Editing IPCC:AR6/SROCC/Chapter-6 (section)

=== 6.4.2 Impacts on Natural, Physical and Human Systems ===

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==== 6.4.2.1 Impacts on Marine Organisms and Ecosystems ====

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Temperature plays an essential role in the biology and ecology of marine organisms (e.g., Pörtner, 2002; Pörtner and Knust, 2007 <sup>[[#fn:r396|396]]</sup> ; Poloczanska et al., 2013 <sup>[[#fn:r397|397]]</sup> ; Hoegh-Guldberg et al., 2014 <sup>[[#fn:r398|398]]</sup> ), and therefore extreme high ocean temperature can have large impacts on marine ecosystems. Recent studies show that MHWs have strongly impacted marine organisms and ecosystem services in all ocean basins (Smale et al., 2019 <sup>[[#fn:r399|399]]</sup> ) over the last two decades. Impacts include coral bleaching and mortality (Hughes et al., 2017b <sup>[[#fn:r400|400]]</sup> ; Hughes et al., 2018a <sup>[[#fn:r401|401]]</sup> ; Hughes et al., 2018b <sup>[[#fn:r402|402]]</sup> ), loss of seagrass and kelp forests (Smale et al., 2019 <sup>[[#fn:r403|403]]</sup> ), shifts in species range (Smale and Wernberg, 2013 <sup>[[#fn:r404|404]]</sup> ), and local (Wernberg et al., 2013 <sup>[[#fn:r405|405]]</sup> ; Wernberg et al., 2016 <sup>[[#fn:r406|406]]</sup> ) and potentially global extinctions of coral species (Brainard et al., 2011 <sup>[[#fn:r407|407]]</sup> ).

A growing number of studies have reported that MHWs negatively affect corals and coral reefs through bleaching, disease, and mortality (see Chapter 5 for an extensive discussion on coral reefs and coral bleaching). The recent (2014–2017) high ocean temperatures in the tropics and subtropics triggered a pan-tropical episode of unprecedented mass bleaching of corals (100s of km 2 ), the third global-scale event after 1997–1998 and 2010 (Heron et al., 2016 <sup>[[#fn:r408|408]]</sup> ; Eakin et al., 2017 <sup>[[#fn:r409|409]]</sup> ; Hughes et al., 2017b <sup>[[#fn:r410|410]]</sup> ; Eakin et al., 2018 <sup>[[#fn:r411|411]]</sup> ; Hughes et al., 2018a <sup>[[#fn:r412|412]]</sup> ). The heat stress during this event was sufficient to cause bleaching at 75% of global reefs (Hughes et al., 2018a; Figure 6.3b) and mortality at 30% (Eakin et al., 2017 <sup>[[#fn:r414|414]]</sup> ), much more than any previously documented global bleaching event. In some locations, many reefs bleached extensively for the first time on record, and over half of the reefs bleached multiple times during the three year event. However, there were distinct geographical variations in bleaching, mainly determined by the spatial pattern and magnitude of the MHW (Figure 6.3b). For example, bleaching was extensive and severe in the northern regions of the Great Barrier Reef, with 93% of the northern Australian Great Barrier Reef coral suffering bleaching in 2016, but impacts were moderate at the southern coral reefs of the Great Barrier Reef (Brainard et al., 2018 <sup>[[#fn:r415|415]]</sup> ; Stuart-Smith et al., 2018 <sup>[[#fn:r416|416]]</sup> ).

