Observation-based estimates of the global oceanic CO2 sink: Spatiotemporal analysis, quantification of uncertainties, processes description

Since the beginning of the industrial revolution, a large amount of greenhouse gases such as carbon dioxide (CO2) have been emitted into the atmosphere due to human activities. One of the main consequences of these emissions is a rapid increase in atmospheric CO2 concentration and a profound modification of the Earth's climate system. The ocean plays an important role in the Earth radiative balance since it acts as an important CO2 sink for the atmosphere. By currently absorbing about 25 % of the CO2 emitted by humans it considerably slows down climate change. Understanding the present-day spatial and temporal dynamics of the air-sea CO2 exchange and the different processes that govern this exchange is of critical importance to anticipate the evolution of the oceanic CO2 sink in the future.This thesis was realized in this context and focused on an improved quantification of the exchange of CO2 through the air-sea interface (FCO2) of the global ocean, embracing open ocean waters and coastal regions. The main objective was to fill knowledge gaps in our understanding of the processes that govern the spatial and temporal distribution of FCO2. This objective was mainly achieved through observational approaches and addressed three main aspects: a quantification of the different sources of FCO2 uncertainties at the global scale, an analysis of spatial distribution of the oceanic CO2 exchange with a strong focus on the coastal ocean and a first assessment of the coastal seasonal FCO2 dynamics and its underlying drivers. The latter relied on a data-model fusion approach allowing to decompose the FCO2 seasonality into its main physical and biogeochemical drivers. The quantification of the oceanic FCO2 from observations consists in calculating an air-sea partial pressure CO2 gradient (ΔpCO2) between the atmosphere and the sea surface. Global monthly continuous partial pressure of CO2 (pCO2) products can for example be derived from observational pCO2 databases and statistical interpolation methods. This ΔpCO2 is then multiplied by a gas exchange transfer rate coefficient (k), which depends on wind speed. However, the parametrization of k is still entailed with poorly quantified uncertainties. From a literature review of all k parameterizations available in the literature over the past 25 years, I first quantified the FCO2 uncertainties associated with k globally and regionally for the open ocean. I also quantified the uncertainties associated with the choice of a wind product over another. Our results show that the range of global FCO2, calculated with these k relationships, diverge by 12 % when using CCMP, ERA or NCEP1. Regional discrepancies in FCO2 are more pronounced than global. These global and regional differences significantly increase when using NCEP2 or other k formulations. To minimize uncertainties associated with the choice of wind product, it is possible to recalculate the parametrization of k globally for a given wind product and its spatio-temporal resolution, in order to match the last evaluation of the global k value. In a second step, we improved the quantification and analysis of the dominant patterns and drivers of the FCO2 spatial distribution for the coastal ocean worldwide. This analysis was performed globally (at 0.25° spatial resolution), using a regional segmentation of the coastal ocean, and latitudinally. I found that coastal regions at high latitudes act as a CO2 sink while tropical regions and along the equator tend to act as an atmospheric CO2 source. Globally integrated, I quantified that the coastal seas act currently as a CO2 sink with a value of -0.20 ± 0.02 Pg C yr-1. For the first time, I also compared the spatial patterns of coastal FCO2 to that of the adjacent open ocean, globally. With the exception of some regions such as those dominated by riverine inputs, I demonstrated that they present similar latitudinal distribution of their FCO2 density per unit of surface area, suggesting analogous responses to increasing atmospheric CO2. I also reevaluated the global ocean CO2 budget and estimated a global anthropogenic CO2 uptake ranging between -2.6 ± 0.4 Pg C yr-1 and -2.9 ± 0.5 Pg C yr-1 for the 1998-2015 period. In a third step, I contributed to the first continuous observational pCO2 data product merging the coastal and open ocean in a consistent manner. This study showed that difference between open ocean and coastal ocean estimates along the overlap area increases with latitude but remains close to 0 µatm globally. Stronger discrepancies, however, exist on the regional level resulting in differences that exceed 10 % of the climatological mean pCO2, particularly in regions constrained by fewer observations, paired with biogeochemical complexity, such as the Peruvian upwelling system and ice covered regions.In a fourth step, a temporal analysis of the FCO2 seasonality was