, and deforestation. Here we show that this picture misses a large shift of natural carbon from the ocean to the atmosphere, with the ocean compensating by taking up nearly twice as much emitted carbon than commonly estimated. We use an ocean circulation model to track emitted carbon through the atmosphere‐ocean system and find that only about 45% of the atmospheric CO2 increase is emitted carbon, with the remaining 55% being natural carbon that has outgassed from the ocean. As the ocean absorbs emitted carbon, the surface ocean becomes more “leaky” for carbon as a result of seawater carbon chemistry. Thus, as the ocean absorbs ever more emitted carbon, it increasingly outgasses natural carbon that was dissolved in the ocean before industrial carbon sources played a significant role.
Key Points
Anthropogenically emitted carbon accounts for about half of the atmospheric CO2 increase since preindustrial times
The remaining half of the atmospheric CO2 increase is due to outgassing of preindustrial carbon from the ocean
By 2020, the ocean had lost 1 preindustrial CO2 molecule for every 2.2 anthropogenically emitted CO2 molecules gained
Radiocarbon (Δ14C) and helium isotopes (δ3He) have long been used to constrain the ocean's ventilation rates and to trace regional deep ocean circulation pathways, but they have not been fully ...exploited together to constrain the deep circulation in global models. Here we assimilate Δ14C and δ3He measurements into a global ocean circulation inverse model (OCIM) to jointly constrain the deep ocean circulation and the rate of mantle‐helium injection at seafloor spreading ridges. We find that the new version of the inverse model (OCIM2) matches the observed Δ14C and δ3He distributions much better than a previous version (OCIM1) that assimilated objectively mapped Δ14C but not δ3He. OCIM2 features faster‐ventilated bottom waters and slower‐ventilated intermediate‐depth waters in the Pacific and Indian Oceans. The mean time since last ventilation (ideal mean age) in Pacific bottom waters is up to 150 years younger, while middepth Pacific waters are up to several hundred years older. The δ3He constraints are shown to be important for estimates of the mean time to next ventilation in the Pacific Ocean. The δ3He constraints also favor jet‐like currents in the deep equatorial Pacific to capture realistic westward propagating helium plumes emanating from the East Pacific Rise. The globally integrated mantle‐helium source is 585–672 mol/year, compared to 400–1,000 mol/year from previous estimates. The major regional difference occurs in the Southern Ocean, where the OCIM2 mantle‐helium source is up to threefold smaller than estimates based on ridge spreading rates.
Key Points
Radiocarbon and helium isotope measurements provide complementary constraints on the deep ocean ventilation in a circulation inverse model
The assimilated circulation produces realistic westward propagating 3He plumes in the deep Eastern Tropical Pacific
The jointly assimilated globally integrated mantle‐3He source is 585–672 mol/year
We investigate the sensitivity of the oxygen content and true oxygen utilization of key low‐oxygen regions Ω to pointwise changes in biological production. To understand how the combined water and ...biogenic particle transport controls the sensitivity patterns and the fate of oxygen in the ocean, we develop new relationships that link the steady‐state oxygen content and deficit of Ω to the downstream and upstream oxygen utilization rate (OUR), respectively. We find that the amount of oxygen from Ω that will be lost per unit volume at point r is linked to OUR(r) through the mean oxygen age accumulated in Ω. The geographic sensitivity pattern of the Ω‐integrated oxygen deficit is shaped by where the utilization occurs that causes this deficit. The contribution to the oxygen deficit of Ω from utilization at r is controlled by the mean time that water at r spends in Ω before next ventilation at the surface. We illustrate these relationships and the new transport timescales using a simple steady‐state data‐constrained carbon and oxygen model. We focus on Ω being the global ocean, the Pacific Hypoxic Zone (PHZ, O2 < 62.5 µM), and the North Pacific oxygen minimum zone. The oxygen deficit of the PHZ is most sensitive where mode and intermediate waters form and where increased organic‐matter production directly increases the PHZ's oxygen demand. The fraction of the local oxygen concentration that will be utilized in respiration is as high as 90% in the PHZ and up to 70% in the water column beneath it.
Plain Language Summary
Dissolved oxygen is vital for marine animals. Oxygen in the ocean comes primarily from the atmosphere and is consumed throughout the water column by microbes that break down organic matter produced by photosynthesis near the surface. In certain parts of the ocean interior, where oxygen consumption is high and the supply of freshly oxygenated waters is low, the oxygen concentration is lethally low for many animals. As the surface production of organic matter changes due to environmental factors, so does the oxygen demand in the ocean interior. Here we develop new relationships for understanding the fate of oxygen in the ocean and illustrate them with an ocean model. By tracing the oxygen deficit of a given subsurface region back in time to where the oxygen was consumed, we show that the sensitivity of the region's oxygen content to changes in biological productivity is controlled mainly by the time water spends in the region before getting re‐oxygenated at the surface. By tracing oxygen forward in time to where it gets utilized, we show that 70%–90% of the oxygen in the North Pacific below a few hundred meters depth gets utilized in the breakdown of organic matter.
Key Points
We map out where the oxygen deficit of low‐oxygen regions originates and link deficit and utilization rates with a new mean transport time
The sensitivity pattern of low‐oxygen regions to production changes is shaped by the time to next ventilation spent in the regions
In and below the hypoxic North Pacific 70%–90% of the oxygen concentration will get utilized before next ventilation
Hydrothermal vents along the ocean's tectonic ridge systems inject superheated water and large amounts of dissolved metals that impact the deep ocean circulation and the oceanic cycling of trace ...metals. The hydrothermal fluid contains dissolved mantle helium that is enriched in 3He relative to the atmosphere, providing an isotopic tracer of the ocean's deep circulation and a marker of hydrothermal sources. This work investigates the potential for the 3He/4He isotope ratio to constrain the ocean's mantle 3He source and to provide constraints on the ocean's deep circulation. We use an ensemble of 11 data-assimilated steady-state ocean circulation models and a mantle helium source based on geographically varying sea-floor spreading rates. The global source distribution is partitioned into 6 regions, and the vertical profile and source amplitude of each region are varied independently to determine the optimal 3He source distribution that minimizes the mismatch between modeled and observed δ3He. In this way, we are able to fit the observed δ3He distribution to within a relative error of ∼15%, with a global 3He source that ranges from 640 to 850 mol yr−1, depending on circulation. The fit captures the vertical and interbasin gradients of the δ3He distribution very well and reproduces its jet-sheared saddle point in the deep equatorial Pacific. This demonstrates that the data-assimilated models have much greater fidelity to the deep ocean circulation than other coarse-resolution ocean models. Nonetheless, the modelled δ3He distributions still display some systematic biases, especially in the deep North Pacific where δ3He is overpredicted by our models, and in the southeastern tropical Pacific, where observed westward-spreading δ3He plumes are not well captured. Sources inferred by the data-assimilated transport with and without isopycnally aligned eddy diffusivity differ widely in the Southern Ocean, in spite of the ability to match the observed distributions of CFCs and radiocarbon for either eddy parameterization.
•The ocean's mantle 3He source is estimated using data-assimilated circulations.•Radiocarbon and CFC constrained circulations allow δ3He fits with mere ∼15% relative error.•The jet-sheared structure of δ3He in the deep tropical Pacific is captured by the circulations used.•Southern Ocean ventilation and eddy diffusion are critically important in the inference of the mantle helium sources.•Biases in modeled δ3He suggest the need to data-assimilate δ3He into the circulation.
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