1.5.2.2 Interactions between ice sheets and AMOC

The AMOC, Greenland Ice Sheet (GrIS), and West Antarctic Ice Sheet (WAIS) are key tipping systems and are threatened by increasing CO₂ emissions and temperatures (Armstrong McKay et al., 2022; Pörtner et al., 2019). Moreover, GrIS, AMOC, and WAIS interact on very different timescales, ranging from decades to multiple centuries. While some of those links might be stabilising, others are destabilising and would allow for the possibility of large-scale cascading events.

Greenland Ice Sheet to AMOC 

The AMOC depends on the formation of dense, salty water in the high latitudes of the North Atlantic. As GrIS melting increases (1.2.2.1), the associated discharge of salt-free freshwater in the ocean will decrease surface water salinity and thereby density, inhibiting the formation of dense waters and weakening the circulation. As less salt is transported to the North Atlantic, the salt-advection feedback implies a self-sustained freshening of the high latitudes of the North Atlantic, which, in the worst case, can result in the collapse of the AMOC (1.4.2.1). On top of this classic positive/amplifying feedback, there exists a wide range of other feedbacks related to the AMOC, either negative (heat advection feedback) or positive (evaporation feedback). 

An overall destabilising impact of GrIS melting on the AMOC is mostly consistent across models, where adding freshwater in the North Atlantic (Jackson and Wood, 2018; Mecking et al., 2016; Stouffer et al., 2007), also in combination with increasing CO₂ emissions (Bakker et al., 2016; Swingedouw et al., 2006), leads to a substantial weakening of the circulation. Importantly, in the case of AMOC collapse, some models suggest it does not recover within century timescales (Jackson and Wood, 2018; Mecking et al., 2016). Note, however, that estimated melt rates of the GrIS are generally smaller than the amount of freshwater additions in models necessary to collapse the AMOC (Sinet et al., 2023, Jackson and Wood 2018), and it is currently a smaller contributor than increased Arctic precipitation.

West Antarctic Ice Sheet to AMOC

In the case of freshwater release in the Southern Hemisphere originating from West Antarctica, different opposing processes are at play that could affect the AMOC. These effects have been identified to act on different timescales and depend on the state of the circulation (Berk et al., 2021; Swingedouw et al., 2009). First, the weakening of Antarctic Bottom Water (AABW; see 1.4.2.2) formation might lead to enhancement of the AMOC through the so-called ‘ocean bipolar seesaw’. This describes the tendency for opposing temperature changes in the Southern and Northern Hemisphere, with ocean bottom water changes in response to ice sheet melt in either hemisphere taking a long time to affect the other hemisphere. 

Second, the increase in wind intensity over the Southern Hemisphere, related to an increase in sea ice cover, might also help to enhance the AMOC (Li et al., 2023; Swingedouw et al., 2008). Third, the release of freshwater in the Southern Ocean might eventually reach the North Atlantic on a longer timescale (centuries), possibly weakening the AMOC. As a result, the impact of a WAIS collapse on the AMOC is still unclear, as most models show either a slight weakening (e.g. Stouffer et al., 2007; Seidov et al., 2005) or a slight strengthening (e.g. Swingedouw et al., 2009) of the circulation. Notably, some studies also found that a sufficient freshwater release into the Southern Ocean allows for delaying an AMOC collapse (Sadai et al., 2020), or a recovery from it (Weaver et al., 2003).

AMOC to ice sheets

An AMOC collapse would decrease northward heat transport, leading to a substantial cooling of the Northern Hemisphere, and warming in the Southern Hemisphere (Pedro et al., 2018; Jackson et al., 2015; Stouffer et al., 2006). Cooling the high latitudes of the North Atlantic could stabilise the GrIS. Conversely, the related warming of the Southern Ocean represents a destabilising impact on the WAIS, being susceptible to these warmer ocean waters via the ice shelves and their buttressing effect on upstream ice flow (Favier et al., 2014; Joughin et al., 2014).

Direct interactions between Greenland and West Antarctic ice sheets via sea level feedbacks

It is known that an increase in sea level has an overall destabilising influence on marine-based sectors of ice sheets, possibly triggering or enhancing the retreat of their grounding line (Schoof, 2007; Weertman, 1974). In the case of ice sheet collapse, the induced sea level rise would vary locally depending on gravitational effects (with sea level falling near the former ice sheet as less water is attracted towards it), rotational effects, and mantle deformation (Kopp et al., 2010; Mitrovica et al., 2009). Overall, sea level rise is expected to negatively impact both the GrIS and WAIS, but more strongly the latter, where most of the bedrock lies well below sea level (Gomez et al., 2020).