performed for the coastal ocean based on an observational approach. I analyzed and quantified the FCO2 seasonal dynamics globally and for different latitudinal bands. Globally, coastal regions act as a CO2 sink with a more intense uptake occurring in summer (-21 Tg C month-1) because of the disproportionate influence of high latitude shelves in the Northern Hemisphere. I also estimated the contribution of different drivers (sea-ice coverage, wind speed, and ΔpCO2 change) to the FCO2 seasonal amplitude. This data-driven approach allowed me to conclude that the ΔpCO2 is the main driver of the FCO2 variability at the seasonal timescale. I then used a global oceanic biogeochemical model to decompose the seasonal coastal pCO2 variability further into its driving physical and biological processes. From a first qualitative assessment, I concluded that the thermal effect associated to sea surface temperature changes is the main effect governing the coastal seasonal pCO2 variability except at high latitudes where the non-thermal effect associated to changes in biology, circulation, fresh water and the air-sea CO2 exchange itself dominate. I also found that, overall, the thermal effect alone should lead to larger seasonal fluctuations, but its influence is partly offset by the non-thermal effect. Throughout this thesis, I also evaluated the extent to which the continuous observational pCO2 products derived from an artificial neuronal network approach and from the global ocean biogeochemical model MOM6-COBALT could reproduce the raw pCO2 fields extracted from global databases. Overall, I showed that at the regional scale, the two products are in relatively good agreement compared to observations. I also identified regions where discrepancies are the largest and where future observational data are needed in the future, as well as regions where agreement is the most satisfactory and, thus, most suitable for further process-based analyses.Depuis le début de la révolution industrielle, une grande quantité de gaz à effet de serre tels que le dioxyde de carbone (CO2) a été émise dans l'atmosphère en raison des activités humaines. L'une des principales conséquences de ces émissions est une augmentation rapide de la concentration en CO2 atmosphérique et une modification profonde du système climatique de la Terre. L'océan joue un rôle important dans l'équilibre radiatif de la Terre car il agit comme un important puits de CO2 pour l'atmosphère. En absorbant actuellement environ 25 % du CO2 émis par l'homme, il ralentit considérablement le changement climatique. Comprendre la dynamique spatiale et temporelle actuelle de l'échange de CO2 air-mer et les différents processus qui régissent cet échange est d'une importance cruciale pour anticiper l'évolution du puits océanique de CO2 à l'avenir.Cette thèse a été réalisée dans ce contexte et s'est concentrée sur une meilleure quantification de l'échange de CO2 à travers l'interface air-mer (FCO2) de l'océan global, considérant à la fois l’océan ouvert et les régions côtières. L'objectif principal était de combler les lacunes dans notre compréhension des processus qui régissent la distribution spatiale et temporelle du FCO2. Cet objectif a été principalement atteint grâce à des approches observationnelles et a abordé trois aspects principaux: une quantification des différentes sources d'incertitudes du FCO2 à l'échelle globale, une analyse de la distribution spatiale de l'échange de CO2 océanique avec un fort accent sur l'océan côtier et une première évaluation de la dynamique saisonnière du FCO2 côtier et de ses moteurs sous-jacents. Ce dernier s'est appuyé sur une approche de fusion de modèles et d’approches observationnelles permettant de décomposer la saisonnalité du FCO2 en ses principaux moteurs physiques et biogéochimiques.La quantification du FCO2 océanique à partir d’observations consiste à calculer un gradient de pression partielle air-mer de CO2 (ΔpCO2) entre l'atmosphère et la surface de la mer. Des produits globaux continus mensuels de la pression partielle de CO2 (pCO2) peuvent par exemple être dérivés à partir de bases de données observationnelles de pCO2 et de méthodes d'interpolation statistique. ΔpCO2 est ensuite multiplié par un coefficient de vitesse de transfert d'échange gazeux (k), qui dépend de la vitesse du vent. Cependant, la paramétrisation de k est sujette à de larges incertitudes et mal quantifiées. À partir d'une synthèse de la littérature de toutes les paramétrisations de k disponibles dans la littérature au cours des 25 dernières années, j'ai d'abord quantifié les incertitudes sur FCO2 associées à k à l'échelle globale et régionale pour l'océan ouvert. J'ai également quantifié les incertitudes associées au choix d'un produit éolien par rapport à un autre. Nos résultats montrent que la gamme du FCO2 global, calculée avec ces différentes paramétrisations de k, diverge de 12 % lors de l'utilisation de CCMP, ERA ou NCEP1. En raison des différences dans les pattern de vent régionaux, les différences