Apart from strong impacts on corals, recent MHWs have demonstrated their potential impacts on other marine ecosystems and ecosystems services (Ummenhofer and Meehl, 2017 <sup>[[#fn:r417|417]]</sup> ; Smale et al., 2019 <sup>[[#fn:r418|418]]</sup> ). Two of the best studied MHWs with extensive ecological implications are the Western Australia 2011 MHW and the Northeast Pacific 2013–2015 MHW. The Western Australia 2011 MHW resulted in a regime shift of the temperate reef ecosystem (Wernberg et al., 2013 <sup>[[#fn:r419|419]]</sup> ; Wernberg et al., 2016 <sup>[[#fn:r420|420]]</sup> ). The abundance of the dominant habitat-forming seaweeds ''Scytohalia dorycara'' and ''Ecklonia radiata'' became significantly reduced and ''Ecklonia'' kelp forest was replaced by small turf-forming algae with wide ranging impacts on associated sessile invertebrates and demersal fish. The sea grass ''Amphibolis antarctica'' in Shark Bay underwent defoliation after the MHW (Fraser et al., 2014 <sup>[[#fn:r421|421]]</sup> ), and together with the loss of other sea grass species, these lead to releases of 2–9 Tg CO 2 to the atmosphere during the subsequent three years after the MHW (Arias-Ortiz et al., 2018 <sup>[[#fn:r422|422]]</sup> ). In addition, coral bleaching and adverse impacts on invertebrate fisheries were documented (Depczynski et al., 2013 <sup>[[#fn:r423|423]]</sup> ; Caputi et al., 2016 <sup>[[#fn:r424|424]]</sup> ). The Northeast Pacific 2013–2015 MHW also caused extensive alterations to open ocean and coastal ecosystems (Cavole et al., 2016 <sup>[[#fn:r425|425]]</sup> ). Impacts included increased mortality events of sea birds (Jones et al., 2018 <sup>[[#fn:r426|426]]</sup> ), salmon and marine mammals (Cavole et al., 2016 <sup>[[#fn:r427|427]]</sup> ), very low ocean primary productivity (Whitney, 2015 <sup>[[#fn:r428|428]]</sup> ; Jacox et al., 2016 <sup>[[#fn:r429|429]]</sup> ), an increase in warm water copepod species (Di Lorenzo and Mantua, 2016) and novel species compositions (Peterson et al., 2017 <sup>[[#fn:r430|430]]</sup> ). In addition, a coast wide bloom of the toxigenic diatom ''Pseudo-nitzschia'' resulted in the largest ever recorded outbreak of domoic acid along the North American west coast (McCabe et al., 2016 <sup>[[#fn:r431|431]]</sup> ). Domoic acid was detected in many marine mammals, such as whales, dolphins, porpoises, seals and sea lions. The elevated toxins in commercially harvested fish and invertebrates resulted in prolonged and geographically extensive closure of razor clam and crab fisheries.

Other MHWs also demonstrated the vulnerability of marine organisms and ecosystems to extremely high ocean temperatures. The Northwest Atlantic 2012 MHW strongly impacted coastal ecosystems by causing a northward movement of warm water species and local migrations of some species (e.g., lobsters) earlier in the season (Mills et al., 2013 <sup>[[#fn:r432|432]]</sup> ; Pershing et al., 2015) <sup>[[#fn:r433|433]]</sup> . The Mediterranean Sea 2003 MHW lead to mass mortalities of macro-invertebrate species (Garrabou et al., 2009 <sup>[[#fn:r434|434]]</sup> ) and the Tasman Sea 2015–2016 MHW had impacts on sessile, sedentary and cultured species in the shallow, near-shore environment including outbreaks of disease in commercially viable species (Oliver et al., 2017 <sup>[[#fn:r435|435]]</sup> ). ''Vibrio'' outbreaks were also observed in the Baltic Sea in response to elevated SSTs (Baker-Austin et al., 2013 <sup>[[#fn:r436|436]]</sup> ). The Alaskan Sea 2016 MHW favoured some phytoplankton species, leading to harmful algal blooms, shellfish poisoning events and mortality events in seabirds (Walsh et al., 2018 <sup>[[#fn:r437|437]]</sup> ; see chapter 3 for more details). Also, lower than average size of multiple groundfish species were observed including Pollock, Pacific cod, and Chinook salmon (Zador and Siddon, 2016 <sup>[[#fn:r438|438]]</sup> ). The Yellow Sea/East China Sea 2016 MHW killed a large number of different marine organisms in coastal and bay areas around South Korea (Kim and Han, 2017 <sup>[[#fn:r439|439]]</sup> ) and the Southwest Atlantic 2017 MHW lead to toxic algal blooms (Manta et al., 2018 <sup>[[#fn:r440|440]]</sup> ). The Coastal Peruvian 2017 MHW affected anchovies, which showed decreased fat content and early spawning as a reproductive strategy (IMPARPE, 2017), a behaviour usually seen during warm El Niño conditions (Ñiquen and Bouchon, 2004 <sup>[[#fn:r442|442]]</sup> ).

Based on the examples described above we conclude with ''very high confidence'' that a range of organisms and ecosystems have been impacted by MHWs across all ocean basins over the last two decades. Given that MHWs will ''very likely'' increase in intensity and frequency with further climate warming, we conclude with ''high confidence'' that this will push some marine organisms, fisheries and ecosystem beyond the limits of their resilience. These impacts will occur on top of those expected from a progressive shift in global mean ocean temperatures.

