Abstract
Abstract The role of sea ice in the carbon cycle is minimally represented in current Earth System Models (ESMs). Among potentially important flaws, mentioned by several authors and generally overlooked during ESM design, is the link between sea-ice growth and melt and oceanic dissolved inorganic carbon (DIC) and total alkalinity (TA). Here we investigate whether this link is indeed an important feature of the marine carbon cycle misrepresented in ESMs. We use an ocean general circulation model (NEMO-LIM-PISCES) with sea-ice and marine carbon cycle components, forced by atmospheric reanalyses, adding a first-order representation of DIC and TA storage and release in/from sea ice. Our results suggest that DIC rejection during sea-ice growth releases several hundred Tg C yr−1 to the surface ocean, of which < 2% is exported to depth, leading to a notable but weak redistribution of DIC towards deep polar basins. Active carbon processes (mainly CaCO3 precipitation but also ice-atmosphere CO2 fluxes and net community production) increasing the TA/DIC ratio in sea-ice modified ocean-atmosphere CO2 fluxes by a few Tg C yr−1 in the sea-ice zone, with specific hemispheric effects: DIC content of the Arctic basin decreased but DIC content of the Southern Ocean increased. For the global ocean, DIC content increased by 4 Tg C yr−1 or 2 Pg C after 500 years of model run. The simulated numbers are generally small compared to the present-day global ocean annual CO2 sink (2.6 ± 0.5 Pg C yr−1). However, sea-ice carbon processes seem important at regional scales as they act significantly on DIC redistribution within and outside polar basins. The efficiency of carbon export to depth depends on the representation of surface-subsurface exchanges and their relationship with sea ice, and could differ substantially if a higher resolution or different ocean model were used.
Highlights
For the sake of simplicity, the current representation of the role of sea ice in the carbon cycle in Earth System Models (ESMs) is minimalist (Orr et al, 2001)
Among potentially important flaws, mentioned by several authors and generally overlooked during ESM design, is the link between sea-ice growth and melt and oceanic dissolved inorganic carbon (DIC) and total alkalinity (TA). We investigate whether this link is an important feature of the marine carbon cycle misrepresented in ESMs
Our results suggest that DIC rejection during sea-ice growth releases several hundred Tg C yr−1 to the surface ocean, of which < 2% is exported to depth, leading to a notable but weak redistribution of DIC towards deep polar basins
Summary
For the sake of simplicity, the current representation of the role of sea ice in the carbon cycle in Earth System Models (ESMs) is minimalist (Orr et al, 2001). The small remainder of DIC and TA that is not rejected is stored within sea ice, transported by ice drift, and released to the ocean, impacting surface water properties in the regions of net ice melt. Besides these physical contributions, there are active carbon processes in the ice. DIC can be exchanged with the atmosphere in the form of ice-atmosphere CO2 fluxes (e.g., Miller et al, 2011; Papakyriakou and Miller, 2011; Geilfus et al, 2012; Nomura et al, 2013; see Figure 1). Net community production (net primary production minus community respiration) occurs within sea ice through microbial communities, impacting DIC and TA
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