Editing IPCC:AR6/SR15/Chapter-3 (section)

==== 3.4.4.4 Ocean circulation ====

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The movement of water within the ocean is essential to its biology and ecology, as well to the circulation of heat, water and nutrients around the planet (Section 3.3.7). The movement of these factors drives local and regional climates, as well as primary productivity and food production. Firmly attributing recent changes in the strength and direction of ocean currents to climate change, however, is complicated by long-term patterns and variability (e.g., Pacific decadal oscillation, PDO; Signorini et al., 2015) <sup>[[#fn:r582|582]]</sup> and a lack of records that match the long-term nature of these changes in many cases (Lluch-Cota et al., 2014) <sup>[[#fn:r583|583]]</sup> . An assessment of the literature since AR5 (Sydeman et al., 2014) <sup>[[#fn:r584|584]]</sup> , however, concluded that (overall) upwelling-favourable winds have intensified in the California, Benguela and Humboldt upwelling systems, but have weakened in the Iberian system and have remained neutral in the Canary upwelling system in over 60 years of records (1946–2012) ( ''medium confidence'' ). These conclusions are consistent with a growing consensus that wind-driven upwelling systems are ''likely'' to intensify under climate change in many upwelling systems (Sydeman et al., 2014; Bakun et al., 2015; Di Lorenzo, 2015) <sup>[[#fn:r585|585]]</sup> , with potentially positive and negative consequences (Bakun et al., 2015) <sup>[[#fn:r586|586]]</sup> .

Changes in ocean circulation can have profound impacts on marine ecosystems by connecting regions and facilitating the entry and establishment of species in areas where they were unknown before (e.g., ‘tropicalization’ of temperate ecosystems; Wernberg et al., 2012; Vergés et al., 2014, 2016; Zarco-Perello et al., 2017) <sup>[[#fn:r587|587]]</sup> , as well as the arrival of novel disease agents ( ''low-medium confidence'' ) (Burge et al., 2014; Maynard et al., 2015; Weatherdon et al., 2016) <sup>[[#fn:r588|588]]</sup> . For example, the herbivorous sea urchin ''Centrostephanus rodgersii'' has been reached Tasmania from the Australian mainland, where it was previously unknown, owing to a strengthening of the East Australian Current (EAC) that connects the two regions ( ''high confidence'' ) (Ling et al., 2009) <sup>[[#fn:r589|589]]</sup> ''.'' As a consequence, the distribution and abundance of kelp forests has rapidly decreased, with implications for fisheries and other ecosystem services (Ling et al., 2009) <sup>[[#fn:r590|590]]</sup> . These risks to marine ecosystems are projected to become greater at 1.5°C, and more so at 2°C ( ''medium confidence'' ) (Cheung et al., 2009; Pereira et al., 2010; Pinsky et al., 2013; Burrows et al., 2014) <sup>[[#fn:r591|591]]</sup> .

Changes to ocean circulation can have even larger influence in terms of scale and impacts. Weakening of the Atlantic Meridional Overturning Circulation (AMOC), for example, is projected to be highly disruptive to natural and human systems as the delivery of heat to higher latitudes via this current system is reduced (Collins et al., 2013) <sup>[[#fn:r592|592]]</sup> . Evidence of a slowdown of AMOC has increased since AR5 (Smeed et al., 2014; Rahmstorf et al., 2015a, b; Kelly et al., 2016) <sup>[[#fn:r593|593]]</sup> , yet a strong causal connection to climate change is missing ( ''low confidence'' ) (Section 3.3.7).

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