Editing IPCC:AR6/WGI/Chapter-8 (section)

=== 8.6.1 Abrupt Water Cycle Responses to a Collapse of Atlantic Meridional Overturning Circulation ===

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Multiple lines of evidence, including both paleoclimate reconstructions and simulations, suggest that a severe weakening or collapse of Atlantic Meridional Overturning Circulation (AMOC, see Glossary) causes abrupt and profound changes in the global hydrological cycle ( [[#Chiang--2005|Chiang and Bitz, 2005]] ; [[#Broccoli--2006|Broccoli et al., 2006]] ; [[#Chiang--2012|Chiang and Friedman, 2012]] ; [[#Jackson--2015|Jackson et al., 2015]] ; [[#Renssen--2018|Renssen et al., 2018]] ). Deep water formation in the North Atlantic is dependent on a delicate balance of heat and salt fluxes ( [[#Buckley--2016|Buckley and Marshall, 2016]] ); disruption in either of these due to melting ice sheets, a change in precipitation and evaporation, or ocean circulation can force AMOC to cross a tipping point (SROCC; [[#Drijfhout--2015|Drijfhout et al., 2015]] ). During the last deglacial transition, one such slowdown in AMOC – during the Younger Dryas event (12,800–11,700 years ago) – caused worldwide changes in precipitation patterns. These included a southward migration of the tropical ITCZ (Peterson et al., 2000; [[#McGee--2014|McGee et al., 2014]] ; [[#Schneider--2014|Schneider et al., 2014]] ; [[#Mohtadi--2016|Mohtadi et al., 2016]] ; [[#Reimi--2016|Reimi and Marcantonio, 2016]] ; P.X. [[#Wang--2017|]] [[#Wang--2017|]] [[#Wang--2017|Wang et al., 2017]] ) and systematic weakening of the African and Asian monsoons (Tierney and DeMenocal, 2013; Otto-Bliesner et al. , 2014; Cheng et al. , 2016; Grandey et al. , 2016; Wurtzel et al. , 2018). Conversely, the Southern Hemisphere (SH) monsoon systems intensified (Cruz et al. , 2005; Ayliffe et al. , 2013; Stríkis et al. , 2015, 2018; Campos et al. , 2019) . Drying occurred in Meso-America (Lachniet et al., 2013) while the North American monsoon system was largely unaffected (Bhattacharya et al., 2018). The mid-latitude region in North America was wetter (Polyak et al. , 2004; Grimm et al. , 2006; Wagner et al. , 2010; Voelker et al. , 2015) , while Europe was drier (Genty et al., 2006; [[#Rach--2017|Rach et al., 2017]] ; [[#Naughton--2019|Naughton et al., 2019]] ). A transient coupled climate model simulation was able to reproduce the large-scale precipitation response to such an event (Figure 8.27a; [[#Liu--2009|Liu et al., 2009]] ).

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[[File:660261de89307e1162d5893b09404793 IPCC_AR6_WGI_Figure_8_27.png]]

'''Figure 8.27 |''' '''Comparison of reconstructed past and idealized future annual mean precipitation responses to an Atlantic Meridional Overturning Circulation (AMOC) collapse. (a)''' Model simulation of precipitation response to the Younger Dryas event relative to the preceding warm Bølling-Allerød period (base colours, calculated as the difference between 12,600–11,700 years before the present (BP) and 14,500–12,900 BP from the Transient Climate Evolution (TraCE) paleoclimate simulation of [[#Liu--2009|Liu et al., 2009]] ), with paleoclimate proxy evidence superimposed on top (dots). '''(b)''' Model simulation of precipitation response to an abrupt collapse in AMOC under a doubling of 1990 CO <sub>2</sub> levels (after W. [[#Liu--2017|]] [[#Liu--2017|Liu et al., 2017]] ). Regions with rainfall rates below 20 mm yr <sup>–1</sup> are masked. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

These patterns of past hydroclimatic change are relevant for future projections because it is ''very likely'' that AMOC will weaken by 2100 in response to increased greenhouse gas emissions ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.3.1|Section 9.2.3.1]] ; Weaver et al. , 2012; Drijfhout et al. , 2015; Bakker et al. , 2016; Reintges et al. , 2017) . Furthermore, there is ''medium confidence'' that the decline in AMOC will not involve an abrupt collapse before 2100 ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.3.1|Section 9.2.3.1]] ). The response of precipitation to hypothetical AMOC collapse under elevated greenhouse gases bears resemblance to the paleoclimate response during the Younger Dryas event, with some important differences due to effects of increased CO <sub>2</sub> on global precipitation patterns (Figure 8.27b). As with the paleoclimate events, AMOC collapse results in a southward shift in the ITCZ that is most pronounced in the tropical Atlantic. This could cause drying in the Sahel region (Defrance et al., 2017) as well as Meso-America and northern Amazonia (Parsons et al., 2014; Y. [[#Chen--2018|]] [[#Chen--2018|]] [[#Chen--2018|Chen et al., 2018]] ). AMOC collapse also causes the Asian monsoon systems to weaken (Figure 8.27b; W. [[#Liu--2017|]] [[#Liu--2017|Liu et al., 2017]] ) counteracting the strengthening expected in response to elevated greenhouse gases (see [[#8.4.2|Section 8.4.2]] ). Europe is projected to experience moderate drying in response to AMOC collapse (Jackson et al., 2015).

In summary, given that there is ''medium confidence'' that the decline in AMOC will not involve an abrupt collapse before 2100, there is ''low confidence'' that an AMOC-driven abrupt change in the water cycle will occur by 2100. However, if AMOC collapse does occur, it is ''very likely'' that there would be large regional impacts on the water cycle.

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