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==== 6.4.2.2 Impacts on the Physical System ====

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MHWs can impact weather patterns over land via teleconnections causing drought, heavy precipitation or heat wave events. For example, the Northeast Pacific 2013–2015 MHW and the associated persistent atmospheric high-pressure ridge prevented normal winter storms from reaching the West Coast of the US and may have contributed to the drought conditions across the entire West Coast (Seager et al., 2015 <sup>[[#fn:r443|443]]</sup> ; Di Lorenzo and Mantua, 2016). The Tasman Sea 2015–2016 MHW has increased the intensity of rainfall that caused flooding in northeast Tasmania in January 2016 (see Box 6.1) and the Coastal Peruvian 2017 MHW caused heavy rainfall and flooding on the west coast of tropical South America (ENFEN, 2017 <sup>[[#fn:r444|444]]</sup> ; Echevin et al., 2018 <sup>[[#fn:r445|445]]</sup> ; Garreaud, 2018 <sup>[[#fn:r446|446]]</sup> ; Takahashi et al., 2018 <sup>[[#fn:r447|447]]</sup> ). Similarly, MHWs in the Mediterranean Sea may have amplified heatwaves (Feudale and Shukla, 2007 <sup>[[#fn:r448|448]]</sup> ; García-Herrera et al., 2010 <sup>[[#fn:r449|449]]</sup> ) and heavy precipitation events over central Europe (Messmer et al., 2017 <sup>[[#fn:r450|450]]</sup> ), as well as trigger intense ETCs over the Mediterranean Sea (González ‐ Alemán et al., 2019 <sup>[[#fn:r451|451]]</sup> ). Such physical changes induced by MHWs may then also affect ecosystems and human systems on land (Reimer et al., 2015 <sup>[[#fn:r452|452]]</sup> ).

It should be noted that past and future impacts of MHWs on weather patterns over land depend not only on the duration and intensity of MHWs, but also on a wide range of different additional processes in the climate system such as the large-scale circulation of the atmosphere and oceans, and changes in the mean climate. Therefore, we conclude that there is currently ''low confidence'' in how MHWs impact the weather systems over land.

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==== 6.4.2.3 Impacts on the Human System ====

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MHWs can also lead to significant socioeconomic ramifications when affecting aquaculture or important fishery species, or when triggering heavy rain or drought events on land. The Northwest Atlantic 2012 MHW, for example, had major economic impacts on the US lobster industry in 2015 (Mills et al., 2013). The MHWs lead to changes in lobster fishing practices and harvest patterns, because the lobsters moved from the deep offshore waters into shallower coastal areas much earlier in the season than usual causing a rapid rise in lobster catch rates. Together with a supply chain bottleneck, the record catch outstripped market demand and contributed to a collapse in lobster prices (Mills et al., 2013 <sup>[[#fn:r453|453]]</sup> ). Even though high catch volumes were reported, the price collapse threatened the economic viability of many US and Canadian lobster fisheries. Economic impacts through changes in fisheries were also reported during the Northeast Pacific 2013–2015 MHW and the Alaskan Sea 2016 MHW. The Northeast Pacific 2013–2015 MHW led to closing of both commercial and recreational fisheries resulting in millions of USD in losses among fishing industries (Cavole et al., 2016 <sup>[[#fn:r455|455]]</sup> ). In addition, the toxin produced by the harmful algal blooms can be transferred through the marine food web and humans who eat contaminated fish, shellfish or crustaceans (Berdalet et al., 2016 <sup>[[#fn:r456|456]]</sup> ; Du et al., 2016 <sup>[[#fn:r457|457]]</sup> ; McCabe et al., 2016 <sup>[[#fn:r458|458]]</sup> ). The ingestion of such contaminated seafood products, the inhalation of aerosolised toxins or the skin contact with toxin-contaminated water may cause toxicity in humans. Symptoms in human associated with the ingestion of the contaminated seafood range from mild gastrointestinal distress to seizures, coma, permanent short-term memory loss and death (Perl et al., 1990 <sup>[[#fn:r459|459]]</sup> ). The ecological changes associated with the Alaskan Sea 2016 MHW impacted subsistence and commercial activities. For example, ice-based harvesting of seals, crabs and fish in western Alaska was delayed due to the lack of winter sea ice. MHWs can also impact the socioeconomic and human system through changes to weather patterns. For example, heavy rain associated with the Coastal Peruvian 2017 MHW triggered numerous landslides and flooding, which resulted in a death toll of several hundred, and widespread damage to infrastructure and civil works (United Nations, 2017 <sup>[[#fn:r460|460]]</sup> ).

Studies on the impact of MHWs on human systems are still relatively scarce, even though many show negative impacts on human health and economy. We therefore conclude with ''medium confidence'' that MHWs can negatively impact human health and economy.

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