26 Jun 2023
 | 26 Jun 2023
Status: this preprint is currently under review for the journal ESSD.

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

Alizée Roobaert, Pierre Regnier, Peter Landschützer, and Goulven G. Laruelle

Abstract. 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, allows establishing regional carbon budgets requiring high-resolution coastal flux estimates and provides new constraints for closing the global carbon cycle.

Alizée Roobaert et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on essd-2023-228', Zelun Wu, 17 Aug 2023
  • RC2: 'Comment on essd-2023-228', Anonymous Referee #2, 17 Sep 2023

Alizée Roobaert et al.

Data sets

A novel sea surface partial pressure of carbon dioxide (pCO2) data product for the global coastal ocean resolving trends over the 1982-2020 period (NCEI Accession 0279118) Alizée Roobaert, Pierre Regnier, Peter Landschützer and Goulven G. Laruelle

Alizée Roobaert et al.


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Short summary
Advancements in understanding the coastal air-sea CO2 exchange (FCO2) have been made, but long-term temporal trends remain unclear. Based on observations and a machine learning approach, we reconstructs the longest global time series of coastal FCO2 (1982 to 2020). Results show the coastal ocean acts as a CO2 sink, with increasing intensity over time. This new coastal FCO2 product allows establishing regional carbon budgets and provides new constraints for closing the global carbon cycle.