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

=== 3.4.4 Ocean Ecosystems ===

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The ocean plays a central role in regulating atmospheric gas concentrations, global temperature and climate. It also provides habitat to a large number of organisms and ecosystems that provide goods and services worth trillions of USD per year (e.g., Costanza et al., 2014; Hoegh-Guldberg et al., 2015) <sup>[[#fn:r559|559]]</sup> . Together with local stresses (Halpern et al., 2015) <sup>[[#fn:r560|560]]</sup> , climate change poses a major threat to an increasing number of ocean ecosystems (e.g., warm water or tropical coral reefs: ''virtually certain'' , WGII AR5) and consequently to many coastal communities that depend on marine resources for food, livelihoods and a safe place to live. Previous sections of this report have described changes in the ocean, including rapid increases in ocean temperature down to a depth of at least 700 m (Section 3.3.7). In addition, anthropogenic carbon dioxide has decreased ocean pH and affected the concentration of ions in seawater such as carbonate (Sections 3.3.10 and 3.4.4.5), both over a similar depth range. Increased ocean temperatures have intensified storms in some regions (Section 3.3.6), expanded the ocean volume and increased sea levels globally (Section 3.3.9), reduced the extent of polar summer sea ice (Section 3.3.8), and decreased the overall solubility of the ocean for oxygen (Section 3.3.10). Importantly, changes in the response to climate change rarely operate in isolation. Consequently, the effect of global warming of 1.5°C versus 2°C must be considered in the light of multiple factors that may accumulate and interact over time to produce complex risks, hazards and impacts on human and natural systems.

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<span id="observed-impacts"></span>
==== 3.4.4.1 Observed impacts ====

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Physical and chemical changes to the ocean resulting from increasing atmospheric CO <sub>2</sub> and other GHGs are already driving significant changes to ocean systems ( ''very high confidence'' ) and will continue to do so at 1.5°C, and more so at 2°C, of global warming above pre-industrial temperatures (Section 3.3.11). These changes have been accompanied by other changes such as ocean acidification, intensifying storms and deoxygenation (Levin and Le Bris, 2015) <sup>[[#fn:r561|561]]</sup> . Risks are already significant at current greenhouse gas concentrations and temperatures, and they vary significantly among depths, locations and ecosystems, with impacts being singular, interactive and/or cumulative (Boyd et al., 2015) <sup>[[#fn:r562|562]]</sup> .

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<span id="warming-and-stratification-of-the-surface-ocean"></span>
==== 3.4.4.2 Warming and stratification of the surface ocean ====

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As atmospheric greenhouse gases have increased, the global mean surface temperature (GMST) has reached about 1°C above the pre-industrial period, and oceans have rapidly warmed from the ocean surface to the deep sea ( ''high confidence'' ) (Sections 3.3.7; Hughes and Narayanaswamy, 2013; Levin and Le Bris, 2015; Yasuhara and Danovaro, 2016; Sweetman et al., 2017) <sup>[[#fn:r563|563]]</sup> . Marine organisms are already responding to these changes by shifting their biogeographical ranges to higher latitudes at rates that range from approximately 0 to 40 km yr <sup>–1</sup> (Burrows et al., 2014; Chust, 2014; Bruge et al., 2016; Poloczanska et al., 2016) <sup>[[#fn:r564|564]]</sup> , which has consequently affected the structure and function of the ocean, along with its biodiversity and foodwebs ( ''high confidence'' ). Movements of organisms does not necessarily equate to the movement of entire ecosystems. For example, species of reef-building corals have been observed to shift their geographic ranges, yet this has not resulted in the shift of entire coral ecosystems ( ''high confidence'' ) (Woodroffe et al., 2010; Yamano et al., 2011) <sup>[[#fn:r565|565]]</sup> . In the case of ‘less mobile’ ecosystems (e.g., coral reefs, kelp forests and intertidal communities), shifts in biogeographical ranges may be limited, with mass mortalities and disease outbreaks increasing in frequency as the exposure to extreme temperatures increases ( ''very high confidence'' ) (Hoegh-Guldberg, 1999; Garrabou et al., 2009; Rivetti et al., 2014; Maynard et al., 2015; Krumhansl et al., 2016; Hughes et al., 2017b; see also Box 3.4) <sup>[[#fn:r566|566]]</sup> . These trends are projected to become more pronounced at warming of 1.5°C, and more so at 2°C, above the pre-industrial period (Hoegh-Guldberg et al., 2007; Donner, 2009; Frieler et al., 2013; Horta E Costa et al., 2014; Vergés et al., 2014, 2016; Zarco-Perello et al., 2017) <sup>[[#fn:r567|567]]</sup> and are ''likely'' to result in decreases in marine biodiversity at the equator but increases in biodiversity at higher latitudes (Cheung et al., 2009; Burrows et al., 2014) <sup>[[#fn:r568|568]]</sup> .

