Decadal variations and trends of the global ocean carbon sink

Abstract

We investigate the variations of the ocean CO2 sink during the past three decades using global surface ocean maps of the partial pressure of CO2 reconstructed from observations contained in the Surface Ocean CO2 Atlas Version 2. To create these maps, we used the neural network-based data interpolation method of Landschützer et al. (2014) but extended the work in time from 1998 to 2011 to the period from 1982 through 2011. Our results suggest strong decadal variations in the global ocean carbon sink around a long-term increase that corresponds roughly to that expected from the rise in atmospheric CO2. The sink is estimated to have weakened during the 1990s toward a minimum uptake of only -0.8 ± 0.5 Pg C yr-1 in 2000 and thereafter to have strengthened considerably to rates of more than -2.0 ± 0.5 Pg C yr-1. These decadal variations originate mostly from the extratropical oceans, while the tropical regions contribute primarily to interannual variations. Changes in sea surface temperature affecting the solubility of CO2 explain part of these variations, particularly at subtropical latitudes. But most of the higher-latitude changes are attributed to modifications in the surface concentration of dissolved inorganic carbon and alkalinity, induced by decadal variations in atmospheric forcing, with patterns that are reminiscent of those of the Northern and Southern Annular Modes. These decadal variations lead to a substantially smaller cumulative anthropogenic CO2 uptake of the ocean over the 1982 through 2011 period (reduction of 7.5 ± 5.5 Pg C) relative to that derived by the Global Carbon Budget. © 2016. American Geophysical Union. All Rights Reserved

Global high-resolution monthly <i>p</i>CO<sub>2</sub> climatology for the coastal ocean derived from neural network interpolation

“…This approach facilitates future integration with existing global ocean data products (e.g., Landschützer et al, 2016;Rödenbeck et al, 2015) and model outputs, which typically struggle to represent the shallowest parts of the ocean (Bourgeois et al, 2016). The outer limit used here is given by whichever point is the furthest from the coast: either 300 km distance from the coastline (which roughly corresponds to the outer edge of territorial waters; Crossland et al, 2005) or the 1000 m isobaths .…”

“…These calculations are done iteratively using a sigmoid activation function on an incomplete dataset in order to perform an assessment on the remaining data after each iteration, until an optimal relationship is found. Additionally, as performed in Landschützer et al (2015), the output pCO 2 data were smoothed using the spatial and temporal mean of each point's neighboring pixels in both time and space within the 3-pixel neighborhood domain. This operation is performed iteratively and does not significantly alter the results, but it ensures smoother transitions in the pCO 2 field at the boundaries between the provinces.…”

“…By filling in the spatial and temporal gaps, these products greatly facilitate the calculation of the air-sea CO 2 exchange, as they do not require separate assumptions about the surface ocean pCO 2 in areas lacking data. Such methods are well suited to resolve spatial gradients, and they also permit us to determine seasonal and interannual variations and trends in pCO 2 (e.g., Landschützer et al, 2014Landschützer et al, , 2015Landschützer et al, , 2016Zeng et al, 2014). Because of the small relative contribution of the coastal ocean to the total oceanic surface area and the relatively coarse spatial resolution of the ANN-based surface ocean pCO 2 products so far, they are not well suited to resolve the high spatiotemporal variations of the surface ocean pCO 2 fields along the shelves.…”

“…The definition of the coastal/open-oceanic boundary varies strongly from one study to the other (Walsh, 1988;Laruelle et al, 2013), with a potentially large impact on the shelf CO 2 budget (Laruelle et al, 2010). Here, we use a very wide definition for this boundary (i.e., 300 km width or 1000 m depth) to secure spatial continuity between our new shelf pCO 2 product and those already existing for the open ocean (Landschützer et al, , 2016Rödenbeck et al, 2015). Our approach leads to the first continuous and monthly resolved pCO 2 maps over the 1998-2015 period across the global shelf region, permitting us to study the seasonal dynamics of these regions in relationship to that of the adjacent open ocean.…”

Uncertainty in the global oceanic CO<sub>2</sub> uptake induced by wind forcing: quantification and spatial analysis

“…The pCO 2 fields are based on measurements of the Surface Ocean CO 2 Atlas version 2 (SOCATv2) dataset ) using a two-step artificial neuronal network (Landschützer et al, 2015a) to generate continuous monthly 1 • × 1 • resolution pCO 2 maps for the global ocean excluding the Arctic Ocean, coastal regions and marginal seas. A more detailed description of the method and its extensive evaluation can be found in Landschützer et al ( , 2014Landschützer et al ( , 2016. Four global wind speed datasets are used to evaluate the sensitivity of F CO 2 to the choice of one wind product over the other.…”