Long-term trends and anthropogenic forcing of surface ocean carbon storage and acidification.

The Chukchi Sea is an increasing CO2 sink driven by rapid climate changes. Understanding the seasonal variation of air-sea CO2 exchange and the underlying mechanisms of biogeochemical dynamics is important for predicting impacts of climate change on and feedbacks by the ocean. Here, we present a unique data set of underway sea surface partial pressure of CO2 (pCO2) and discrete samples of biogeochemical properties collected in five consecutive cruises in 2014 and examine the seasonal variations in air-sea CO2 flux and net community production (NCP). We found that thermal and non-thermal effects have different impacts on sea surface pCO2 and thus the air-sea CO2 flux in different water masses. The Bering summer water combined with meltwater has a significantly greater atmospheric CO2 uptake potential than that of the Alaskan Coastal Water in the southern Chukchi Sea in summer, due to stronger biological CO2 removal and a weaker thermal effect. By analyzing the seasonal drawdown of dissolved inorganic carbon (DIC) and nutrients, we found that DIC-based NCP was higher than nitrate-based NCP by 66%-84% and attributable to partially decoupled C and N uptake because of a variable phytoplankton stoichiometry. A box model with a non-Redfield C:N uptake ratio can adequately reproduce observed pCO2 and DIC, which reveals that, during the intensive growing season (late spring to early summer), 30%-46% CO2 uptake in the Chukchi Sea was supported by a flexible stoichiometry of phytoplankton. These findings have important ramification for forecasting the responses of CO2 uptake of the Chukchi ecosystem to climate change.

The seasonal evolution of dissolved inorganic carbon (DIC) and CO2 air-sea fluxes in the Jiaozhou Bay was investigated by means of a data set from four cruises covering a seasonal cycle during 2003 and 2004. The results revealed that DIC had no obvious seasonal variation, with an average concentration of 2035 µmol kg-1 C in surface water. However, the sea surface partial pressure of CO2 changed with the season. pCO2 was 695 µatm in July and 317 µatm in February. Using the gas exchange coefficient calculated with Wanninkhof’s model, it was concluded that the Jiaozhou Bay was a source of atmospheric CO2 in spring, summer, and autumn, whereas it was a sink in winter. The Jiaozhou Bay released 2.60 x 1011 mmol C to the atmosphere in spring, 6.18 x 1011 mmol C in summer, and 3.01 x 1011 mmol C in autumn, whereas it absorbed 5.32 x 1010 mmol C from the atmosphere in winter. A total of 1.13 x 1012 mmol C was released to the atmosphere over one year. The behaviour as a carbon source/sink obviously varied in the different regions of the Jiaozhou Bay. In February, the inner bay was a carbon sink, while the bay mouth and the outer bay were carbon sources. In June and July, the inner and outer bay were carbon sources, but the strength was different, increasing from the inner to the outer bay. In November, the inner bay was a carbon source, but the bay mouth was a carbon sink. The outer bay was a weaker CO2 source. These changes are controlled by many factors, the most important being temperature and phytoplankton. Water temperature in particular was the main factor controlling the carbon dioxide system and the behaviour of the Jiaozhou Bay as a carbon source/sink. The Jiaozhou Bay is a carbon dioxide source when the water temperature is higher than 6.6°C. Otherwise, it is a carbon sink. Phytoplankton is another controlling factor that may play an important role in behaviour as a carbon source or sink in regions where the source or sink nature is weaker.