While the impacts of species shifting their ranges are mostly negative for human communities and industry, there are instances of short-term gains. Fisheries, for example, may expand temporarily at high latitudes in the Northern Hemisphere as the extent of summer sea ice recedes and NPP increases ( ''medium confidence'' ) (Cheung et al., 2010; Lam et al., 2016; Weatherdon et al., 2016) <sup>[[#fn:r569|569]]</sup> . High-latitude fisheries are not only influenced by the effect of temperature on NPP but are also strongly influenced by the direct effects of changing temperatures on fish and fisheries (Section 3.4.4.9; Barange et al., 2014; Pörtner et al., 2014; Cheung et al., 2016b; Weatherdon et al., 2016 <sup>[[#fn:r570|570]]</sup> ). Temporary gains in the productivity of high-latitude fisheries are offset by a growing number of examples from low and mid-latitudes where increases in sea temperature are driving decreases in NPP, owing to the direct effects of elevated temperatures and/or reduced ocean mixing from reduced ocean upwelling, that is, increased stratification ( ''low-medium'' ''confidence'' ) (Cheung et al., 2010; Ainsworth et al., 2011; Lam et al., 2012, 2014, 2016; Bopp et al., 2013; Boyd et al., 2014; Chust et al., 2014; Hoegh-Guldberg et al., 2014; Poloczanska et al., 2014; Pörtner et al., 2014; Signorini et al., 2015) <sup>[[#fn:r571|571]]</sup> . Reduced ocean upwelling has implications for millions of people and industries that depend on fisheries for food and livelihoods (Bakun et al., 2015; FAO, 2016; Kämpf and Chapman, 2016) <sup>[[#fn:r572|572]]</sup> , although there is ''low confidence'' in the projection of the size of the consequences at 1.5°C. It is also important to appreciate these changes in the context of large-scale ocean processes such as the ocean carbon pump. The export of organic carbon to deeper layers of the ocean increases as NPP changes in the surface ocean, for example, with implications for foodwebs and oxygen levels (Boyd et al., 2014; Sydeman et al., 2014; Altieri and Gedan, 2015; Bakun et al., 2015; Boyd, 2015) <sup>[[#fn:r573|573]]</sup> .

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<span id="storms-and-coastal-runoff"></span>
==== 3.4.4.3 Storms and coastal runoff ====

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Storms, wind, waves and inundation can have highly destructive impacts on ocean and coastal ecosystems, as well as the human communities that depend on them (IPCC, 2012; Seneviratne et al., 2012) <sup>[[#fn:r574|574]]</sup> . The intensity of tropical cyclones across the world’s oceans has increased, although the overall number of tropical cyclones has remained the same or decreased ( ''medium confidence'' ) (Section 3.3.6; Elsner et al., 2008; Holland and Bruyère, 2014) <sup>[[#fn:r575|575]]</sup> . The direct force of wind and waves associated with larger storms, along with changes in storm direction, increases the risks of physical damage to coastal communities and to ecosystems such as mangroves ( ''low to medium confidence'' ) (Long et al., 2016; Primavera et al., 2016; Villamayor et al., 2016; Cheal et al., 2017) <sup>[[#fn:r576|576]]</sup> and tropical coral reefs (De’ath et al., 2012; Bozec et al., 2015; Cheal et al., 2017) <sup>[[#fn:r577|577]]</sup> . These changes are associated with increases in maximum wind speed, wave height and the inundation, although trends in these variables vary from region to region (Section 3.3.5). In some cases, this can lead to increased exposure to related impacts, such as flooding, reduced water quality and increased sediment runoff ( ''medium-high confidence'' ) (Brodie et al., 2012; Wong et al., 2014; Anthony, 2016 <sup>[[#fn:r578|578]]</sup> ; AR5, Table 5.1).

