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The temporal variation of the carbonate system, air–sea CO 2 fluxes, and pH is analyzed in the southern Indian Ocean, south of the polar front, based on in situ data obtained from 1985 to 2021 at a fixed station (50°40 ′ S–68°25 ′ E) and results from a neural network model that reconstructs the fugacity of CO 2 (fCO2) and fluxes at monthly scale. Anthropogenic CO 2 (C ant) is estimated in the water column and is detected down to the bottom (1600 m) in 1985, resulting in an aragonite saturation horizon at 600 m that migrated up to 400 m in 2021 due to the accumulation of C ant. At the subsurface, the trend of C ant is estimated at +0.53±0.01 µ mol kg -1 yr -1 with a detectable increase in the trend in recent years. At the surface during austral winter the oceanic fCO2 increased at a rate close to or slightly lower than in the atmosphere. To the contrary, in summer, we observed contrasting fCO2 and dissolved inorganic carbon (C T) trends depending on the decade and emphasizing the role of biological drivers on air–sea CO 2 fluxes and pH inter-annual variability. The regional air–sea CO 2 fluxes evolved from an annual source to the atmosphere of 0.8 molC m -2 yr -1 in 1985 to a sink of -0.5 molC m -2 yr -1 in 2020. Over 1985–2020, the annual pH trend in surface waters of -0.0165±0.0040 per decade was mainly controlled by the accumulation of anthropogenic CO 2 , but the summer pH trends were modulated by natural processes that reduced the acidification rate in the last decade. Using historical data from November 1962, we estimated the long-term trend for fCO2 , C T , and pH, confirming that the progressive acidification was driven by the atmospheric CO 2 increase. In 59 years this led to a diminution of 11 % for both aragonite and calcite saturation state. As atmospheric CO 2 is expected to increase in the future, the pH and carbonate saturation state will decrease at a faster rate than observed in recent years. A projection of future C T concentrations for a high emission scenario (SSP5-8.5) indicates that the surface pH in 2100 would decrease to 7.32 in winter. This is up to -0.86 lower than pre-industrial pH and -0.71 lower than pH observed in 2020. The aragonite undersaturation in surface waters would be reached as soon as 2050 (scenario SSP5-8.5) and 20 years later for a stabilization scenario (SSP2-4.5) with potential impacts on phytoplankton species and higher trophic levels in the rich ecosystems of the Kerguelen Islands area. [ABSTRACT FROM AUTHOR]

Observation-based data reconstructions of global surface ocean carbonate system variables play an essential role in monitoring the recent status of ocean carbon uptake and ocean acidification, as well as their impacts on marine organisms and ecosystems. So far, ongoing efforts are directed towards exploring new approaches to describe the complete marine carbonate system and to better recover its fine-scale features. In this respect, our research activities within the Copernicus Marine Environment Monitoring Service (CMEMS) aim to develop a sustainable production chain of observation-derived global ocean carbonate system datasets at high space–time resolutions. As the start of the long-term objective, this study introduces a new global 0.25 ∘ monthly reconstruction, namely CMEMS-LSCE (Laboratoire des Sciences du Climat et de l'Environnement) for the period 1985–2021. The CMEMS-LSCE reconstruction derives datasets of six carbonate system variables, including surface ocean partial pressure of CO2 (pCO2), total alkalinity (AT), total dissolved inorganic carbon (CT), surface ocean pH, and saturation states with respect to aragonite (Ωar) and calcite (Ωca). Reconstructing pCO2 relies on an ensemble of neural network models mapping gridded observation-based data provided by the Surface Ocean CO2 ATlas (SOCAT). Surface ocean AT is estimated with a multiple-linear-regression approach, and the remaining carbonate variables are resolved by CO2 system speciation given the reconstructed pCO2 and AT ; 1 σ uncertainty associated with these estimates is also provided. Here, σ stands for either the ensemble standard deviation of pCO2 estimates or the total uncertainty for each of the five other variables propagated through the processing chain with input data uncertainty. We demonstrate that the 0.25 ∘ resolution pCO2 product outperforms a coarser spatial resolution (1 ∘) thanks to higher data coverage nearshore and a better description of horizontal and temporal variations in pCO2 across diverse ocean basins, particularly in the coastal–open-ocean continuum. Product qualification with observation-based data confirms reliable reconstructions with root-mean-square deviation from observations of less than 8 %, 4 %, and 1 % relative to the global mean of pCO2 , AT (CT), and pH. The global average 1 σ uncertainty is below 5 % and 8 % for pCO2 and Ωar (Ωca), 2 % for AT and CT , and 0.4 % for pH relative to their global mean values. Both model–observation misfit and model uncertainty indicate that coastal data reproduction still needs further improvement, wherein high temporal and horizontal gradients of carbonate variables and representative uncertainty from data sampling would be taken into account as a priority. This study also presents a potential use case of the CMEMS-LSCE carbonate data product in tracking the recent state of ocean acidification. The data associated with this study are available at 10.14768/a2f0891b-763a-49e9-af1b-78ed78b16982 (Chau et al., 2023). [ABSTRACT FROM AUTHOR]