We present BIORYS4, a new Global Ocean Biogeochemical Reanalysis developed at Mercator Ocean International. within the Copernicus Marine Service framework. BIORYS4 provides 3D biogeochemical (BGC) fields at a quarter degree horizontal resolution and 75 vertical levels, spanning from 1993 to the present. The BGC fields are simulated using the PISCES-v2 (Pelagic Interactions Scheme for Carbon and Ecosystem Studies, version 2) model (Aumont et al. 2015), forced by the Global Ocean Physics Reanalysis, also developed at Mercator Ocean International within the Copernicus Marine Service. The BGC simulation is constrained through two approaches: climatological relaxation and data assimilation using a Singular Evolutive Extended Kalman Filter (SEEK) implemented in the Mercator Assimilation System. Dissolved inorganic nitrate, phosphate, silicate and iron, dissolved organic carbonand dissolved oxygen are relaxed toward monthly climatologies, and total alkalinity is relaxed toward a annual climatology using a 1-year relaxation timescale to preserve the model's internal interannual variability. Dissolved inorganic carbon is additionally relaxed toward interannual fields to account for anthropogenic emissions. To further constrain the model, the chlorophyll concentration derived from Ocean Color data is assimilated, updating the chlorophyll, nitrate, and silicate representations over the mixed layer. We constrain carbonate system variables through the assimilation of dissolved inorganic carbon and total alkalinity derived from a neural network product based on the Surface Ocean CO2 Atlas (SOCAT). Comparisons with a wide range of observational datasets demonstrate that BIORYS4 provides a robust representation of global biogeochemical processes. In particular, the assimilation of SOCAT-based carbonate variables significantly improves the simulated surface partial pressure of CO2 and air-sea CO2 fluxes, highlighting that direct assimilation of these fluxes could further enhance model accuracy and better resolve regional and temporal dynamics. The BIORYS4 global ocean biogeochemical reanalysis will be available freely through the Copernicus Marine Service, serving diverse scientific and operational user communities.

Abstract. Information on changes in the oceanic carbon dioxide (CO2) concentration and air–sea CO2 flux as well as on ocean acidification in the Indian Ocean is very limited. In this study, temporal changes of the inorganic carbon system in the eastern equatorial Indian Ocean (EIO, 5° N–5° S, 90–95° E) are examined using partial pressure of carbon dioxide (pCO2) data collected in May 2012, historical pCO2 data since 1962, and total alkalinity (TA) data calculated from salinity. Results show that sea surface pCO2 in the equatorial belt (2° N–2° S, 90–95° E) increased from ∼307 μatm in April 1963 to ∼373 μatm in May 1999, ∼381 μatm in April 2007, and ∼385 μatm in May 2012. The mean rate of pCO2 increase in this area (∼1.56 μatm yr−1) was close to that in the atmosphere (∼1.46 μatm yr−1). Despite the steady pCO2 increase in this region, no significant change in air–sea CO2 fluxes was detected during this period. Ocean acidification as indicated by pH and saturation states for carbonate minerals has indeed taken place in this region. Surface water pH (total hydrogen scale) and saturation state for aragonite (Ωarag), calculated from pCO2 and TA, decreased significantly at rates of −0.0016 ± 0.0001 and −0.0095 ± 0.0005 yr−1, respectively. The respective contributions of temperature, salinity, TA, and dissolved inorganic carbon (DIC) to the increase in surface pCO2 and the decreases in pH and Ωarag are quantified. We find that the increase in DIC dominated these changes, while contributions from temperature, salinity, and TA were insignificant. The increase in DIC was most likely associated with the increasing atmospheric CO2 concentration, and the transport of accumulated anthropogenic CO2 from a CO2 sink region via basin-scale ocean circulations. These two processes may combine to drive oceanic DIC to follow atmospheric CO2 increase.

Ocean acidification is likely to impact marine ecosystems and human societies adversely and is a carbon cycle issue of great concern. Projecting the degree of ocean acidification and the carbon-climate feedback will require understanding the current status, variability, and trends of ocean inorganic carbon system variables and the ocean carbon sink. With this goal in mind, we reconstructed total alkalinity (TA), dissolved inorganic carbon (DIC), CO2 partial pressure (pCO2sea), sea–air CO2 flux, pH, and aragonite saturation state (Ωarg) for the global ocean based on measurements of pCO2sea and TA. We used a multiple linear regression approach to derive relationships to explain TA and DIC and obtained monthly 1° × 1° gridded values of TA and DIC for the period 1993–2018. These data were converted to pCO2sea, pH, and Ωarg, and monthly sea-air CO2 fluxes were obtained in combination with atmospheric CO2. Mean annual sea–air CO2 flux and its rate of change were estimated to be − 2.0 ± 0.5 PgC year−1 and − 0.3 (PgC year−1) decade−1, respectively. Our analysis revealed that oceanic CO2 uptake decreased during the 1990s and has been increasing since 2000. Our estimate of the globally averaged rate of pH change, − 0.0181 ± 0.0001 decade−1, was consistent with that expected from the trend of atmospheric CO2 growth. However, rates of decline of pH were relatively slow in the Southern Ocean (− 0.0165 ± 0.0001·decade−1) and in the western equatorial Pacific (− 0.0148 ± 0.0002·decade−1). Our estimate of the globally averaged rate of pH change can be used to verify Indicator 14.3.1 of Sustainable Development Goals.