Sea level rise also amplifies the impacts of storms and wave action (Section 3.3.9), with ''robust evidence'' that storm surges and damage are already penetrating farther inland than a few decades ago, changing conditions for coastal ecosystems and human communities. This is especially true for small islands (Box 3.5) and low-lying coastal communities, where issues such as storm surges can transform coastal areas (Section 3.4.5; Brown et al., 2018a) <sup>[[#fn:r579|579]]</sup> . Changes in the frequency of extreme events, such as an increase in the frequency of intense storms, have the potential (along with other factors, such as disease, food web changes, invasive organisms and heat stress-related mortality; Burge et al., 2014; Maynard et al., 2015; Weatherdon et al., 2016; Clements et al., 2017) <sup>[[#fn:r580|580]]</sup> to overwhelm the capacity for natural and human systems to recover following disturbances. This has recently been seen for key ecosystems such as tropical coral reefs (Box 3.4), which have changed from coral-dominated ecosystems to assemblages dominated by other organisms such as seaweeds, with changes in associated organisms and ecosystem services ( ''high confidence'' ) (De’ath et al., 2012; Bozec et al., 2015; Cheal et al., 2017; Hoegh-Guldberg et al., 2017; Hughes et al., 2017a, b) <sup>[[#fn:r581|581]]</sup> . The impacts of storms are amplified by sea level rise (Section 3.4.5), leading to substantial challenges today and in the future for cities, deltas and small island states in particular (Sections 3.4.5.2 to 3.4.5.4), as well as for coastlines and their associated ecosystems (Sections 3.4.5.5 to 3.4.5.7).

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<span id="ocean-circulation"></span>
==== 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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<span id="ocean-acidification"></span>
==== 3.4.4.5 Ocean acidification ====

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Ocean chemistry encompasses a wide range of phenomena and chemical species, many of which are integral to the biology and ecology of the ocean (Section 3.3.10; Gattuso et al., 2014, 2015; Hoegh-Guldberg et al., 2014; Pörtner et al., 2014) <sup>[[#fn:r594|594]]</sup> . While changes to ocean chemistry are ''likely'' to be of central importance, the literature on how climate change might influence ocean chemistry over the short and long term is limited ( ''medium confidence'' ). By contrast, numerous risks from the specific changes associated with ocean acidification have been identified (Dove et al., 2013; Kroeker et al., 2013; Pörtner et al., 2014; Gattuso et al., 2015; Albright et al., 2016) <sup>[[#fn:r595|595]]</sup> , with the consensus that resulting changes to the carbonate chemistry of seawater are having, and are ''likely'' to continue to have, fundamental and substantial impacts on a wide variety of organisms ( ''high confidence'' ). Organisms with shells and skeletons made out of calcium carbonate are particularly at risk, as are the early life history stages of a large number of organisms and processes such as de-calcification, although there are some taxa that have not shown high-sensitivity to changes in CO <sub>2</sub> , pH and carbonate concentrations (Dove et al., 2013; Fang et al., 2013; Kroeker et al., 2013; Pörtner et al., 2014; Gattuso et al., 2015) <sup>[[#fn:r596|596]]</sup> . Risks of these impacts also vary with latitude and depth, with the greatest changes occurring at high latitudes as well as deeper regions. The aragonite saturation horizon (i.e., where concentrations of calcium and carbonate fall below the saturation point for aragonite, a key crystalline form of calcium carbonate) is decreasing with depth as anthropogenic CO <sub>2</sub> penetrates deeper into the ocean over time. Under many models and scenarios, the aragonite saturation is projected to reach the surface by 2030 onwards, with a growing list of impacts and consequences for ocean organisms, ecosystems and people (Orr et al., 2005; Hauri et al., 2016) <sup>[[#fn:r597|597]]</sup> .

Further, it is difficult to reliably separate the impacts of ocean warming and acidification. As ocean waters have increased in sea surface temperature (SST) by approximately 0.9°C they have also decreased by 0.2 pH units since 1870–1899 (‘pre-industrial’; Table 1 in Gattuso et al., 2015; Bopp et al., 2013) <sup>[[#fn:r598|598]]</sup> . As CO <sub>2</sub> concentrations continue to increase along with other GHGs, pH will decrease while sea temperature will increase, reaching 1.7°C and a decrease of 0.2 pH units (by 2100 under RCP4.5) relative to the pre-industrial period. These changes are ''likely'' to continue given the negative correlation of temperature and pH. Experimental manipulation of CO <sub>2</sub> , temperature and consequently acidification indicate that these impacts will continue to increase in size and scale as CO <sub>2</sub> and SST continue to increase in tandem (Dove et al., 2013; Fang et al., 2013; Kroeker et al., 2013) <sup>[[#fn:r599|599]]</sup> .

