2.2.4.1 Atlantic Meridional Overturning Circulation

The AMOC is a system of ocean currents that transports warm waters northwards in the Atlantic (Buckley and Marshall, 2016). The AMOC is considered to be a tipping element of the climate system (see Chapter 1.4). However, although it is considered very likely to weaken over the next century due to anthropogenic climate change (Fox-Kemper et al., 2021), it is considered unlikely to collapse.

If the AMOC were to collapse, it would have significant global consequences. The overall impact would depend on the level of global warming that had already occurred by the time of collapse. Large-scale temperature changes are likely to be additive (Vellinga and Wood, 2008), so the large cooling seen from an AMOC collapse over the North Atlantic ocean is likely to dominate the warming, however changes over land are more uncertain. Yet, for other impacts, an AMOC collapse may exacerbate changes caused by global warming.

Since the AMOC transports heat northwards in the Atlantic, a collapse would tend to cause a significant cooling in the North Atlantic Ocean, which would drive cooler temperatures over much of the Northern Hemisphere, especially Europe and North America, and potentially across the whole hemisphere (Bellomo et al., 2021; Jackson et al., 2015; Stouffer et al., 2006). This, however, would compete with the effects of global warming, with the net effect depending on the magnitude of the latter. The reduced heat transport would slightly add to warming in the Southern Hemisphere. Cooler ocean temperatures in the North Atlantic would drive reduced evaporation and hence less atmospheric water vapour for precipitation (Bellomo et al., 2021; Jackson et al., 2015; Stouffer et al., 2006). They would also result in an increase in Arctic sea ice. 

Changes in sea surface temperature (SST) gradients also affect atmospheric circulation patterns, which have significant impacts on regional climate. One major change due to AMOC collapse would be a southwards shift in the Intertropical Convergence Zone (ITCZ), which is a region in the tropics where north and south trade winds meet and there is heavy rainfall (Bellomo et al., 2021; Jackson et al., 2015; Stouffer et al., 2006). Changes in SST patterns in the North Atlantic have also been shown to affect the North Atlantic Oscillation (NAO), which affects weather over Europe (Bellomo et al., 2022; Jackson et al., 2015; Brayshaw, 2009). 

Another large-scale impact is from changes to sea level associated with the changing ocean currents. A collapse of the AMOC would cause significant increases to sea level throughout the North Atlantic, which would have impacts on the western coasts of Europe and the eastern coasts of North America (Little et al., 2019; Hu et al., 2009; Leverman et al., 2005; Kienert and Rahmstorf, 2012; Lorbacher et al., 2010)

In Europe and North America we would generally expect colder winters as a result of an AMOC collapse, with more precipitation falling as snow and more cold extremes (Wang et al., 2022; Jacob, 2005; Vellinga and Wood, 2002). Although there would be less precipitation in general, the shift to more positive NAO would lead to more winter storms (Bellomo et al., 2022; Jackson et al., 2015; Brayshaw, 2009) and hence windier weather with more precipitation on western coasts of northern Europe (Bellomo et al.,  2022; Jackson et al., 2015). In the summer, an AMOC collapse would cause a reduction in cloud amount and an anomalous high pressure system over northern Europe, resulting in more precipitation over southern Europe and less over northern Europe (Jackson et al., 2015). In Britain, this could lead to a widespread cessation of arable farming, causing large reductions in water supply and losses of agricultural output an order of magnitude larger than those arising from climate change without AMOC collapse (Figure 2.2.7; Ritchie et al., 2020).

In the tropics, an AMOC collapse would cause a southwards shift of the ITCZ, and hence a shift of the monsoon rains in central/southern America and West Africa. There is also evidence that there would be shifts for the South Asian and Indian monsoons. Shifts in monsoons would cause significant changes in seasonal precipitation, with some regions receiving much more rain, some much less, and some with shift of rain to different seasons, potentially causing severe regional impacts (Sandeep et al., 2020; Defrance, 2017; Marzin, 2013; Parsons et al., 2013; Chang et al., 2008; Zhang and Delworth, 2005). The large shifts in monsoon rainfall over the tropics associated with AMOC collapse would be expected to have major impacts on vegetation productivity worldwide, including crop productivity, with decreases in many regions such as Western and Central Africa, Central America, Northern South America and eastern Europe, but increases in other regions such as north-east South America and southern Africa  (Vellinga and Wood, 2002).