Global Ocean Biogeochemical Model Research Articles

The Shelf‐To‐Basin Transport of Iron From the Northern U.S. West Coast to the Pacific Ocean

AbstractRelease of iron (Fe) from continental shelves is a major source of this limiting nutrient for phytoplankton in the open ocean, including productive Eastern Boundary Upwelling Systems. The mechanisms governing the transport and fate of Fe along continental margins remain poorly understood, reflecting interaction of physical and biogeochemical processes that are crudely represented by global ocean biogeochemical models. Here, we use a submesoscale‐permitting physical‐biogeochemical model to investigate processes governing the delivery of shelf‐derived Fe to the open ocean along the northern U.S. West Coast. We find that a significant fraction (∼20%) of the Fe released by sediments on the shelf is transported offshore, fertilizing the broader Northeast Pacific Ocean. This transport is governed by two main pathways that reflect interaction between the wind‐driven ocean circulation and Fe release by low‐oxygen sediments: the first in the surface boundary layer during upwelling events; the second in the bottom boundary layer, associated with pervasive interactions of the poleward California Undercurrent with bottom topography. In the water column interior, transient and standing eddies strengthen offshore transport, counteracting the onshore pull of the mean upwelling circulation. Several hot‐spots of intense Fe delivery to the open ocean are maintained by standing meanders in the mean current and enhanced by transient eddies and seasonal oxygen depletion. Our results highlight the importance of fine‐scale dynamics for the transport of Fe and shelf‐derived elements from continental margins to the open ocean, and the need to improve representation of these processes in biogeochemical models used for climate studies.

An Assessment of CO2 Storage and Sea‐Air Fluxes for the Atlantic Ocean and Mediterranean Sea Between 1985 and 2018

AbstractAs part of the second phase of the Regional Carbon Cycle Assessment and Processes project (RECCAP2), we present an assessment of the carbon cycle of the Atlantic Ocean, including the Mediterranean Sea, between 1985 and 2018 using global ocean biogeochemical models (GOBMs) and estimates based on surface ocean carbon dioxide (CO2) partial pressure (pCO2 products) and ocean interior dissolved inorganic carbon observations. Estimates of the basin‐wide long‐term mean net annual CO2 uptake based on GOBMs and pCO2 products are in reasonable agreement (−0.47 ± 0.15 PgC yr−1 and −0.36 ± 0.06 PgC yr−1, respectively), with the higher uptake in the GOBM‐based estimates likely being a consequence of a deficit in the representation of natural outgassing of land derived carbon. In the GOBMs, the CO2 uptake increases with time at rates close to what one would expect from the atmospheric CO2 increase, but pCO2 products estimate a rate twice as fast. The largest disagreement in the CO2 flux between GOBMs and pCO2 products is found north of 50°N, coinciding with the largest disagreement in the seasonal cycle and interannual variability. The mean accumulation rate of anthropogenic CO2 (Cant) over 1994–2007 in the Atlantic Ocean is 0.52 ± 0.11 PgC yr−1 according to the GOBMs, 28% ± 20% lower than that derived from observations. Around 70% of this Cant is taken up from the atmosphere, while the remainder is imported from the Southern Ocean through lateral transport.

Assessment of Global Ocean Biogeochemistry Models for Ocean Carbon Sink Estimates in RECCAP2 and Recommendations for Future Studies