While many risks have been defined through laboratory and mesocosm experiments, there is a growing list of impacts from the field ( ''medium confidence'' ) that include community-scale impacts on bacterial assemblages and processes (Endres et al., 2014) <sup>[[#fn:r600|600]]</sup> , coccolithophores (K.J.S. Meier et al., 2014) <sup>[[#fn:r601|601]]</sup> , pteropods and polar foodwebs (Bednaršek et al., 2012, 2014) <sup>[[#fn:r602|602]]</sup> , phytoplankton (Moy et al., 2009; Riebesell et al., 2013; Richier et al., 2014) <sup>[[#fn:r603|603]]</sup> , benthic ecosystems (Hall-Spencer et al., 2008; Linares et al., 2015) <sup>[[#fn:r604|604]]</sup> , seagrass (Garrard et al., 2014) <sup>[[#fn:r605|605]]</sup> , and macroalgae (Webster et al., 2013; Ordonez et al., 2014) <sup>[[#fn:r606|606]]</sup> , as well as excavating sponges, endolithic microalgae and reef-building corals (Dove et al., 2013; Reyes-Nivia et al., 2013; Fang et al., 2014) <sup>[[#fn:r607|607]]</sup> , and coral reefs (Box 3.4; Fabricius et al., 2011; Allen et al., 2017) <sup>[[#fn:r608|608]]</sup> . Some ecosystems, such as those from bathyal areas (i.e., 200–3000 m below the surface), are ''likely'' to undergo very large reductions in pH by the year 2100 (0.29 to 0.37 pH units), yet evidence of how deep-water ecosystems will respond is currently limited despite the potential planetary importance of these areas ( ''low to medium confidence'' ) (Hughes and Narayanaswamy, 2013; Sweetman et al., 2017) <sup>[[#fn:r609|609]]</sup> .

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<span id="deoxygenation"></span>
==== 3.4.4.6 Deoxygenation ====

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Oxygen levels in the ocean are maintained by a series of processes including ocean mixing, photosynthesis, respiration and solubility (Boyd et al., 2014, 2015; Pörtner et al., 2014; Breitburg et al., 2018) <sup>[[#fn:r610|610]]</sup> . Concentrations of oxygen in the ocean are declining ( ''high confidence'' ) owing to three main factors related to climate change: (i) heat-related stratification of the water column (less ventilation and mixing), (ii) reduced oxygen solubility as ocean temperature increases, and (iii) impacts of warming on biological processes that produce or consume oxygen such as photosynthesis and respiration ( ''high confidence'' ) (Bopp et al., 2013; Pörtner et al., 2014; Altieri and Gedan, 2015; Deutsch et al., 2015; Schmidtko et al., 2017; Shepherd et al., 2017; Breitburg et al., 2018) <sup>[[#fn:r611|611]]</sup> . Further, a range of processes (Section 3.4.11) are acting synergistically, including factors not related to climate change, such as runoff and coastal eutrophication (e.g., from coastal farming and intensive aquaculture). These changes can lead to increased phytoplankton productivity as a result of the increased concentration of dissolved nutrients. Increased supply of organic carbon molecules from coastal run-off can also increase the metabolic activity of coastal microbial communities (Altieri and Gedan, 2015; Bakun et al., 2015; Boyd, 2015) <sup>[[#fn:r612|612]]</sup> . Deep sea areas are ''likely'' to experience some of the greatest challenges, as abyssal seafloor habitats in areas of deep-water formation are projected to experience decreased water column oxygen concentrations by as much as 0.03 mL L <sup>–1</sup> by 2100 (Levin and Le Bris, 2015; Sweetman et al., 2017) <sup>[[#fn:r613|613]]</sup> .

The number of ‘dead zones’ (areas where oxygenated waters have been replaced by hypoxic conditions) has been growing strongly since the 1990s (Diaz and Rosenberg, 2008; Altieri and Gedan, 2015; Schmidtko et al., 2017) <sup>[[#fn:r614|614]]</sup> . While attribution can be difficult because of the complexity of the processes involved, both related and unrelated to climate change, some impacts associated to deoxygenation ( ''low-medium confidence'' ) include the expansion of oxygen minimum zones (OMZ) (Turner et al., 2008; Carstensen et al., 2014; Acharya and Panigrahi, 2016; Lachkar et al., 2018) <sup>[[#fn:r615|615]]</sup> , physiological impacts (Pörtner et al., 2014) <sup>[[#fn:r616|616]]</sup> , and mortality and/or displacement of oxygen dependent organisms such as fish (Hamukuaya et al., 1998; Thronson and Quigg, 2008; Jacinto, 2011) <sup>[[#fn:r617|617]]</sup> and invertebrates (Hobbs and Mcdonald, 2010; Bednaršek et al., 2016; Seibel, 2016; Altieri et al., 2017) <sup>[[#fn:r618|618]]</sup> . In addition, deoxygenation interacts with ocean acidification to present substantial separate and combined challenges for fisheries and aquaculture ( ''medium confidence'' ) (Hamukuaya et al., 1998; Bakun et al., 2015; Rodrigues et al., 2015; Feely et al., 2016; S. Li et al., 2016; Asiedu et al., 2017a; Clements and Chopin, 2017; Clements et al., 2017; Breitburg et al., 2018) <sup>[[#fn:r619|619]]</sup> . Deoxygenation is expected to have greater impacts as ocean warming and acidification increase ( ''high confidence'' ), with impacts being larger and more numerous than today (e.g., greater challenges for aquaculture and fisheries from hypoxia), and as the number of hypoxic areas continues to increase. Risks from deoxygenation are ''virtually certain'' to increase as warming continues, although our understanding of risks at 1.5°C versus 2°C is incomplete ( ''medium confidence'' ). Reducing coastal pollution, and consequently the penetration of organic carbon into deep benthic habitats, is expected to reduce the loss of oxygen in coastal waters and hypoxic areas in general ( ''high confidence'' ) (Breitburg et al., 2018) <sup>[[#fn:r620|620]]</sup> .