régionales sur le FCO2 sont plus prononcés que globalement. Ces différences globales et régionales augmentent de manière significative lors de l'utilisation de NCEP2 ou d'autres formulations de k. Afin de réduire les incertitudes associées au choix du produit de vent, il est possible de recalculer la paramétrisation de k pour un produit de vent donné et à une résolution spatio temporelle.Dans un deuxième temps, nous avons amélioré la quantification et l'analyse des principaux pattern et des différents processus sur la distribution spatiale du FCO2 pour l’ensemble des régions côtières. Cette analyse a été réalisée à l'échelle globale (à une résolution spatiale de 0.25°), en utilisant une segmentation régionale de l'océan côtier, et latitudinalement. J'ai trouvé que les régions côtières aux hautes latitudes agissent comme un puits de CO2 tandis que les régions côtières tropicales et le long de l'équateur ont tendance à agir comme une source de CO2 atmosphérique. Globalement, j'ai quantifié que les régions côtières agissent actuellement en tant que puits de CO2 avec une valeur de -0.20 ± 0.02 Pg C an-1. Pour la première fois, j'ai également comparé la distribution spatiale du FCO2 côtier à celle de l'océan ouvert adjacent, à l'échelle globale. À l'exception de certaines régions telles que celles dominées par les apports fluviaux, j'ai démontré que les régions côtières et l’océan ouvert adjacent présentaient une distribution latitudinale similaire sur leur densité de FCO2 par unité de surface, suggérant des réponses analogues à l'augmentation du CO2 atmosphérique. J'ai également réévalué le budget mondial de CO2 de l'océan et estimé une absorption mondiale de CO2 anthropique comprise entre -2.6 ± 0.4 Pg C an-1 et -2.9 ± 0.5 Pg C an-1 pour la période 1998-2015. Dans un troisième temps, j'ai contribué à la création du premier produit continu de pCO2 observationnelles fusionnant le domaine côtier et l'océan ouvert de manière cohérente. Cette étude a montré que la différence entre les estimations provenant du produit de pCO2 de l’océan ouvert à celles dérivant du produit de pCO2 de l’océan côtier le long de leur zone de chevauchement augmente avec la latitude mais reste proche de 0 µatm globallement. Des divergences plus fortes existent cependant au niveau régional, entraînant des différences qui dépassent 10 % sur la moyenne climatologique de pCO2, en particulier dans les régions contraintes par moins d'observations, associées à une complexité biogéochimique, comme le système d'upwelling péruvien et les régions couvertes de glace.Dans une quatrième étape, une analyse temporelle de la saisonnalité du FCO2 a été réalisée pour l'océan côtier sur la base d'une approche observationnelle. J'ai analysé et quantifié la dynamique saisonnière du FCO2 à l'échelle globale et pour différentes bandes latitudinales. À l'échelle globale, les régions côtières agissent comme un puits de CO2 avec une absorption plus intense se produisant en été (-21 Tg C mois-1) en raison de l'influence disproportionnée des régions côtières des hautes latitudes dans l'hémisphère Nord. J'ai également estimé la contribution de différents processus (couverture de glace de mer, vitesse du vent et changement de ΔpCO2) à l'amplitude saisonnière du FCO2. Cette approche basée sur les données observationnelles m'a permis de conclure que ΔpCO2 est le principal moteur de la variabilité du FCO2 à l'échelle saisonnière. J'ai ensuite utilisé un modèle biogéochimique océanique global pour décomposer davantage la variabilité saisonnière du pCO2 côtier en ses processus physiques et biologiques. À partir d'une première évaluation qualitative, j'ai conclu que l'effet thermique associé aux changements de température de la surface de la mer est le principal effet régissant la variabilité côtière saisonnière du pCO2 sauf aux hautes latitudes où l'effet non thermique associé aux changements de biologie, de circulation, d'eau douce et de l’échange de CO2 air-mer domine. J'ai également constaté que, globalement, l'effet thermique à lui seul devrait entraîner des fluctuations saisonnières plus importantes, mais son influence est en partie compensée par l'effet non thermique.Tout au long de cette thèse, j'ai également évalué dans quelle mesure les produits continus de pCO2 observationnelles dérivés d'une approche de réseau de neurones artificiels et du modèle biogéochimique océanique global MOM6-COBALT pourraient reproduire les champs de pCO2 bruts extraits des bases de données globale. Dans l'ensemble, j'ai montré qu'à l'échelle régionale, les deux produits sont relativement en bon accord par rapport aux observations. J'ai également identifié les régions où les différences sont les plus importantes et où de futures données observationnelles sont nécessaires à l'avenir, ainsi que les régions où les deux produits présentent un accord le plus satisfaisant et, par conséquent, le plus approprié pour de futures analyses de compréhension des différents processus.Doctorat en Sciencesinfo:eu-repo/semantics/nonPublishe