AbstractThe ocean is a major carbon sink and takes up 25%–30% of the anthropogenically emitted CO2. A state‐of‐the‐art method to quantify this sink are global ocean biogeochemistry models (GOBMs), but their simulated CO2 uptake differs between models and is systematically lower than estimates based on statistical methods using surface ocean pCO2 and interior ocean measurements. Here, we provide an in‐depth evaluation of ocean carbon sink estimates from 1980 to 2018 from a GOBM ensemble. As sources of inter‐model differences and ensemble‐mean biases our study identifies (a) the model setup, such as the length of the spin‐up, the starting date of the simulation, and carbon fluxes from rivers and into sediments, (b) the simulated ocean circulation, such as Atlantic Meridional Overturning Circulation and Southern Ocean mode and intermediate water formation, and (c) the simulated oceanic buffer capacity. Our analysis suggests that a late starting date and biases in the ocean circulation cause a too low anthropogenic CO2 uptake across the GOBM ensemble. Surface ocean biogeochemistry biases might also cause simulated anthropogenic fluxes to be too low, but the current setup prevents a robust assessment. For simulations of the ocean carbon sink, we recommend in the short‐term to (a) start simulations at a common date before the industrialization and the associated atmospheric CO2 increase, (b) conduct a sufficiently long spin‐up such that the GOBMs reach steady‐state, and (c) provide key metrics for circulation, biogeochemistry, and the land‐ocean interface. In the long‐term, we recommend improving the representation of these metrics in the GOBMs.

Unraveling the Physical and Biological Controls of the Global Coastal CO2 Sink

AbstractThe drivers governing the air‐sea CO2 exchange and its variability in the coastal ocean are poorly understood. Using a global ocean biogeochemical model, this study quantifies the influences of thermal changes, oceanic transport, freshwater fluxes, and biological activity on the spatial and seasonal variability of CO2 sources/sinks in the global coastal ocean. We identify five typical coastal behaviors (dominated by biological drawdown, vertical transport, land imprint, intracoastal alongshore currents, and weak CO2 sources and sinks coastal regions) and propose a new processed‐based delineation of the coastal ocean based on the quantification of these controlling processes. We find that the spatiotemporal variability of CO2 sources/sinks is dominated by strong exchanges with the open ocean and intracoastal processes, while continental influences are restricted to hotspot regions. In addition, where thermal changes appear to drive the seasonal CO2 variability, it often results from compensating effects between individual non‐thermal terms, especially biological drawdown and vertical transport.

The Impact of Recent Climate Change on the Global Ocean Carbon Sink

AbstractIn recent decades, the ocean CO2 uptake has increased in response to rising atmospheric CO2. Yet, physical climate change also affects the ocean CO2 uptake, but magnitude and driving processes are poorly understood. Using a global ocean biogeochemistry model, we find that without climate change, the mean carbon uptake 2000–2019 would have been 13% higher and the trend 1958–2019 would have been 27% higher. Changes in wind are the dominant driver of the climate effect on CO2 uptake as they affect advective carbon transport and mixing, but the effect of warming increases over time. Roughly half of the globally integrated wind‐driven trend stems from the subpolar Southern Ocean and polar oceans in both hemispheres. Warming reduces the solubility of CO2 and acts rather homogeneously over the world oceans. However, the warming effect on pCO2 is dampened by limited exchange of surface and deep waters.

Impact of Increased Horizontal Resolution of an Ocean Model on Carbon Circulation in the North Pacific Ocean

AbstractThe impact of resolving western boundary currents and mesoscale eddies on a carbon circulation simulation for the North Pacific Ocean is investigated to evaluate the merits of using high‐resolution ocean biogeochemical models for climate projections. Simulations by a 100‐km resolution global ocean biogeochemical model with and without embedding a 10‐km resolution model in the North Pacific Ocean are compared. The major improvement in the high‐resolution simulation is the representation of the Kuroshio, its extension current, and the recirculation gyres formed to its south and north, resulting in a proper representation of the North Pacific subtropical mode water (STMW) and an increase in storage of the anthropogenic CO2 (Canth) in STMW by about two‐thirds. The larger storage rate in STMW is caused by supply of a larger amount of warm surface water containing rich Canth to the formation region by the intensified Kuroshio. A huge buoyancy loss from this warm water results in the increased formation of STMW that occupies a vast area in the western subtropical gyre. The surface uptake of Canth in the formation region of STMW is slightly increased but is largely comparable to that of the low‐resolution model. Moreover, there is no structural difference in Canth uptake in other parts of the subtropical region. Thus, the improvement of Canth distribution can be understood as a redistribution of water mass in the subtropical gyre by the improved circulation. The present assessment motivates the use of a high horizontal resolution ocean model in next‐generation projection experiments with carbon cycles.