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<span id="loss-of-sea-ice"></span>
==== 3.4.4.7 Loss of sea ice ====

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Sea ice is a persistent feature of the planet’s polar regions (Polyak et al., 2010) <sup>[[#fn:r621|621]]</sup> and is central to marine ecosystems, people (e.g., food, culture and livelihoods) and industries (e.g., fishing, tourism, oil and gas, and shipping). Summer sea ice in the Arctic, however, has been retreating rapidly in recent decades (Section 3.3.8), with an assessment of the literature revealing that a fundamental transformation is occurring in polar organisms and ecosystems, driven by climate change ( ''high confidence'' ) (Larsen et al., 2014) <sup>[[#fn:r622|622]]</sup> . These changes are strongly affecting people in the Arctic who have close relationships with sea ice and associated ecosystems, and these people are facing major adaptation challenges as a result of sea level rise, coastal erosion, the accelerated thawing of permafrost, changing ecosystems and resources, and many other issues (Ford, 2012; Ford et al., 2015) <sup>[[#fn:r623|623]]</sup> .

There is considerable and compelling evidence that a further increase of 0.5°C beyond the present-day average global surface temperature will lead to multiple levels of impact on a variety of organisms, from phytoplankton to marine mammals, with some of the most dramatic changes occurring in the Arctic Ocean and western Antarctic Peninsula (Turner et al., 2014, 2017b; Steinberg et al., 2015; Piñones and Fedorov, 2016) <sup>[[#fn:r624|624]]</sup> .

The impacts of climate change on sea ice are part of the focus of the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC), due to be released in 2019, and hence are not covered comprehensively here. However, there is a range of responses to the loss of sea ice that are occurring and which increase at 1.5°C and further so with 2°C of global warming. Some of these changes are described briefly here. Photosynthetic communities, such macroalgae, phytoplankton and microalgae dwelling on the underside of floating sea ice are changing, owing to increased temperatures, light and nutrient levels. As sea ice retreats, mixing of the water column increases, and phototrophs have increased access to seasonally high levels of solar radiation ( ''medium confidence'' ) (Dalpadado et al., 2014; W.N. Meier et al., 2014) <sup>[[#fn:r625|625]]</sup> . These changes are expected to stimulate fisheries productivity in high-latitude regions by mid-century ( ''high confidence'' ) (Cheung et al., 2009, 2010, 2016b; Lam et al., 2014) <sup>[[#fn:r626|626]]</sup> , with evidence that this is already happening for several high-latitude fisheries in the Northern Hemisphere, such as the Bering Sea, although these ‘positive’ impacts may be relatively short-lived (Hollowed and Sundby, 2014; Sundby et al., 2016) <sup>[[#fn:r627|627]]</sup> . In addition to the impact of climate change on fisheries via impacts on net primary productivity (NPP), there are also direct effects of temperature on fish, which may in turn have a range of impacts (Pörtner et al., 2014) <sup>[[#fn:r628|628]]</sup> . Sea ice in Antarctica is undergoing changes that exceed those seen in the Arctic (Maksym et al., 2011; Reid et al., 2015) <sup>[[#fn:r629|629]]</sup> , with increases in sea ice coverage in the western Ross Sea being accompanied by strong decreases in the Bellingshausen and Amundsen Seas (Hobbs et al., 2016) <sup>[[#fn:r630|630]]</sup> . While Antarctica is not permanently populated, the ramifications of changes to the productivity of vast regions, such as the Southern Ocean, have substantial implications for ocean foodwebs and fisheries globally.

