A Synthesis of Global Coastal Ocean Greenhouse Gas Fluxes
Laure Resplandy, Allison Hogikyan, Hermann W. Bange, Daniele Bianchi, Thomas Weber, Wei‐Jun Cai, Scott C. Doney, Katja Fennel, Marion Gehlen, Judith Hauck, Fabrice Lacroix, Peter Landschützer, Corinne Le Quéré, Jens Daniel Müller, Raymond G. Najjar, Alizée Roobaert, Sarah Berthet, Laurent Bopp, Trang Thi-Tuyet Chau, Minhan Dai, Nicolas Gruber, Tatiana Ilyina, Annette Kock, Manfredi Manizza, Zouhair Lachkar, Goulven G. Laruelle, Enhui Liao, Ivan D. Lima, Cara Nissen, Christian Rödenbeck, Roland Séférian, Jörg Schwinger, Katsuya Toyama, Hiroyuki Tsujino, Pierre Regnier
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). Major advances have improved our understanding of the coastal air-sea exchanges of these three gasses since the first phase of the Regional Carbon Cycle Assessment and Processes (RECCAP in 2013), but a comprehensive view that integrates the three gasses at the global scale is still lacking. In this second phase (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/yr, 1998-2018, coastal ocean 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 global coastal ocean is a major source of N2O (+0.70 PgCO2-e /yr in observational product and +0.54 PgCO2-e /yr in model median) and of CH4 (+0.21 PgCO2-e /yr in observational product), which offsets a substantial proportion of the net radiative effect of coastal \co uptake (35-58% in CO2-equivalents). Data products and models need improvement to better resolve the spatio-temporal variability and long term trends in CO2, N2O and CH4 in the global coastal ocean.