Uncertainty in the global oceanic CO2_2 uptake induced by wind forcing: quantification and spatial analysis

International audienceThe calculation of the air–water CO$_2$ exchange (FCO$_2$) in the ocean not only depends on the gradient in CO$_2$ partial pressure at the air–water interface but also on the parameterization of the gas exchange transfer velocity ($k$) and the choice of wind product. Here, we present regional and global-scale quantifications of the uncertainty in FCO$_2$ induced by several widely used k formulations and four wind speed data products (CCMP, ERA, NCEP1 and NCEP2). The analysis is performed at a 1°  ×  1° resolution using the sea surface pCO$_2$ climatology generated by Landschützer et al. (2015a) for the 1991–2011 period, while the regional assessment relies on the segmentation proposed by the Regional Carbon Cycle Assessment and Processes (RECCAP) project. First, we use $k$ formulations derived from the global $^{14}$C inventory relying on a quadratic relationship between k and wind speed ($k = c$⋅$U_{{10}^2}$; Sweeney et al., 2007; Takahashi et al., 2009; Wanninkhof, 2014), where c is a calibration coefficient and $U_{10}$ is the wind speed measured 10 m above the surface. Our results show that the range of global FCO$_2$, calculated with these k relationships, diverge by 12 % when using CCMP, ERA or NCEP1. Due to differences in the regional wind patterns, regional discrepancies in FCO$_2$ are more pronounced than global. These global and regional differences significantly increase when using NCEP2 or other k formulations which include earlier relationships (i.e., Wanninkhof, 1992; Wanninkhof et al., 2009) as well as numerous local and regional parameterizations derived experimentally. To minimize uncertainties associated with the choice of wind product, it is possible to recalculate the coefficient c globally (hereafter called c$^∗$) for a given wind product and its spatio-temporal resolution, in order to match the last evaluation of the global k value. We thus performed these recalculations for each wind product at the resolution and time period of our study but the resulting global FCO$_2$ estimates still diverge by 10 %. These results also reveal that the Equatorial Pacific, the North Atlantic and the Southern Ocean are the regions in which the choice of wind product will most strongly affect the estimation of the FCO$_2$, even when using c$^∗$

A novel sea surface pCO2_2-product for the global coastal ocean resolvingtrends over 1982--2020