Global Carbon Budget 2023

Abstract. Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere in a changing climate is critical to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe and synthesize data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (EFOS) are based on energy statistics and cement production data, while emissions from land-use change (ELUC), mainly deforestation, are based on land-use and land-use change data and bookkeeping models. Atmospheric CO2 concentration is measured directly, and its growth rate (GATM) is computed from the annual changes in concentration. The ocean CO2 sink (SOCEAN) is estimated with global ocean biogeochemistry models and observation-based fCO2 products. The terrestrial CO2 sink (SLAND) is estimated with dynamic global vegetation models. Additional lines of evidence on land and ocean sinks are provided by atmospheric inversions, atmospheric oxygen measurements, and Earth system models. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and incomplete understanding of the contemporary carbon cycle. All uncertainties are reported as ±1σ. For the year 2022, EFOS increased by 0.9 % relative to 2021, with fossil emissions at 9.9±0.5 Gt C yr−1 (10.2±0.5 Gt C yr−1 when the cement carbonation sink is not included), and ELUC was 1.2±0.7 Gt C yr−1, for a total anthropogenic CO2 emission (including the cement carbonation sink) of 11.1±0.8 Gt C yr−1 (40.7±3.2 Gt CO2 yr−1). Also, for 2022, GATM was 4.6±0.2 Gt C yr−1 (2.18±0.1 ppm yr−1; ppm denotes parts per million), SOCEAN was 2.8±0.4 Gt C yr−1, and SLAND was 3.8±0.8 Gt C yr−1, with a BIM of −0.1 Gt C yr−1 (i.e. total estimated sources marginally too low or sinks marginally too high). The global atmospheric CO2 concentration averaged over 2022 reached 417.1±0.1 ppm. Preliminary data for 2023 suggest an increase in EFOS relative to 2022 of +1.1 % (0.0 % to 2.1 %) globally and atmospheric CO2 concentration reaching 419.3 ppm, 51 % above the pre-industrial level (around 278 ppm in 1750). Overall, the mean of and trend in the components of the global carbon budget are consistently estimated over the period 1959–2022, with a near-zero overall budget imbalance, although discrepancies of up to around 1 Gt C yr−1 persist for the representation of annual to semi-decadal variability in CO2 fluxes. Comparison of estimates from multiple approaches and observations shows the following: (1) a persistent large uncertainty in the estimate of land-use changes emissions, (2) a low agreement between the different methods on the magnitude of the land CO2 flux in the northern extra-tropics, and (3) a discrepancy between the different methods on the strength of the ocean sink over the last decade. This living-data update documents changes in methods and data sets applied to this most recent global carbon budget as well as evolving community understanding of the global carbon cycle. The data presented in this work are available at https://doi.org/10.18160/GCP-2023 (Friedlingstein et al., 2023).

Contrasting carbon dioxide removal potential and nutrient feedbacks of simulated ocean alkalinity enhancement and macroalgae afforestation

Alongside cuts to emissions, hundreds of gigatons of carbon dioxide removal (CDR) are likely to be required to limit global warming to below 1.5 °C or 2 °C this century. Ocean alkalinity enhancement (OAE) and macroalgae afforestation have received considerable attention within the portfolio of potential CDR options, but their efficacy and constraints remain uncertain. Here we compare the CDR potential and biogeochemical impacts of OAE and macroalgae afforestation in exclusive economic zones (EEZs) using a global high-resolution ocean biogeochemical model. Globally, our simulations indicate the CDR potential of OAE is more than seven times that of macroalgae afforestation for an equivalent mass of either dissolved olivine or harvested wet macroalgae biomass. This difference is predominately attributable to the respective alkalinity content of olivine and carbon content of wet macroalgae biomass. Accounting for potential nutrient impacts on phytoplankton production increases this disparity between the CDR efficiency of OAE and afforestation, and in both cases can result in regions of negative CDR. EEZs with higher CDR in response to OAE consistently exhibit higher CDR in response to macroalgae afforestation. However, nutrient feedbacks are shown to have different EEZ-specific impacts on phytoplankton net primary production. Our simulations indicate that ∼62% of the CDR flux associated with OAE occurs in the EEZ application regions, decreasing to ∼54% if olivine contains iron and silicate and ∼45% for macroalgae afforestation. This suggests that monitoring, reporting, and verification may be problematic for both techniques, as might the allocation of credits toward nationally determined contributions.

