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When freshwater injection on the Southern Ocean is halted, global temperature jumps back within two decades to the value it would have had without any freshwater addition (Fig. 10a). Quick recovery is consistent with the Southern Ocean-centric15picture of the global overturning circulation (Fig. 4, Talley, 2013), as the Southern Meridional Overturning Circulation (SMOC), driven by AABW formation, responds to change of the vertical stability of the ocean column near Antarctica (see Sect. 4) and the ocean mixed layer and sea ice have limited thermal inertia.Global cooling due to ice melt causes a large increase in Earth’s energy imbalance20(Fig. 10b), adding about+2 W m−2, which is larger than the imbalance caused by in-creasing GHGs. Thus, although the cold fresh water from ice sheet disintegration pro-vides a negative feedback on regional and global surface temperature, it increases the planet’s energy imbalance, thus providing more energy for ice melt (Hansen, 2005).This added energy is pumped into the ocean. Southern Ocean Upwelling North Atlantic Deep Water is primarily upwelled at the southern end of the Atlantic transect, in the Southern Ocean.[8] This upwelling comprises the majority of upwelling normally associated with AMOC, and links it with the global circulation[1]. On a global scale, observations suggest 80% of deepwater upwells in the Southern Ocean. [28] This upwelling supplies large quantities of nutrients to the surface, which supports biological activity. Surface supply of nutrients is critical to the ocean's functioning as a carbon sink on long timescales. Furthermore, upwelled water has low concentrations of dissolved carbon, as the water is typically 1000 years old and has not been sensitive to anthropogenic CO2 increases in the atmosphere.[29] Because of its low carbon concentration, this upwelling functions as a carbon sink. Variability in the carbon sink over the observational period has been closely studied and debated[30]. The size of the sink is understood to have decreased until 2002, and then increased until 2012.[31] After upwelling, the water is understood to take one of two pathways: water surfacing near to sea-ice generally forms dense bottomwater and is committed to AMOC's lower cell; water surfacing at lower latitudes moves further northward due to Ekman transport and is committed to the upper cell. [8][32]
