Global coastal ocean CO2 trends over the 1982-2020 period
Roobaert, A.; Regnier, P.; Landschützer, P.; Laruelle, G.G. (2024). Global coastal ocean CO2 trends over the 1982-2020 period, in: EGU General Assembly 2024. Vienna, Austria & Online, 14-19 April 2024. pp. EGU24-14775. https://dx.doi.org/10.5194/egusphere-egu24-14775
In: (2024). EGU General Assembly 2024. Vienna, Austria & Online, 14-19 April 2024. European Geosciences Union: [s.l.].
The development of high-quality controlled databases of sea surface partial pressure of CO2 (pCO2) combined with robust machine learning-based mapping methods that fill pCO2 gaps in time and space, enable us to quantify the oceanic air-sea CO2 exchange and its spatiotemporal variability only based on in-situ observations (pCO2-products). However, most existing pCO2-products do not explicitly include the coastal ocean or have a spatial resolution that is too coarse (e.g., 1°) to capture the highly heterogeneous spatiotemporal dynamics of pCO2 in these regions thus limiting our ability to resolve long-term trends and the interannual variability of the coastal air-sea CO2 exchange (FCO2).To address this limitation, we updated the global coastal pCO2-product of Laruelle et al. (2017) using a 2-step machine learning interpolation technique (relying on Self Organizing Maps and a Feed Forward neural Network) combined with the most extensive monthly time series for coastal waters from the Surface Ocean CO2 Atlas (SOCAT), spanning from 1982 to 2020 to reconstruct monthly high spatial resolution (0.25°) continuous coastal pCO2 maps. This updated coastal pCO2-product is then used to reconstruct the temporal evolution of the global coastal FCO2 based on observations.Our results show that since 1982, the extended coastal ocean, covering an area of 77 million km² in this study, has been acting as an atmospheric CO2 sink, removing -0.4 Pg C yr-1 (-0.2 Pg C yr-1 with a narrower coastal domain roughly equivalent to continental shelves) from the atmosphere. Moreover, the intensity of this CO2 sink has been increasing over time at a rate of 0.1 Pg C yr-1 per decade (0.03 Pg C yr-1 decade-1 in the narrower domain). The long-term change in the air-sea CO2 flux is largely driven by the air-sea pCO2 gradient, dominated by the sea surface pCO2, however wind speed and sea-ice coverage play significant roles, regionally. This new coastal pCO2-product provides a valuable constraint for understanding the strengthening of the global coastal ocean CO2 sink, fill the coastal gap in synthesis studies such as the Global Carbon Budget and serves as a benchmark for evaluating emerging results of ocean biogeochemical models.
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