The Southern Ocean Carbon Cycle 1985–2018: Mean, Seasonal Cycle, Trends, and Storage

AbstractWe assess the Southern Ocean CO2 uptake (1985–2018) using data sets gathered in the REgional Carbon Cycle Assessment and Processes Project Phase 2. The Southern Ocean acted as a sink for CO2 with close agreement between simulation results from global ocean biogeochemistry models (GOBMs, 0.75 ± 0.28 PgC yr−1) and pCO2‐observation‐based products (0.73 ± 0.07 PgC yr−1). This sink is only half that reported by RECCAP1 for the same region and timeframe. The present‐day net uptake is to first order a response to rising atmospheric CO2, driving large amounts of anthropogenic CO2 (Cant) into the ocean, thereby overcompensating the loss of natural CO2 to the atmosphere. An apparent knowledge gap is the increase of the sink since 2000, with pCO2‐products suggesting a growth that is more than twice as strong and uncertain as that of GOBMs (0.26 ± 0.06 and 0.11 ± 0.03 Pg C yr−1 decade−1, respectively). This is despite nearly identical pCO2 trends in GOBMs and pCO2‐products when both products are compared only at the locations where pCO2 was measured. Seasonal analyses revealed agreement in driving processes in winter with uncertainty in the magnitude of outgassing, whereas discrepancies are more fundamental in summer, when GOBMs exhibit difficulties in simulating the effects of the non‐thermal processes of biology and mixing/circulation. Ocean interior accumulation of Cant points to an underestimate of Cant uptake and storage in GOBMs. Future work needs to link surface fluxes and interior ocean transport, build long overdue systematic observation networks and push toward better process understanding of drivers of the carbon cycle.

A Regime Shift on Weddell Sea Continental Shelves with Local and Remote Physical and Biogeochemical Implications is Avoidable in a 2°C Scenario

Abstract Tipping points in the Earth system describe critical thresholds beyond which a single component, part of the system, or the system as a whole changes from one stable state to another. In the present-day Southern Ocean, the Weddell Sea constitutes an important dense-water formation site, associated with efficient deep-ocean carbon and oxygen transfer and low ice-shelf basal melt rates. Here, a regime shift will occur when continental shelves are continuously flushed with warm, oxygen-poor offshore waters from intermediate depth, leading to less efficient deep-ocean carbon and oxygen transfer and higher ice-shelf basal melt rates. We use a global ocean–biogeochemistry model including ice-shelf cavities and an eddy-permitting grid in the southern Weddell Sea to address the susceptibility of this region to such a system change for four twenty-first-century emission scenarios. Assessing the projected changes in shelf–open-ocean density gradients, bottom-water properties, and on-shelf heat transport, our results indicate that the Weddell Sea undergoes a regime shift by 2100 in the highest-emission scenario, SSP5–8.5, but not yet in the lower-emission scenarios. The regime shift is imminent by 2100 in the scenarios SSP3–7.0 and SSP2–4.5, but avoidable under the lowest-emission scenario SSP1–2.6. While shelf-bottom waters freshen and acidify everywhere, bottom waters in the Filchner Trough undergo accelerated warming and deoxygenation following the system change, with implications for local ecosystems and ice-shelf basal melt. Additionally, deep-ocean carbon and oxygen transfer decline, implying that the local changes ultimately affect ocean circulation, climate, and ecosystems globally.