Efforts to control climate change require the stabilization of atmospheric CO 2 concentrations. This can only be achieved through a drastic reduction of global CO 2 emissions. Yet fossil fuel emissions increased by 29% between 2000 and 2008, in conjunction with increased contributions from emerging economies, from the production and international trade of goods and services, and from the use of coal as a fuel source. In contrast, emissions from land-use changes were nearly constant. Between 1959 and 2008, 43% of each year's CO 2 emissions remained in the atmosphere on average; the rest was absorbed by carbon sinks on land and in the oceans. In the past 50 years, the fraction of CO 2 emissions that remains in the atmosphere each year has likely increased, from about 40% to 45%, and models suggest that this trend was caused by a decrease in the uptake of CO 2 by the carbon sinks in response to climate change and variability. Changes in the CO 2 sinks are highly uncertain, but they could have a significant influence on future atmospheric CO 2 levels. It is therefore crucial to reduce the uncertainties. © 2009 Macmillan Publishers Limited. All rights reserved.

Our objective was to understand how marine birds respond to oceanographic variability across the Southern Indian Ocean using data collected during an 16-day cruise (4-21 January 2003). We quantified concurrent water mass distributions, ocean productivity patterns, and seabird distributions across a heterogeneous pelagic ecosystem from subtropical to sub-Antarctic waters. We surveyed 5155 km and sighted 15,606 birds from 51 species, and used these data to investigate how seabirds respond to spatial variability in the structure and productivity of the ocean. We addressed two spatial scales: the structure of seabird communities across macro-mega scale (1000 s km) biogeographic domains, and their coarse-scale (10 s km) aggregation at hydrographic and bathymetric gradients. Both seabird density and species composition changed with latitudinal and onshore-offshore gradients in depth, water temperature, and chlorophyll-a concentration. The average seabird density increased across the subtropical convergence (STC) from 2.4 birds km-2 in subtropical waters to 23.8 birds km-2 in sub-Antarctic waters. The composition of the avifauna also differed across biogeographic domains. Prions (Pachyptila spp.) accounted for 57% of all sub-Antarctic birds, wedge-tailed shearwaters (Puffinus pacificus) accounted for 46% of all subtropical birds, and Indian Ocean yellow-nosed albatross (Thallasarche carteri) accounted for 32% of all birds in the STC. While surface feeders were the most abundant foraging guild across the study area, divers were disproportionately more numerous in the sub-Antarctic domain, and plungers were disproportionately more abundant in subtropical waters. Seabird densities were also higher within shallow shelf-slope regions, especially in sub-Antarctic waters, where large numbers of breeding seabirds concentrated. However, we did not find elevated seabird densities along the STC, suggesting that this broad frontal region is not a site of enhanced aggregation. © 2007 Elsevier Ltd. All rights reserved.

The temporal variation of the carbonate system, air-sea CO2 fluxes and pH is analyzed in the Southern Indian Ocean, south of the Polar Front, based on in-situ data obtained from 1985 to 2021 at a fixed station (50°40'S-68°25'E) and results from a neural network model that reconstructs the fugacity of CO2 (fCO2) and fluxes at monthly scale. Anthropogenic CO2 (Cant) was estimated in the water column and detected down to the bottom (1600m) in 1985 resulting in an aragonite saturation horizon at 600m that migrated up to 400m in 2021 due to the accumulation of Cant. In subsurface, the trend of Cant is estimated at +0.53 (±0.01) µmol.kg-1.yr-1 with a detectable increase in recent years. At the surface during austral winter the oceanic fCO2 increased at a rate close or slightly lower than in the atmosphere. To the contrary, in summer, we observed contrasting fCO2 and dissolved inorganic carbon (CT) trends depending on the decade and emphasizing the role of biological drivers on air-sea CO2 fluxes and pH inter-annual variability. The region moved from an annual source of 0.8 molC.m- 2.yr-1 in 1985 to a sink of -0.5 molC.m-2.yr-1 in 2020. In 1985-2020, the annual pH trend in surface of -0.0165 (± 0.0040).decade-1 was mainly controlled by anthropogenic CO2 but the trend was modulated by natural processes. Using historical data from November 1962 we estimated the long-term trend for fCO2, CT and pH confirming that the progressive acidification was driven by atmospheric CO2 increase. In 59 years this leads to a diminution of 11% for both aragonite and calcite saturation state. As atmospheric CO2 will desperately continue rising in the future, the pH and carbonate saturation state will decrease at a faster rate than observed in recent years. A projection of future CT concentrations for a high emission scenario (SSP5-8.5) indicates that the surface pH in 2100 would decrease to 7.32 in winter. This is up to -0.86 lower than pre-industrial pH and -0.71 lower than pH observed in 2020. The aragonite under-saturation in surface waters would be reached as soon as 2050 (scenario SSP5-8.5) and 20 years later for a stabilization scenario (SSP2-4.5) with potential impacts on phytoplankton species and higher trophic levels in the rich ecosystems of the Kerguelen Island area. [ABSTRACT FROM AUTHOR]