Weekly and bi-monthly carbonate system parameters and ancillary data were collected from 2008 to 2020 in three coastal ecosystems of the southern Western English Channel (sWEC) (SOMLIT-pier and SOMLIT-offshore) and Bay of Brest (SOMLIT-Brest) located in the North East Atlantic Ocean. The main drivers of seasonal and interannual partial pressure of CO2 (pCO2) and dissolved inorganic carbon (DIC) variabilities were the net ecosystem production (NEP) and thermodynamics. Differences were observed between stations, with a higher biological influence on pCO2 and DIC in the near-shore ecosystems, driven by both benthic and pelagic communities. The impact of riverine inputs on DIC dynamics was more pronounced at SOMLIT-Brest (7%) than at SOMLIT-pier (3%) and SOMLIT-offshore (<1%). These three ecosystems acted as a weak source of CO2 to the atmosphere of 0.18 ± 0.10, 0.11 ± 0.12, and 0.39 ± 0.08 mol m–2 year–1, respectively. Interannually, air-sea CO2 fluxes (FCO2) variability was low at SOMLIT-offshore and SOMLIT-pier, whereas SOMLIT-Brest occasionally switched to weak annual sinks of atmospheric CO2, driven by enhanced spring NEP compared to annual means. Over the 2008–2018 period, monthly total alkalinity (TA) and DIC anomalies were characterized by significant positive trends (p-values < 0.001), from 0.49 ± 0.20 to 2.21 ± 0.39 μmol kg−1 year−1 for TA, and from 1.93 ± 0.28 to 2.98 ± 0.39 μmol kg–1 year–1 for DIC. These trends were associated with significant increases of calculated seawater pCO2, ranging from +2.95 ± 1.04 to 3.52 ± 0.47 μatm year–1, and strong reductions of calculated pHin situ, with a mean pHin situ decrease of 0.0028 year–1. This ocean acidification (OA) was driven by atmospheric CO2 forcing (57–66%), Sea surface temperature (SST) increase (31–37%), and changes in salinity (2–5%). Additional pHin situ data extended these observed trends to the 2008–2020 period and indicated an acceleration of OA, reflected by a mean pHin situ decrease of 0.0046 year–1 in the sWEC for that period. Further observations over the 1998–2020 period revealed that the climatic indices North Atlantic Oscillation (NAO) and Atlantic Multidecadal Variability (AMV) were linked to trends of SST, with cooling during 1998–2010 and warming during 2010–2020, which might have impacted OA trends at our coastal stations. These results suggested large temporal variability of OA in coastal ecosystems of the sWEC and underlined the necessity to maintain high-resolution and long-term observations of carbonate parameters in coastal ecosystems.