Seasonal Variability of the Surface Ocean Carbon Cycle: A Synthesis

AbstractThe seasonal cycle is the dominant mode of variability in the air‐sea CO2 flux in most regions of the global ocean, yet discrepancies between different seasonality estimates are rather large. As part of the Regional Carbon Cycle Assessment and Processes Phase 2 project (RECCAP2), we synthesize surface ocean pCO2 and air‐sea CO2 flux seasonality from models and observation‐based estimates, focusing on both a present‐day climatology and decadal changes between the 1980s and 2010s. Four main findings emerge: First, global ocean biogeochemistry models (GOBMs) and observation‐based estimates (pCO2 products) of surface pCO2 seasonality disagree in amplitude and phase, primarily due to discrepancies in the seasonal variability in surface DIC. Second, the seasonal cycle in pCO2 has increased in amplitude over the last three decades in both pCO2 products and GOBMs. Third, decadal increases in pCO2 seasonal cycle amplitudes in subtropical biomes for both pCO2 products and GOBMs are driven by increasing DIC concentrations stemming from the uptake of anthropogenic CO2 (Cant). In subpolar and Southern Ocean biomes, however, the seasonality change for GOBMs is dominated by Cant invasion, whereas for pCO2 products an indeterminate combination of Cant invasion and climate change modulates the changes. Fourth, biome‐aggregated decadal changes in the amplitude of pCO2 seasonal variability are largely detectable against both mapping uncertainty (reducible) and natural variability uncertainty (irreducible), but not at the gridpoint scale over much of the northern subpolar oceans and over the Southern Ocean, underscoring the importance of sustained high‐quality seasonally resolved measurements over these regions.

Ocean iron fertilization may amplify climate change pressures on marine animal biomass for limited climate benefit.

Climate change scenarios suggest that large-scale carbon dioxide removal (CDR) will be required to maintain global warming below 2°C, leading to renewed attention on ocean iron fertilization (OIF). Previous OIF modelling has found that while carbon export increases, nutrient transport to lower latitude ecosystems declines, resulting in a modest impact on atmospheric CO2 . However, the interaction of these CDR responses with ongoing climate change is unknown. Here, we combine global ocean biogeochemistry and ecosystem models to show that, while stimulating carbon sequestration, OIF may amplify climate-induced declines in tropical ocean productivity and ecosystem biomass under a high-emission scenario, with very limited potential atmospheric CO2 drawdown. The 'biogeochemical fingerprint' of climate change, that leads to depletion of upper ocean major nutrients due to upper ocean stratification, is reinforced by OIF due to greater major nutrient consumption. Our simulations show that reductions in upper trophic level animal biomass in tropical regions due to climate change would be exacerbated by OIF within ~20 years, especially in coastal exclusive economic zones (EEZs), with potential implications for fisheries that underpin the livelihoods and economies of coastal communities. Any fertilization-based CDR should therefore consider its interaction with ongoing climate-driven changes and the ensuing ecosystem impacts in national EEZs.

Interaction matters: Bottom-up driver interdependencies alter the projected response of phytoplankton communities to climate change.

