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 (CO 2 ) and releasing nitrous oxide (N 2 O) and methane (CH 4 ). In this second phase of the Regional Carbon Cycle Assessment and Processes (RECCAP2), we quantify global coastal ocean fluxes of CO 2 , N 2 O and CH 4 using an ensemble of global gap‐filled observation‐based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO 2 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 km 2 ). We attribute most of this model‐product difference to the seasonality in sea surface CO 2 partial pressure at mid‐ and high‐latitudes, where models simulate stronger winter CO 2 uptake. The coastal ocean CO 2 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 N 2 O (+0.70 PgCO 2 ‐e year −1 in observational product and +0.54 PgCO 2 ‐e year −1 in model median) and CH 4 (+0.21 PgCO 2 ‐e year −1 in observational product), which offsets a substantial proportion of the coastal CO 2 uptake in the net radiative balance (30%–60% in CO 2 ‐equivalents), highlighting the importance of considering the three greenhouse gases when examining the influence of the coastal ocean on climate.