Abstract. ​​​​​​​Ocean CO2 uptake and acidification in response to human activities are driven primarily by the rise in atmospheric CO2 but are also modulated by climate change. Existing work suggests that this “climate effect” influences the uptake and storage of anthropogenic carbon and acidification via the global increase in ocean temperature, although some regional responses have been attributed to changes in circulation or biological activity. Here, we investigate spatial patterns in the climate effect on surface ocean acidification (and the closely related carbonate chemistry) in an Earth system model under a rapid CO2-increase scenario and identify a different driving process. We show that the amplification of the hydrological cycle, a robustly simulated feature of climate change, is largely responsible for the spatial patterns in this climate effect at the sea surface. This “hydrological effect” can be understood as a subset of the total climate effect, which includes warming, hydrological cycle amplification, circulation, and biological changes. We demonstrate that it acts through two primary mechanisms: (i) directly diluting or concentrating dissolved ions by adding or removing freshwater and (ii) altering the sea surface temperature, which influences the solubility of dissolved inorganic carbon (DIC) and acidity of seawater. The hydrological effect opposes acidification in salinifying regions, most notably the subtropical Atlantic, and enhances acidification in freshening regions such as the western Pacific. Its single strongest effect is to dilute the negative ions that buffer the dissolution of CO2, quantified as alkalinity. The local changes in alkalinity, DIC, and pH linked to the pattern of hydrological cycle amplification are as strong as the (largely uniform) changes due to warming, explaining the weak increase in pH and DIC seen in the climate effect in the subtropical Atlantic Ocean.

We used an offline tracer transport model, driven by reanalysis ocean currents and coupled to a simple biogeochemical model, to synthesize the surface ocean pCO2 and air–sea CO2 flux of the global ocean from 1996 to 2004, using a variational assimilation method. This oceanic CO2 flux analysis system was developed at the National Institute for Environmental Studies (NIES), Japan, as part of a project that provides prior fluxes for atmospheric inversions using CO2 measurements made from an on-board instrument attached to the Greenhouse gas Observing SATellite (GOSAT). Nearly 250 000 pCO2 observations from the database of Takahashi et al. (2007) have been assimilated into the model with a strong constraint provide by ship-track observations while maintaining a weak constraint of 20% on global averages of monthly mean pCO2 in regions where observations are limited. The synthesized global air–sea CO2 flux shows a net sink of 1.48 PgC yr-1. The Southern Ocean air–sea CO2 flux is a sink of 0.41 PgC yr-1. The interannual variability of synthesized CO2 flux from the El Niño region suggests a weaker source (by an amplitude of 0.4 PgC yr-1) during the El Niño events in 1997/1998 and 2003/2004. The assimilated air–sea CO2 flux shows remarkable correlations with the CO2 fluxes obtained from atmospheric inversions on interannual time-scales.

The Arctic Ocean is undergoing rapid transformations due to the loss of sea ice, shifts in its heat budget and physical structure, and the “greening” of the polar surface ocean. These changes have profound implications for ocean biogeochemistry, the carbon cycle, and ocean acidification (OA). As part of the U.S. Synoptic Arctic Survey (SAS), we conducted a transect from the Chukchi Sea shelf to the North Pole during late summer 2022, enabling comprehensive sampling of the ocean carbon cycle in the seldom-sampled high Arctic. Discrete samples of Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA) were collected from CTD-hydrocasts spanning surface to deep waters, complemented by higher-frequency underway measurements of DIC, TA, and pH. These observations establish a critical baseline for tracking future changes in Arctic carbon dynamics, biogeochemistry, and acidification. Additionally, the 2022 US SAS dataset allows for comparison with earlier observations, including the 1994 Arctic Ocean Section (AOS), the 2005 Beringia expedition, and the 2015 GEOTRACES Arctic cruise. Our synthesis reveals significant and ongoing changes in the Arctic Ocean carbon cycle, including: (1) substantial uptake of anthropogenic CO₂; (2) alterations in the driving force for air-sea CO₂ exchange; (3) a decreasing capacity of the Arctic Ocean to absorb atmospheric CO₂; and (4) intensified impacts on surface pH and ocean acidification. These findings underscore the accelerating pace of carbon cycle changes in the high Arctic and highlight the importance of sustained monitoring.

