A Synthesis of Global Coastal Ocean Greenhouse Gas Fluxes

Abstract

International audienceThe 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