Carbon Fluxes in the Coastal Ocean: Synthesis, Boundary Processes, and Future Trends

Abstract

This review examines the current understanding of the global coastal ocean carbon cycle and provides a new quantitative synthesis of air-sea CO2 exchange. This reanalysis yields an estimate for the globally integrated coastal ocean CO2 flux of −0.25 ± 0.05 Pg C year−1, with polar and subpolar regions accounting for most of the CO2 removal (>90%). A framework that classifies river-dominated ocean margin (RiOMar) and ocean-dominated margin (OceMar) systems is used to conceptualize coastal carbon cycle processes. The carbon dynamics in three contrasting case study regions, the Baltic Sea, the Mid-Atlantic Bight, and the South China Sea, are compared in terms of the spatio-temporal variability of surface pCO2. Ocean carbon models that range from box models to three-dimensional coupled circulation-biogeochemical models are reviewed in terms of the ability to simulate key processes and project future changes in different continental shelf regions. Common unresolved challenges remain for implementation of these models across RiOMar and OceMar systems. The long-term trends in coastal ocean carbon fluxes for different coastal systems under anthropogenic stress that are emerging in observations and numerical simulations are highlighted. Knowledge gaps in projecting future perturbations associated with before and after net-zero CO2 emissions in the context of concurrent changes in the land-ocean-atmosphere coupled system pose a key challenge. ▪ A new synthesis yields an estimate for globally integrated coastal ocean carbon sink of −0.25 Pg C year−1, with greater than 90% of atmospheric CO2 removal occurring in polar and subpolar regions. ▪ The sustained coastal and open ocean carbon sink is vital in mitigating climate change and meeting the target set by the Paris Agreement. ▪ Uncertainties in the future coastal ocean carbon cycle are associated with concurrent trends and changes in the land-ocean-atmosphere coupled system. ▪ The major gaps and challenges identified for current coastal ocean carbon research have important implications for climate and sustainability policies. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 50 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Mismatch of N release from the permafrost and vegetative uptake opens pathways of increasing nitrous oxide emissions in the high Arctic

“…Lateral water and biogeochemical exports from the permafrost are, however, still poorly constrained (Lacroix, Ilyina, Mathis, et al, 2021; Lacroix et al, 2020), and difficult to assess in our model‐based study. These lateral exports could however have important implications for high‐Arctic freshwater systems (Sanders et al, 2022) and Arctic shelves (Dai et al, 2022; Lacroix, Ilyina, Laruelle, et al, 2021; Terhaar et al, 2021).…”

Spatial reconstruction of long-term (2003–2020) sea surface pCO2 in the South China Sea using a machine learning based regression method aided by empirical orthogonal function analysis

“…In addition to the above observational data, we selected four independent surveys corresponding to four seasons, and a continuous observation dataset during 2003-2019 at the Southeast Asia Time-Series (SEATs) station (Dai et al, 2022) as two important independent testing datasets. 1.…”

“…The data (the reconstructed CO2 data, the Observational CO2 data, and the remote sensing derived CO2 data) for this paper are available under the link https://github.com/Elricriven/co2data (Wang et al, 2022).…”

“…However, it remains largely unknown whether and by how much the ocean will continue to act as a sink for anthropogenic CO2, i.e., the extent of its climate change mitigation capacity to understand climate-carbon coupled systems and develop zero-emission strategies and actions. Ocean margins, an essential part of the land-ocean continuum, contributed ~10-20% of the global ocean CO2 sequestration with only 7% of the surface area and represent a particularly challenging regime (e.g., Chen and Borges, 2009;Dai et al 2022;Laruelle et al, 2014). This is primarily attributed to the ocean margins' extremely complex and dynamic processes, often characterized by large spatial and temporal variability of air-sea CO2 fluxes that lead to even larger uncertainty in their prediction than those occurring in the open ocean (Dai et al, 2013(Dai et al, , 2022Cao et al, 2020;Laruelle et al, 2014;Chen and Borges, 2009 and the references therein).…”

“…Ocean margins, an essential part of the land-ocean continuum, contributed ~10-20% of the global ocean CO2 sequestration with only 7% of the surface area and represent a particularly challenging regime (e.g., Chen and Borges, 2009;Dai et al 2022;Laruelle et al, 2014). This is primarily attributed to the ocean margins' extremely complex and dynamic processes, often characterized by large spatial and temporal variability of air-sea CO2 fluxes that lead to even larger uncertainty in their prediction than those occurring in the open ocean (Dai et al, 2013(Dai et al, , 2022Cao et al, 2020;Laruelle et al, 2014;Chen and Borges, 2009 and the references therein). Limited spatiotemporal coverage of observational data is an important source of these uncertainties.…”