Abstract
The Arctic Ocean is particularly vulnerable to ocean acidification, a process that is mainly driven by the uptake of anthropogenic carbon (Cant) from the atmosphere. Although Cant concentrations cannot be measured directly in the ocean, they have been estimated using data‐based methods such as the transient time distribution (TTD) approach, which characterizes the ventilation of water masses with inert transient tracers, such as CFC‐12. Here, we evaluate the TTD approach in the Arctic Ocean using an eddying ocean model as a test bed. When the TTD approach is applied to simulated CFC‐12 in that model, it underestimates the same model's directly simulated Cant concentrations by up to 12%, a bias that stems from its idealized assumption of gas equilibrium between atmosphere and surface water, both for CFC‐12 and anthropogenic CO2. Unlike the idealized assumption, the simulated partial pressure of CFC‐12 (pCFC‐12) in Arctic surface waters is undersaturated relative to that in the atmosphere in regions and times of deep‐water formation, while the simulated equivalent for Cant is supersaturated. After accounting for the TTD approach's negative bias, the total amount of Cant in the Arctic Ocean in 2005 increases by 8% to 3.3 ± 0.3 Pg C. By combining the adjusted TTD approach with scenarios of future atmospheric CO2, it is estimated that all Arctic waters, from surface to depth, would become corrosive to aragonite by the middle of the next century even if atmospheric CO2 could be stabilized at 540 ppm.
Highlights
Ocean uptake of anthropogenic carbon (Cant) drives reductions in ocean pH and carbonate ion concentration, the latter of which reduces the calcium carbonate (CaCO3) saturation state (Orr et al, 2005)
The transient time distribution (TTD) approach could not be evaluated in some parts of the Arctic Ocean because of our simulation strategy, which initialized all variables in the high-resolution configuration ORCA025 in 1958 with results from the coarse-resolution configuration ORCA05
When evaluating the TTD approach in the model's younger waters that are affected little by the change in resolution in 1958, the calculated CTanTtD concentrations are seen to underestimate the reference CNanEtMO concentrations by 7 ± 2% (2.5 ± 0.9 μmol kg−1 ) in Summer Pacific Water (SPW), by 12 ± 3% (4.9 ± 1.3 μmol kg−1) in Winter Pacific Water (WPW), by 4 ± 2% (1.1 ± 0.6 μmol kg−1) in Atlantic Water (AW), and by 12 ± 2% (5.0 ± 1.1 μmol kg−1) in Barents Sea Water (BSW) (Table S1)
Summary
Ocean uptake of anthropogenic carbon (Cant) drives reductions in ocean pH and carbonate ion concentration, the latter of which reduces the calcium carbonate (CaCO3) saturation state (Orr et al, 2005). The Arctic ASH is projected to shoal by ∼100 m during the 21st century based on a coupled carbon-climate model forced under the SRES A2 scenario (Steinacher et al, 2009). The same model projects that the aragonite undersaturation in surface waters will propagate downwards rapidly, merging with the shoaling ASH at about 1,800 m by the end of the century. A data-based approach that imposes exponentially increasing atmospheric CO2 suggests that the deep ASH will shoal by much more (∼900 m) and will merge with the deepening undersaturation from the surface at about 1,000 m by the end of the century (Anderson et al, 2010)
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