Phytoplankton growth is controlled by multiple environmental drivers, which are all modified by climate change. While numerous experimental studies identify interactive effects between drivers, large-scale ocean biogeochemistry models mostly account for growth responses to each driver separately and leave the results of these experimental multiple-driver studies largely unused. Here, we amend phytoplankton growth functions in a biogeochemical model by dual-driver interactions (CO2 and temperature, CO2 and light), based on data of a published meta-analysis on multiple-driver laboratory experiments. The effect of this parametrization on phytoplankton biomass and community composition is tested using present-day and future high-emission (SSP5-8.5) climate forcing. While the projected decrease in future total global phytoplankton biomass in simulations with driver interactions is similar to that in control simulations without driver interactions (5%-6%), interactive driver effects are group-specific. Globally, diatom biomass decreases more with interactive effects compared with the control simulation (-8.1% with interactions vs. no change without interactions). Small-phytoplankton biomass, by contrast, decreases less with on-going climate change when the model accounts for driver interactions (-5.0% vs. -9.0%). The response of global coccolithophore biomass to future climate conditions is even reversed when interactions are considered (+33.2% instead of -10.8%). Regionally, the largest difference in the future phytoplankton community composition between the simulations with and without driver interactions is detected in the Southern Ocean, where diatom biomass decreases (-7.5%) instead of increases (+14.5%), raising the share of small phytoplankton and coccolithophores of total phytoplankton biomass. Hence, interactive effects impact the phytoplankton community structure and related biogeochemical fluxes in a future ocean. Our approach is a first step to integrate the mechanistic understanding of interacting driver effects on phytoplankton growth gained by numerous laboratory experiments into a global ocean biogeochemistry model, aiming toward more realistic future projections of phytoplankton biomass and community composition.

Sparse observations induce large biases in estimates of the global ocean CO2 sink: an ocean model subsampling experiment.

Estimates of ocean [Formula: see text] uptake from global ocean biogeochemistry models and [Formula: see text]-based data products differ substantially, especially in high latitudes and in the trend of the [Formula: see text] uptake since 2000. Here, we assess the effect of data sparsity on two [Formula: see text]-based estimates by subsampling output from a global ocean biogeochemistry model. The estimates of the ocean [Formula: see text] uptake are improved from a sampling scheme that mimics present-day sampling to an ideal sampling scheme with 1000 evenly distributed sites. In particular, insufficient sampling has given rise to strong biases in the trend of the ocean carbon sink in the [Formula: see text] products. The overestimation of the [Formula: see text] flux trend by 20-35% globally and 50-130% in the Southern Ocean with the present-day sampling is reduced to less than [Formula: see text] with the ideal sampling scheme. A substantial overestimation of the decadal variability of the Southern Ocean carbon sink occurs in one product and appears related to a skewed data distribution in [Formula: see text] space. With the ideal sampling, the bias in the mean [Formula: see text] flux is reduced from 9-12% to 2-9% globally and from 14-26% to 5-17% in the Southern Ocean. On top of that, discrepancies of about [Formula: see text] (15%) persist due to uncertainties in the gas-exchange calculation. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Process controlling iron-manganese regulation of the Southern Ocean biological carbon pump.

Iron (Fe) is a key limiting nutrient driving the biological carbon pump and is routinely represented in global ocean biogeochemical models. However, in the Southern Ocean, the potential role for other micronutrients has not received the same attention. For example, although manganese (Mn) is essential to photosynthetic oxygen production and combating oxidative stress, it is not included in ocean models and a clear understanding of its interaction with Fe in the region is lacking. This is especially important for the Southern Ocean because both Mn and Fe are strongly depleted. We use a hierarchical modelling approach to explore how the physiological traits associated with Fe and Mn contribute to driving the footprint of micronutrient stress across different phytoplankton functional types (PFTs). We find that PFT responses are driven by physiological traits associated with their physiological requirements and acclimation to environmental conditions. Southern Ocean-specific adaptations to prevailing low Fe, such as large photosynthetic antenna sizes, are of major significance for the regional biological carbon pump. Other traits more strongly linked to Mn, such as dealing with oxidative stress, may become more important under a changing Fe supply regime. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Ecosystem impacts of marine heat waves in the northeast Pacific