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<span id="sea-level-rise"></span>
==== 3.4.4.8 Sea level rise ====

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Mean sea level is increasing (Section 3.3.9), with substantial impacts already being felt by coastal ecosystems and communities (Wong et al., 2014) <sup>[[#fn:r631|631]]</sup> ( ''high confidence'' ). These changes are interacting with other factors, such as strengthening storms, which together are driving larger storm surges, infrastructure damage, erosion and habitat loss (Church et al., 2013; Stocker et al., 2013; Blankespoor et al., 2014) <sup>[[#fn:r632|632]]</sup> . Coastal wetland ecosystems such as mangroves, sea grasses and salt marshes are under pressure from rising sea level ( ''medium confidence'' ) (Section 3.4.5; Di Nitto et al., 2014; Ellison, 2014; Lovelock et al., 2015; Mills et al., 2016; Nicholls et al., 2018) <sup>[[#fn:r633|633]]</sup> , as well as from a wide range of other risks and impacts unrelated to climate change, with the ongoing loss of wetlands recently estimated at approximately 1% per annum across a large number of countries (Blankespoor et al., 2014; Alongi, 2015) <sup>[[#fn:r634|634]]</sup> . While some ecosystems (e.g., mangroves) may be able to shift shoreward as sea levels increase, coastal development (e.g., buildings, seawalls and agriculture) often interrupts shoreward shifts, as well as reducing sediment supplies down some rivers (e.g., dams) due to coastal development (Di Nitto et al., 2014; Lovelock et al., 2015; Mills et al., 2016) <sup>[[#fn:r635|635]]</sup> .

Responses to sea level rise challenges for ocean and coastal systems include reducing the impact of other stresses, such as those arising from tourism, fishing, coastal development, reduced sediment supply and unsustainable aquaculture/agriculture, in order to build ecological resilience (Hossain et al., 2015; Sutton-Grier and Moore, 2016; Asiedu et al., 2017a) <sup>[[#fn:r636|636]]</sup> . The available literature largely concludes that these impacts will intensify under a 1.5°C warmer world but will be even higher at 2°C, especially when considered in the context of changes occurring beyond the end of the current century. In some cases, restoration of coastal habitats and ecosystems may be a cost-effective way of responding to changes arising from increasing levels of exposure to rising sea levels, intensifying storms, coastal inundation and salinization (Section 3.4.5 and Box 3.5; Arkema et al., 2013) <sup>[[#fn:r637|637]]</sup> , although limitations of these strategies have been identified (e.g., Lovelock et al., 2015; Weatherdon et al., 2016) <sup>[[#fn:r638|638]]</sup> .

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<span id="projected-risks-and-adaptation-options-for-oceans-under-global-warming-of-1.5c-or-2c-above-pre-industrial-levels"></span>
==== 3.4.4.9 Projected risks and adaptation options for oceans under global warming of 1.5°C or 2°C above pre-industrial levels ====

<div id="section-3-4-4-9-block-1"></div>

A comprehensive discussion of risk and adaptation options for all natural and human systems is not possible in the context and length of this report, and hence the intention here is to illustrate key risks and adaptation options for ocean ecosystems and sectors. This assessment builds on the recent expert consensus of Gattuso et al. (2015) <sup>[[#fn:r639|639]]</sup> by assessing new literature from 2015–2017 and adjusting the levels of risk from climate change in the light of literature since 2014. The original expert group’s assessment (Supplementary Material 3.SM.3.2) was used as input for this new assessment, which focuses on the implications of global warming of 1.5°C as compared to 2°C. A discussion of potential adaptation options is also provided, the details of which will be further explored in later chapters of this special report. The section draws on the extensive analysis and literature presented in the Supplementary Material of this report (3.SM.3.2, 3.SM.3.3) and has a summary in Figures 3.18 and 3.20 which outline the added relative risks of climate change.

<div id="section-3-4-4-10"></div>

<span id="framework-organisms-tropical-corals-mangroves-and-seagrass"></span>
==== 3.4.4.10 Framework organisms (tropical corals, mangroves and seagrass) ====

<div id="section-3-4-4-10-block-1"></div>

Marine organisms (‘ecosystem engineers’), such as seagrass, kelp, oysters, salt marsh species, mangroves and corals, build physical structures or frameworks (i.e., sea grass meadows, kelp forests, oyster reefs, salt marshes, mangrove forests and coral reefs) which form the habitat for a large number of species (Gutiérrez et al., 2012) <sup>[[#fn:r640|640]]</sup> . These organisms in turn provide food, livelihoods, cultural significance, and services such as coastal protection to human communities (Bell et al., 2011, 2018; Cinner et al., 2012; Arkema et al., 2013; Nurse et al., 2014; Wong et al., 2014; Barbier, 2015; Bell and Taylor, 2015; Hoegh-Guldberg et al., 2015; Mycoo, 2017; Pecl et al., 2017) <sup>[[#fn:r641|641]]</sup> .

