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

=== 3.3.7 Ocean Circulation and Temperature ===

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It is ''virtually certain'' that the temperature of the upper layers of the ocean (0–700 m in depth) has been increasing, and that the global mean for sea surface temperature (SST) has been changing at a rate just behind that of GMST. The surfaces of three ocean basins has warmed over the period 1950–2016 (by 0.11°C, 0.07°C and 0.05°C per decade for the Indian, Atlantic and Pacific Oceans, respectively; Hoegh-Guldberg et al., 2014 <sup>[[#fn:r248|248]]</sup> ), with the greatest changes occurring at the highest latitudes. Isotherms (i.e., lines of equal temperature) of sea surface temperature (SST) are shifting to higher latitudes at rates of up to 40 km per year (Burrows et al., 2014; García Molinos et al., 2015) <sup>[[#fn:r249|249]]</sup> . Long-term patterns of variability make detecting signals due to climate change complex, although the recent acceleration of changes to the temperature of the surface layers of the ocean has made the climate signal more distinct (Hoegh-Guldberg et al., 2014) <sup>[[#fn:r250|250]]</sup> . There is also evidence of significant increases in the frequency of marine heatwaves in the observational record (Oliver et al., 2018) <sup>[[#fn:r251|251]]</sup> , consistent with changes in mean ocean temperatures ( ''high confidence'' ). Increasing climate extremes in the ocean are associated with the general rise in global average surface temperature, as well as more intense patterns of climate variability (e.g., climate change intensification of ENSO) (Section 3.5.2.5). Increased heat in the upper layers of the ocean is also driving more intense storms and greater rates of inundation in some regions, which, together with sea level rise, are already driving significant impacts to sensitive coastal and low-lying areas (Section 3.3.6).

Increasing land–sea temperature gradients have the potential to strengthen upwelling systems associated with the eastern boundary currents (Benguela, Canary, Humboldt and Californian Currents; Bakun, 1990) <sup>[[#fn:r252|252]]</sup> . Observed trends support the conclusion that a general strengthening of longshore winds has occurred (Sydeman et al., 2014) <sup>[[#fn:r253|253]]</sup> , but the implications of trends detected in upwelling currents themselves are unclear (Lluch-Cota et al., 2014) <sup>[[#fn:r254|254]]</sup> . Projections of the scale of changes between 1°C and 1.5°C of global warming and between 1.5°C and 2°C are only informed by the changes during the past increase in GMST of 0.5°C ( ''low confidence'' ). However, evidence from GCM projections of future climate change indicates that a general strengthening of the Benguela, Canary and Humboldt upwelling systems under enhanced anthropogenic forcing (D. Wang et al., 2015) <sup>[[#fn:r255|255]]</sup> is projected to occur ( ''medium confidence'' ). This strengthening is projected to be stronger at higher latitudes. In fact, evidence from regional climate modelling is supportive of an increase in long-shore winds at higher latitudes, whereas long-shore winds may decrease at lower latitudes as a consequence of the poleward displacement of the subtropical highs under climate change (Christensen et al., 2007; Engelbrecht et al., 2009) <sup>[[#fn:r256|256]]</sup> .

''It is more likely than not'' that the Atlantic Meridional Overturning Circulation (AMOC) has been weakening in recent decades, given the detection of the cooling of surface waters in the North Atlantic and evidence that the Gulf Stream has slowed since the late 1950s (Rahmstorf et al., 2015b; Srokosz and Bryden, 2015; Caesar et al., 2018) <sup>[[#fn:r257|257]]</sup> . There is only ''limited evidence'' linking the current anomalously weak state of AMOC to anthropogenic warming (Caesar et al., 2018) <sup>[[#fn:r258|258]]</sup> . It is ''very likely'' that the AMOC will weaken over the 21st century. The best estimates and ranges for the reduction based on CMIP5 simulations are 11% (1– 24%) in RCP2.6 and 34% (12– 54%) in RCP8.5 (AR5). There is no evidence indicating significantly different amplitudes of AMOC weakening for 1.5°C versus 2°C of global warming.

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