Abstract. Marine heat waves (MHWs) are a recurrent phenomenon in the northeast Pacific that impact regional ecosystems and are expected to intensify in the future. Prior work showed that these events, including the 2014–2015 “warm blob”, are associated with widespread surface nutrient declines in the subpolar Alaska Gyre (AG) and the North Pacific Transition Zone (NPTZ) but reduced chlorophyll concentrations in the NPTZ only. Here we explain the contrast between these two regions using a global ocean biogeochemical model (MOM6-COBALT) with Argo float and ship-based observations to investigate how MHWs influence marine productivity. We find that phytoplankton and zooplankton production respond relatively modestly to MHWs in both regions. However, differences in the response to seasonal iron and nitrogen limitation between large (>10 µm) and small (<10 µm) phytoplankton size classes explain the differences in ecosystem response to MHWs across the two biomes. During MHWs, reduced nutrient supply limits large phytoplankton production in the NPTZ (−13 % annually) but has a limited impact on the already iron-limited large phytoplankton population in the AG (−2 %). In contrast, MHWs yield a springtime increase in small phytoplankton in both regions due to shallower mixed layers and weaker light limitation. These modest changes are in apparent contradiction with prior estimates suggesting a collapse in net community production during the warm blob. We show, however, that 70 % of the decline in net community production previously calculated from nitrate Argo data can be attributed to artifacts in the method and that only 30 % can be attributed to interannual variability, in line with our model-based results. Although modest, the primary production anomalies associated with MHWs modify the phytoplankton size distribution, resulting in a significant shift towards small phytoplankton production (i.e., lower large-to-small-phytoplankton ratio) and reduced secondary and export production, especially in the NPTZ.

Modified future diurnal variability of the global surface ocean CO2 system.

Our understanding of how increasing atmospheric CO2 and climate change influences the marine CO2 system and in turn ecosystems has increasingly focused on perturbations to carbonate chemistry variability. This variability can affect ocean-climate feedbacks and has been shown to influence marine ecosystems. The seasonal variability of the ocean CO2 system has already changed, with enhanced seasonal variations in the surface ocean pCO2 over recent decades and further amplification projected by models over the 21st century. Mesocosm studies and CO2 vent sites indicate that diurnal variability of the CO2 system, the amplitude of which in extreme events can exceed that of mean seasonal variability, is also likely to be altered by climate change. Here, we modified a global ocean biogeochemical model to resolve physically and biologically driven diurnal variability of the ocean CO2 system. Forcing the model with 3-h atmospheric outputs derived from an Earth system model, we explore how surface ocean diurnal variability responds to historical changes and project how it changes under two contrasting 21st-century emission scenarios. Compared to preindustrial values, the global mean diurnal amplitude of pCO2 increases by 4.8μatm (+226%) in the high-emission scenario but only 1.2μatm (+55%) in the high-mitigation scenario. The probability of extreme diurnal amplitudes of pCO2 and [H+ ] is also affected, with 30- to 60-fold increases relative to the preindustrial under high 21st-century emissions. The main driver of heightened pCO2 diurnal variability is the enhanced sensitivity of pCO2 to changes in temperature as the ocean absorbs atmospheric CO2 . Our projections suggest that organisms in the future ocean will be exposed to enhanced diurnal variability in pCO2 and [H+ ], with likely increases in the associated metabolic cost that such variability imposes.