Risks of climate change impacts for seagrass and mangrove ecosystems were recently assessed by an expert group led by Short et al. (2016) <sup>[[#fn:r642|642]]</sup> . Impacts of climate change were assessed to be similar across a range of submerged and emerged plants. Submerged plants such as sea-grass were affected mostly by temperature extremes (Arias-Ortiz et al., 2018) <sup>[[#fn:r643|643]]</sup> , and indirectly by turbidity, while emergent communities such as mangroves and salt marshes were most susceptible to sea level variability and temperature extremes, which is consistent with other evidence (Di Nitto et al., 2014; Sierra-Correa and Cantera Kintz, 2015; Osorio et al., 2016; Sasmito et al., 2016) <sup>[[#fn:r644|644]]</sup> , especially in the context of human activities that reduce sediment supply (Lovelock et al., 2015) <sup>[[#fn:r645|645]]</sup> or interrupt the shoreward movement of mangroves though the construction of coastal infrastructure. This in turn leads to ‘coastal squeeze’ where coastal ecosystems are trapped between changing ocean conditions and coastal infrastructure (Mills et al., 2016) <sup>[[#fn:r646|646]]</sup> . Projections of the future distribution of seagrasses suggest a poleward shift, which raises concerns that low-latitude seagrass communities may contract as a result of increasing stress levels (Valle et al., 2014) <sup>[[#fn:r647|647]]</sup> .

Climate change (e.g., sea level rise, heat stress, storms) presents risk for coastal ecosystems such as seagrass ( ''high confidence'' ) and reef-building corals ( ''very high confidence'' ) (Figure 3.18, Supplementary Material 3.SM.3.2), with evidence of increasing concern since AR5 and the conclusion that tropical corals may be even more vulnerable to climate change than indicated in assessments made in 2014 (Hoegh-Guldberg et al., 2014; Gattuso et al., 2015) <sup>[[#fn:r648|648]]</sup> . The current assessment also considered the heatwave-related loss of 50% of shallow-water corals across hundreds of kilometres of the world’s largest continuous coral reef system, the Great Barrier Reef. These large-scale impacts, plus the observation of back-to-back bleaching events on the Great Barrier Reef (predicted two decades ago, Hoegh-Guldberg, 1999) <sup>[[#fn:r649|649]]</sup> and arriving sooner than predicted (Hughes et al., 2017b, 2018) <sup>[[#fn:r650|650]]</sup> , suggest that the research community may have underestimated climate risks for coral reefs (Figure 3.18). The general assessment of climate risks for mangroves prior to this special report was that they face greater risks from deforestation and unsustainable coastal development than from climate change (Alongi, 2008; Hoegh-Guldberg et al., 2014; Gattuso et al., 2015) <sup>[[#fn:r651|651]]</sup> . Recent large-scale die-offs (Duke et al., 2017; Lovelock et al., 2017) <sup>[[#fn:r652|652]]</sup> , however, suggest that risks from climate change may have been underestimated for mangroves as well. With the events of the last past three years in mind, risks are now considered to be undetectable to moderate (i.e., moderate risks now start at 1.3°C as opposed to 1.8°C; ''medium confidence'' ). Consequently, when average global warming reaches 1.3°C above pre-industrial levels, the risk of climate change to mangroves are projected to be ''moderate'' (Figure 3.18) while tropical coral reefs will have reached a high level of risk as examplified by increasing damage from heat stress since the early 1980s. At global warming of 1.8°C above pre-industrial levels, seagrasses are projected to reach moderate to high levels of risk (e.g., damage resulting from sea level rise, erosion, extreme temperatures, and storms), while risks to mangroves from climate change are projected to remain moderate (e.g., not keeping up with sea level rise, and more frequent heat stress mortality) although there is ''low certainty'' as to when or if this important ecosystem is ''likely'' to transition to higher levels of additional risk from climate change (Figure 3.18).

Warm water (tropical) coral reefs are projected to reach a very high risk of impact at 1.2°C (Figure 3.18), with most available evidence suggesting that coral-dominated ecosystems will be non-existent at this temperature or higher ( ''high confidence'' ). At this point, coral abundance will be near zero at many locations and storms will contribute to ‘flattening’ the three-dimensional structure of reefs without recovery, as already observed for some coral reefs (Alvarez-Filip et al., 2009) <sup>[[#fn:r653|653]]</sup> . The impacts of warming, coupled with ocean acidification, are expected to undermine the ability of tropical coral reefs to provide habitat for thousand of species, which together provide a range of ecosystem services (e.g., food, livelihoods, coastal protection, cultural services) that are important for millions of people ( ''high confidence'' ) (Burke et al., 2011) <sup>[[#fn:r654|654]]</sup> .

