A Synthesis of Global Coastal Ocean Greenhouse Gas Fluxes

Abstract

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.

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

“…Coastal CO 2 sources are largely located in the tropics and subtropics (40°S-40°N), while CO 2 sinks are found at temperate (between 40°and 60°of latitude) and high (poleward of 60°) latitudes in both hemispheres (Figures 1a and 1b), in agreement with prior studies (e.g., Borges et al, 2005;Cai, 2011; C. T. A. Chen et al, 2013;Dai et al, 2022;Laruelle et al, 2010Laruelle et al, , 2014Resplandy et al, 2024;Roobaert et al, 2019). The strongest CO 2 sources (FCO 2 values > 0.5 mol C m 2 yr 1 ) are mainly encountered along upwelling currents, while the strongest sinks (FCO 2 values < 0.5 mol C m 2 yr 1 ) mainly occur in temperate and high latitudes of the Northern Hemisphere, along river plumes (e.g., the Amazon River) as well as along intracoastal alongshore currents (e.g., the Gulf stream).…”

“…T. A. Chen & Borges, 2009; C. T. A. Chen et al, 2013;Dai et al, 2013;Laruelle et al, 2010), with the latest estimates pointing toward the lower estimates (Dai et al, 2022;Regnier et al, 2022;Resplandy et al, 2024;Roobaert et al, 2019). This global uptake is the cumulated effect of strong coastal CO 2 sinks in temperate regions and at high latitudes, partially compensated by weak coastal CO 2 sources in sub-tropical and tropical regions (e.g., Borges et al, 2005;Cai, 2011;Cao et al, 2020; C. T. A.…”

“…This global uptake is the cumulated effect of strong coastal CO 2 sinks in temperate regions and at high latitudes, partially compensated by weak coastal CO 2 sources in sub-tropical and tropical regions (e.g., Borges et al, 2005;Cai, 2011;Cao et al, 2020; C. T. A. Chen et al, 2013;Dai et al, 2022;Laruelle et al, 2010Laruelle et al, , 2014Resplandy et al, 2024;Roobaert et al, 2019). This large-scale latitudinal gradient suggests that sea surface temperature (SST) is a first order control on the sea surface partial pressure of CO 2 (called hereafter pCO 2 ) and hence on the coastal air-sea CO 2 flux (FCO 2 ) (e.g., Laruelle et al, 2014).…”

“…While the spatial distribution of the coastal FCO 2 is now relatively well constrained thanks to the significant increase in field measurements (19 million of coastal pCO 2 data in the last release of the Surface Ocean CO 2 Atlas data product, SOCATv2022, Bakker et al, 2022) and the popularization of advanced interpolation techniques allowing the production of data driven spatially continuous monthly pCO 2 climatologies (Chau et al, 2022;Fay et al, 2021;Landschützer et al, 2020;Laruelle et al, 2017;Roobaert et al, 2024), on seasonal timescale its variability is less known at the global scale and only recently started to be investigated (e.g., Cao et al, 2020;Dai et al, 2022;Laruelle et al, 2014;Resplandy et al, 2024;Roobaert et al, 2019). At first order, FCO 2 seasonal amplitude is large at temperate and high latitudes and decreases equatorward.…”

High-resolution Neural Network Demonstrates Strong CO2 Source-Sink Juxtaposition in the Coastal Zone

“…The global ocean takes up nearly a quarter of anthropogenic carbon dioxide (CO 2 ) emissions annually (Friedlingstein et al, 2023). It has been suggested coastal oceans contribute disproportionately to oceanic CO 2 uptake relative to global ocean by surface area (Bourgeois et al, 2016;Chau et al, 2022;Laruelle et al, 2014;Resplandy et al, 2024;Roobaert et al, 2019Roobaert et al, , 2024, but exhibit far greater heterogeneity in air-sea CO 2 fluxes (Liu et al, 2010) and may be changing at a different rate compared to the open ocean (Laruelle et al, 2018;Resplandy et al, 2024). Coastal oceans serve as an important hub of exchange, outgassing carbon delivered by terrestrial ecosystems to the ocean (Regnier et al, 2022), while facilitating transport between the coast and open ocean, and directly absorbing CO 2 from the atmosphere (Bauer et al, 2013; C.-T.…”