Global Carbon Budget 2022

Abstract. Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere in a changing climate is critical to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe and synthesize data sets and methodologies to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (EFOS) are based on energy statistics and cement production data, while emissions from land-use change (ELUC), mainly deforestation, are based on land use and land-use change data and bookkeeping models. Atmospheric CO2 concentration is measured directly, and its growth rate (GATM) is computed from the annual changes in concentration. The ocean CO2 sink (SOCEAN) is estimated with global ocean biogeochemistry models and observation-based data products. The terrestrial CO2 sink (SLAND) is estimated with dynamic global vegetation models. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as ±1σ. For the year 2021, EFOS increased by 5.1 % relative to 2020, with fossil emissions at 10.1 ± 0.5 GtC yr−1 (9.9 ± 0.5 GtC yr−1 when the cement carbonation sink is included), and ELUC was 1.1 ± 0.7 GtC yr−1, for a total anthropogenic CO2 emission (including the cement carbonation sink) of 10.9 ± 0.8 GtC yr−1 (40.0 ± 2.9 GtCO2). Also, for 2021, GATM was 5.2 ± 0.2 GtC yr−1 (2.5 ± 0.1 ppm yr−1), SOCEAN was 2.9  ± 0.4 GtC yr−1, and SLAND was 3.5 ± 0.9 GtC yr−1, with a BIM of −0.6 GtC yr−1 (i.e. the total estimated sources were too low or sinks were too high). The global atmospheric CO2 concentration averaged over 2021 reached 414.71 ± 0.1 ppm. Preliminary data for 2022 suggest an increase in EFOS relative to 2021 of +1.0 % (0.1 % to 1.9 %) globally and atmospheric CO2 concentration reaching 417.2 ppm, more than 50 % above pre-industrial levels (around 278 ppm). Overall, the mean and trend in the components of the global carbon budget are consistently estimated over the period 1959–2021, but discrepancies of up to 1 GtC yr−1 persist for the representation of annual to semi-decadal variability in CO2 fluxes. Comparison of estimates from multiple approaches and observations shows (1) a persistent large uncertainty in the estimate of land-use change emissions, (2) a low agreement between the different methods on the magnitude of the land CO2 flux in the northern extratropics, and (3) a discrepancy between the different methods on the strength of the ocean sink over the last decade. This living data update documents changes in the methods and data sets used in this new global carbon budget and the progress in understanding of the global carbon cycle compared with previous publications of this data set. The data presented in this work are available at https://doi.org/10.18160/GCP-2022 (Friedlingstein et al., 2022b).

Global Ocean Particulate Organic Phosphorus, Carbon, Oxygen for Respiration, and Nitrogen (GO-POPCORN)

Concentrations and elemental stoichiometry of suspended particulate organic carbon, nitrogen, phosphorus, and oxygen demand for respiration (C:N:P:−O2) play a vital role in characterizing and quantifying marine elemental cycles. Here, we present Version 2 of the Global Ocean Particulate Organic Phosphorus, Carbon, Oxygen for Respiration, and Nitrogen (GO-POPCORN) dataset. Version 1 is a previously published dataset of particulate organic matter from 70 different studies between 1971 and 2010, while Version 2 is comprised of data collected from recent cruises between 2011 and 2020. The combined GO-POPCORN dataset contains 2673 paired surface POC/N/P measurements from 70°S to 73°N across all major ocean basins at high spatial resolution. Version 2 also includes 965 measurements of oxygen demand for organic carbon respiration. This new dataset can help validate and calibrate the next generation of global ocean biogeochemical models with flexible elemental stoichiometry. We expect that incorporating variable C:N:P:-O2 into models will help improve our estimates of key ocean biogeochemical fluxes such as carbon export, nitrogen fixation, and organic matter remineralization.

Variability in the Global Ocean Carbon Sink From 1959 to 2020 by Correcting Models With Observations

AbstractThe ocean reduces human impact on the climate by absorbing and sequestering CO2. From 1950s to the 1980s, observations of pCO2 and related ocean carbon variables were sparse and uncertain. Thus, global ocean biogeochemical models (GOBMs) have been the basis for quantifying the ocean carbon sink. The LDEO‐Hybrid Physics Data product (LDEO‐HPD) interpolates sparse surface ocean pCO2 data to global coverage by using GOBMs as priors, and applying machine learning to estimate full‐coverage corrections. The largest component of the GOBM corrections are climatological. This is consistent with recent findings of large seasonal discrepancies in GOBMs, but contrasts the long‐held view that interannual variability is a major source of GOBM error. This supports extension of the LDEO‐HPD pCO2 product back to 1959, using a climatology of model‐observation misfits prior to 1982. Consistent with previous studies for 1980 onward, air‐sea CO2 fluxes for 1959–2020 demonstrate response to atmospheric pCO2 growth and volcanic eruptions.