We investigate the interannual variability in the flux of CO2 between the atmosphere and the Southern Ocean on the basis of hindcast simulations with a coupled physical‐biogeochemical‐ecological model with particular emphasis on the role of the Southern Annular Mode (SAM). The simulations are run under either pre‐industrial or historical CO2 concentrations, permitting us to separately investigate natural, anthropogenic, and contemporary CO2 flux variability. We find large interannual variability (±0.19 PgC yr−1) in the contemporary air‐sea CO2 flux from the Southern Ocean (<35°S). Forty‐three percent of the contemporary air‐sea CO2 flux variance is coherent with SAM, mostly driven by variations in the flux of natural CO2, for which SAM explains 48%. Positive phases of the SAM are associated with anomalous outgassing of natural CO2 at a rate of 0.1 PgC yr−1 per standard deviation of the SAM. In contrast, we find an anomalous uptake of anthropogenic CO2 at a rate of 0.01 PgC yr−1 during positive phases of the SAM. This uptake of anthropogenic CO2 only slightly mitigates the outgassing of natural CO2, so that a positive SAM is associated with anomalous outgassing in contemporaneous times. The primary cause of the natural CO2 outgassing is anomalously high oceanic partial pressures of CO2 caused by elevated dissolved inorganic carbon (DIC) concentrations. These anomalies in DIC are primarily a result of the circulation changes associated with the southward shift and strengthening of the zonal winds during positive phases of the SAM. The secular, positive trend in the SAM has led to a reduction in the rate of increase of the uptake of CO2 by the Southern Ocean over the past 50 years.

Although they are key components of the surface ocean carbon budget, physical processes inducing carbon fluxes across the mixed‐layer base, i.e., subduction and obduction, have received much less attention than biological processes. Using a global model analysis of the preindustrial ocean, physical carbon fluxes are quantified and compared to the other carbon fluxes in and out of the surface mixed layer, i.e., air‐sea CO2gas exchange and sedimentation of biogenic material. Model‐based carbon obduction and subduction are evaluated against independent data‐based estimates to the extent that was possible. We find that climatological physical fluxes of dissolved inorganic carbon (DIC) are two orders of magnitude larger than the other carbon fluxes and vary over the globe at smaller spatial scale. At temperate latitudes, the subduction of DIC and to a much lesser extent (<10%) the sinking of particles maintain CO2undersaturation, whereas DIC is obducted back to the surface in the tropical band (75%) and Southern Ocean (25%). At the global scale, these two large counter‐balancing fluxes of DIC amount to +275.5 PgC yr−1 for the supply by obduction and −264.5 PgC yr−1 for the removal by subduction which is ∼ 3 to 5 times larger than previous estimates. Moreover, we find that subduction of organic carbon (dissolved and particulate) represents ∼ 20% of the total export of organic carbon: at the global scale, we evaluate that of the 11 PgC yr−1 of organic material lost from the surface every year, 2.1 PgC yr−1 is lost through subduction of organic carbon. Our results emphasize the strong sensitivity of the oceanic carbon cycle to changes in mixed‐layer depth, ocean currents, and wind.

Controlling mechanisms of surface partial pressure of CO2 in Jiaozhou Bay during summer and the influence of heavy rain