Total alkalinity (AT) and total dissolved inorganic carbon (CT) in the oceans are important properties to understand the ocean carbon cycle and its link with climate change (ocean carbon sinks and sources) or global change (ocean acidification). We present a data-base of more than 44 400 AT and CT observations in various ocean regions obtained since 1993 mainly in the frame of French projects. This includes both surface and water columns data acquired in open oceans, coastal zones and in the Mediterranean Sea and either from time-series or punctual cruises. Most AT and CT data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with Certified Reference Material, with an overall accuracy of ± 4 µmol kg-1 for both AT and CT. Given the lack of observations in the Indian and Southern Oceans, we added sea surface underway AT and CT data obtained in 1998-2018 in the frame of OISO cruises and in 2019 during the CLIM-EPARSES cruise measured onboard using the same technique. Separate datasets for the global ocean, and for the Mediterranean Sea are provided in a single format (https://doi.org/10.17882/95414, Metzl et al., 2023) that offers a direct use for regional or global purposes, e.g. AT/Salinity relationships, long-term CT estimates, constraint and validation of diagnostics CT-AT reconstructed fields or ocean carbon and coupled climate/carbon models simulations, as well as data derived from BG-ARGO floats. When associated with other properties, these data can also be used to calculate pH, fugacity of CO2 (fCO2) and other carbon systems properties to derive ocean acidification rates or air-sea CO2 fluxes. [ABSTRACT FROM AUTHOR]

Observation-based data reconstructions of global surface ocean carbonate system variables play an essential role in monitoring the recent status of ocean carbon uptake and ocean acidification as well as their impacts on marine organisms and ecosystems. So far ongoing efforts are directed towards exploring new approaches to describe the complete marine carbonate system and to better recover its fine-scale features. In this respect, our research activities within the Copernicus Marine Environment Monitoring Service (CMEMS) aim at developing a sustainable production chain of observation-derived global ocean carbonate system datasets at high space-time resolution. As the start of the long-term objective, this study introduces a new global 0.25° monthly reconstruction, namely CMEMS-LSCE, for the period 1985-2021. The CMEMS-LSCE reconstruction derives datasets of six carbonate system variables including surface ocean partial pressure of CO2 (pCO2), total alkalinity (AT), total dissolved inorganic carbon (DIC), surface ocean pH, and saturation states with respect to aragonite (Ωar) and calcite (Ωca). Reconstructing pCO2 relies on an ensemble of neural network models mapping gridded observation-based data provided by the Surface Ocean CO2 ATlas (SOCAT). Surface ocean AT is estimated with a multiple linear regression approach, and the remaining carbonate variables are resolved by CO2 system speciation given the reconstructed pCO2 and AT. 1σ-uncertainty associated with these estimates is also provided. Here, σ stands for either ensemble standard deviation of pCO2 estimates or total uncertainty for each of the five other variables propagated through the processing chain with input data uncertainty. We demonstrate that the 0.25°-resolution pCO2 product outperforms a coarser spatial resolution (1°) thanks to a higher data coverage nearshore and a better description of horizontal and temporal variations in pCO2 across diverse ocean basins, particularly in the coastal-open-ocean continuum. Product qualification with observation-based data confirms reliable reconstructions with root-of-mean--square--deviation from observations less than 8%, 4%, and 1% relative to the global mean of pCO2, AT (DIC), and pH. The global average 1σ-uncertainty is below 5% and 8% for pCO2 and Ωar (Ωca), 2% for AT and DIC, and 0.4% for pH relative to their global mean values. Both model-observation misfit and model uncertainty indicate that coastal data reproduction still needs further improvement, wherein high temporal and horizontal gradients of carbonate variables and representative uncertainty from data sampling would be taken into account in priority. This study also presents a potential use case of the CMEMS-LSCE carbonate data product in tracking the recent state of ocean acidification. [ABSTRACT FROM AUTHOR]