In recent years, advancements in machine learning based interpolation methods have enabled the production of high-resolution maps of sea surface partial pressure of CO2 (pCO2) derived from observations extracted from databases such as the Surface Ocean CO2 Atlas (SOCAT). These pCO2-products now allow quantifying the oceanic air–sea CO2 exchange based on observations. However, most of them do not yet explicitly include the coastal ocean. Instead, they simply extend the open ocean values onto the nearshore shallow waters, or their spatial resolution is simply so coarse that they do not accurately capture the highly heterogeneous spatiotemporal pCO2 dynamics of coastal zones. Until today, only one global pCO2-product has been specifically designed for the coastal ocean (Laruelle et al. 2017). This product, however, has shortcomings because it only provides a climatology covering a relatively short period (1998–2015), thus hindering its application to the evaluation of the interannual variability, decadal changes and the long-term trends of the coastal air–sea CO2 exchange, a temporal evolution that is still poorly understood and highly debated. Here we aim at closing this knowledge gap and update the coastal product of Laruelle et al. (2017) to investigate the longest global monthly time series available for the coastal ocean from 1982 to 2020. The method remains based on a two-step Self-Organizing Maps and Feed-Forward Network method adapted for coastal regions, but we include additional environmental predictors and use a larger pool of training and validation data with ∼18 million direct observations extracted from the latest release of the SOCAT database. Our study reveals that the coastal ocean has been acting as an atmospheric CO2 sink of −0.40 Pg C yr−1 (−0.18 Pg C yr−1 with a narrower coastal domain) on average since 1982, and the intensity of this sink has increased at a rate of 0.06 Pg C yr−1 decade−1 (0.02 Pg C yr−1 decade−1 with a narrower coastal domain) over time. Our results also show that the temporal changes in the air–sea pCO2 gradient plays a significant role in the long-term evolution of the coastal CO2 sink, along with wind speed and sea-ice coverage changes that can also play an important role in some regions, particularly at high latitudes. This new reconstructed coastal pCO2-product (https://doi.org/10.25921/4sde-p068; Roobaert et al. 2023) allows us to establish regional carbon budgets requiring high-resolution coastal flux estimates and provides new constraints for closing the global carbon cycle.info:eu-repo/semantics/publishe

A novel sea surface pCO2-product for the global coastal ocean resolving trends over the 1982–2020 period

In recent years, advancements in machine learning based interpolation methods have enabled the production of high-resolution maps of sea surface partial pressure of CO2 (pCO2) derived from observations extracted from databases such as the Surface Ocean CO2 Atlas (SOCAT). These pCO2-products now allow quantifying the oceanic air-sea CO2 exchange based on observations. However, most of them do not yet explicitly include the coastal ocean. Instead, they simply extend the open ocean values onto the nearshore shallow waters, or their spatial resolution is simply so coarse that they do not accurately capture the highly heterogeneous spatiotemporal pCO2 dynamics of coastal zones. Until today, only one global pCO2-product was specifically designed for the coastal ocean (Laruelle et al. 2017). This product however has shortcomings because it only provides a climatology covering a relatively short period (1998–2015), thus hindering its application to the evaluation of the interannual variability and the long-term trends of the coastal air-sea CO2 exchange, a temporal evolution that is still poorly understood and highly debated. Here we aim at closing this knowledge gap and update the coastal product of Laruelle et al. (2017) to investigate the longest global monthly time series available for the coastal ocean from 1982 to 2020. The method remains based on a 2-step Self Organizing Maps and Feed Forward Network method adapted for coastal regions, but we include additional environmental predictors and use a larger pool of training and validation data with ~ 18 million direct observations extracted from the latest release of the SOCAT database. Our study reveals that the coastal ocean has been acting as an atmospheric CO2 sink of -0.4 Pg C yr-1 (-0.2 Pg C yr-1 with a narrower coastal domain) on average since 1982, and the intensity of this sink has increased at a rate of 0.1 Pg C yr-1 decade-1 (0.03 Pg C yr-1 decade-1 with a narrower coastal domain) over time. Our results also show that the temporal trend in the air-sea pCO2 gradient plays a significant role in the decadal evolution of the coastal CO2 sink, along with wind speed and sea-ice coverage changes that can also play an important role in some regions, particularly at high latitudes. This new reconstructed coastal pCO2-product (Roobaert et al. 2023, https://www.ncei.noaa.gov/archive/accession/0279118) allows establishing regional carbon budgets requiring high-resolution coastal flux estimates and provides new constraints for closing the global carbon cycle.info:eu-repo/semantics/publishe