Studies on biogeochemical cycling of carbon in the Chilka Lake, Asia’s largest brackish lagoon on the east coast of India, revealed, for the first time, strong seasonal and spatial variability associated with salinity distribution. The lake was studied twice during May 2005 (premonsoon) and August 2005 (monsoon). It exchanges waters with the sea (Bay of Bengal) and several rivers open into the lake. The lake showed contrasting levels of dissolved inorganic carbon (DIC) and organic carbon (DOC) in different seasons; DIC was higher by ∼22% and DOC was lower by ∼36% in premonsoon than in monsoon due to seasonal variations in their supply from rivers and in situ production/mineralisation. The DIC/DOC ratios in the lake during monsoon were influenced by physical mixing of end member water masses and by intense respiration of organic carbon. A strong relationship between excess DIC and apparent oxygen utilisation showed significant control of biological processes over CO2 production in the lake. Surface partial pressure of CO2 (pCO2), calculated using pH–DIC couple according to Cai and Wang (Limnol and Oceanogr 43:657–668, 1998), exhibited discernable gradients during monsoon through northern (1,033–6,522 μatm), central (391–2,573 μatm) and southern (102–718 μatm) lake. The distribution pattern of pCO2 in the lake seems to be governed by pCO2 levels in rivers and their discharge rates, which were several folds higher during monsoon than premonsoon. The net CO2 efflux, based on gas transfer velocity parameterisation of Borges et al. (Limnol and Oceanogr 49(5):1630–1641, 2004), from entire lake during monsoon (141 mmolC m−2 d−1 equivalent to 2.64 GgC d−1 at basin scale) was higher by 44 times than during premonsoon (9.8 mmolC m−2 d−1 ≈ 0.06 GgC d−1). 15% of CO2 efflux from lake in monsoon was contributed by its supply from rivers and the rest was contributed by in situ heterotrophic activity. Based on oxygen and total carbon mass balance, net ecosystem production (NEP) of lake (−308 mmolC m−2 d−1 ≈ −3.77 GgC d−1) was found to be almost in consistent with the total riverine organic carbon trapped in the lake (229 mmolC m−2 d−1 ≈ 2.80 GgC d−1) suggesting that the strong heterotrophy in the lake is mainly responsible for elevated fluxes of CO2 during monsoon. Further, the pelagic net community production represented 92% of NEP and benthic compartment plays only a minor role. This suggests that Chilka lake is an important region in biological transformation of organic carbon to inorganic carbon and its export to the atmosphere.

The monthly, seasonal, annual, and decadal trends of seven hydro-meteorological variables were analysed for stations in Akwa Ibom State, Nigeria, controlled by the Nigerian Meteorological Agency (NiMet) and the Cross River Basin Development Authority (CRBDA) from 1972 to 2021. At the 5% statistical significance level, the non-parametric Mann-Kendall and Sen's slope estimator techniques were used to detect if there was a positive or negative trend and the magnitude of the trend in hydro-meteorological data. In the present study, there was a significant statistically increasing (positive) trend in mean seasonal and annual rainfall, maximum temperature, minimum temperature, and runoff. However, there was a significant statistically decreasing (negative) trend in average annual relative humidity, solar radiation, and potential evapotranspiration. The magnitudes of the trends were 19.39mm/year, 0.0314oC/year, 0.013oC/year, -0.104%/year, -8.78MJ/m2/year, -1.440mm/year, and 0.028m3/s/year for annual rainfall, maximum temperature, minimum temperature, relative humidity, solar radiation, PET, and runoff, respectively. The rising trends in precipitation, temperature, and runoff in this research area show that this region is subject to climatic variability. The results of the Mann-Kendall and Sen's slope estimator statistical tests revealed the consistency of performance in the detection of the trend for the hydro-meteorological variables.

This study examines historical (1981-2020) and future (2020-2070) trends in rainfall and temperature in the Kilombero Basin using the Mann-Kendall method with Sen's slope estimator. Data were obtained from the Tanzania Meteorological Agency and from simulated historical and future climate data sourced from the Coupled Model Intercomparison Project 6 (CMIP6). The CMIP6 datasets were downscaled and bias-corrected using the CMhyd tool. The basin exhibited a bimodal rainfall pattern with an average of 1400 mm, peaking around April. The CMIP6 models successfully simulated monthly rainfall, Tmax, and Tmin at most stations. No definitive trends in rainfall were observed, but Tmax and Tmin showed significant increases under both SSP2-4.5 and SSP5-8.5 scenarios. More warming is predicted under SSP5-8.5 by the mid-21st century, raising Tmax and Tmin at all stations. This rise in temperature could potentially increase evapotranspiration demand, negatively impacting freshwater availability. The average annual rainfall showed a slightly increasing trend post-2000, from 1403.96mm/year (1981-1999) to 1433.38mm/year (2000-2020), an increase of 2.05%. Sen's slope analysis, however, revealed varying trends across stations, with most showing a decreasing trend. Notably, only Ulanga Met Station showed a significant increasing trend with a slope value of 14.70 and a p-value below 0.05. The study concluded that both temperature and precipitation in the Kilombero Basin are on the rise.