A framework to evaluate and elucidate the driving mechanisms of coastal sea surface pCO2 seasonality using an ocean general circulation model (MOM6-COBALT)

info:eu-repo/semantics/publishe

SeaFlux: harmonization of air–sea CO<sub>2</sub> fluxes from surface <i>p</i>CO<sub>2</sub> data products using a standardized approach

International audienceAbstract. Air–sea flux of carbon dioxide (CO2) is a critical component of the global carbon cycle and the climate system with the ocean removing about a quarter of the CO2 emitted into the atmosphere by human activities over the last decade. A common approach to estimate this net flux of CO2 across the air–sea interface is the use of surface ocean CO2 observations and the computation of the flux through a bulk parameterization approach. Yet, the details for how this is done in order to arrive at a global ocean CO2 uptake estimate vary greatly, enhancing the spread of estimates. Here we introduce the ensemble data product, SeaFlux (Gregor and Fay, 2021, https://doi.org/10.5281/zenodo.5482547​​​​​​​, https://github.com/luke-gregor/pySeaFlux, last access: 9 September 2021​​​​​​​); this resource enables users to harmonize an ensemble of products that interpolate surface ocean CO2 observations to near-global coverage with a common methodology to fill in missing areas in the products. Further, the dataset provides the inputs to calculate fluxes in a consistent manner. Utilizing six global observation-based mapping products (CMEMS-FFNN, CSIR-ML6, JENA-MLS, JMA-MLR, MPI-SOMFFN, NIES-FNN), the SeaFlux ensemble approach adjusts for methodological inconsistencies in flux calculations. We address differences in spatial coverage of the surface ocean CO2 between the mapping products, which ultimately yields an increase in CO2 uptake of up to 17 % for some products. Fluxes are calculated using three wind products (CCMPv2, ERA5, and JRA55). Application of a scaled gas exchange coefficient has a greater impact on the resulting flux than solely the choice of wind product. With these adjustments, we present an ensemble of global surface ocean pCO2 and air–sea carbon flux estimates. This work aims to support the community effort to perform model–data intercomparisons which will help to identify missing fluxes as we strive to close the global carbon budget

A Synthesis of Global Coastal Ocean Greenhouse Gas Fluxes

The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO2) and releasing nitrous oxide (N2O) and methane (CH4). In this second phase of the Regional Carbon Cycle Assessment and Processes (RECCAP2), we quantify global coastal ocean fluxes of CO2, N2O and CH4 using an ensemble of global gap-filled observation-based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO2 in both observational products and models, but the magnitude of the median net global coastal uptake is ∼60% larger in models (−0.72 vs. −0.44 PgC year−1, 1998–2018, coastal ocean extending to 300 km offshore or 1,000 m isobath with area of 77 million km2). We attribute most of this model-product difference to the seasonality in sea surface CO2 partial pressure at mid- and high-latitudes, where models simulate stronger winter CO2 uptake. The coastal ocean CO2 sink has increased in the past decades but the available time-resolving observation-based products and models show large discrepancies in the magnitude of this increase. The global coastal ocean is a major source of N2O (+0.70 PgCO2-e year−1 in observational product and +0.54 PgCO2-e year−1 in model median) and CH4 (+0.21 PgCO2-e year−1 in observational product), which offsets a substantial proportion of the coastal CO2 uptake in the net radiative balance (30%–60% in CO2-equivalents), highlighting the importance of considering the three greenhouse gases when examining the influence of the coastal ocean on climate.ISSN:0886-6236ISSN:1944-922