Strategies for reducing the impact of climate change on framework organisms include reducing stresses not directly related to climate change (e.g., coastal pollution, overfishing and destructive coastal development) in order to increase their ecological resilience in the face of accelerating climate change impacts (World Bank, 2013; Ellison, 2014; Anthony et al., 2015; Sierra-Correa and Cantera Kintz, 2015; Kroon et al., 2016; O’Leary et al., 2017) <sup>[[#fn:r655|655]]</sup> , as well as protecting locations where organisms may be more robust (Palumbi et al., 2014) <sup>[[#fn:r656|656]]</sup> or less exposed to climate change (Bongaerts et al., 2010; van Hooidonk et al., 2013; Beyer et al., 2018) <sup>[[#fn:r657|657]]</sup> . This might involve cooler areas due to upwelling, or involve deep-water locations that experience less extreme conditions and impacts. Given the potential value of such locations for promoting the survival of coral communities under climate change, efforts to prevent their loss resulting from other stresses are important (Bongaerts et al., 2010, 2017; Chollett et al., 2010, 2014; Chollett and Mumby, 2013; Fine et al., 2013; van Hooidonk et al., 2013; Cacciapaglia and van Woesik, 2015; Beyer et al., 2018) <sup>[[#fn:r658|658]]</sup> . A full understanding of the role of refugia in reducing the loss of ecosystems has yet to be developed ( ''low to medium confidence'' ). There is also interest in ''ex situ'' conservation approaches involving the restoration of corals via aquaculture (Shafir et al., 2006; Rinkevich, 2014) <sup>[[#fn:r659|659]]</sup> or the use of ‘assisted evolution’ to help corals adapt to changing sea temperatures (van Oppen et al., 2015, 2017) <sup>[[#fn:r660|660]]</sup> , although there are numerous challenges that must be surpassed if these approaches are to be cost-effective responses to preserving coral reefs under rapid climate change ( ''low confidence'' ) (Hoegh-Guldberg, 2012, 2014a; Bayraktarov et al., 2016) <sup>[[#fn:r661|661]]</sup> .

High levels of adaptation are expected to be required to prevent impacts on food security and livelihoods in coastal populations ( ''medium confidence'' ). Integrating coastal infrastructure with changing ecosystems such as mangroves, seagrasses and salt marsh, may offer adaptation strategies as they shift shoreward as sea levels rise ( ''high confidence'' ). Maintaining the sediment supply to coastal areas would also assist mangroves in keeping pace with sea level rise (Shearman et al., 2013; Lovelock et al., 2015; Sasmito et al., 2016) <sup>[[#fn:r662|662]]</sup> . For this reason, habitat for mangroves can be strongly affected by human actions such as building dams which reduce the sediment supply and hence the ability of mangroves to escape ‘drowning’ as sea level rises (Lovelock et al., 2015) <sup>[[#fn:r663|663]]</sup> . In addition, integrated coastal zone management should recognize the importance and economic expediency of using natural ecosystems such as mangroves and tropical coral reefs to protect coastal human communities (Arkema et al., 2013; Temmerman et al., 2013; Ferrario et al., 2014; Hinkel et al., 2014; Elliff and Silva, 2017) <sup>[[#fn:r664|664]]</sup> . Adaptation options include developing alternative livelihoods and food sources, ecosystem-based management/adaptation such as ecosystem restoration, and constructing coastal infrastructure that reduces the impacts of rising seas and intensifying storms (Rinkevich, 2015; Weatherdon et al., 2016; Asiedu et al., 2017a; Feller et al., 2017) <sup>[[#fn:r665|665]]</sup> . Clearly, these options need to be carefully assessed in terms of feasibility, cost and scalability, as well as in the light of the coastal ecosystems involved (Bayraktarov et al., 2016) <sup>[[#fn:r666|666]]</sup> .

<div id="section-3-4-4-11"></div>

<span id="ocean-foodwebs-pteropods-bivalves-krill-and-fin-fish"></span>
==== 3.4.4.11 Ocean foodwebs (pteropods, bivalves, krill and fin fish) ====

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Ocean foodwebs are vast interconnected systems that transfer solar energy and nutrients from phytoplankton to higher trophic levels, including apex predators and commercially important species such as tuna. Here, we consider four representative groups of marine organisms which are important within foodwebs across the ocean, and which illustrate the impacts and ramifications of 1.5°C or higher levels of warming.

The first group of organisms, pteropods, are small pelagic molluscs that suspension feed and produce a calcium carbonate shell. They are highly abundant in temperate and polar waters where they are an important link in the foodweb between phytoplankton and a range of other organisms including fish, whales and birds. The second group, bivalve molluscs (e.g., clams, oysters and mussels), are filter-feeding invertebrates. These invertebrate organisms underpin important fisheries and aquaculture industries, from polar to tropical regions, and are important food sources for a range of organisms including humans. The third group of organisms considered here is a globally significant group of invertebrates known as ''euphausiid crustaceans'' (krill), which are a key food source for many marine organisms and hence a major link between primary producers and higher trophic levels (e.g., fish, mammals and sea birds). Antarctic krill,  ''Euphausia superba'' , are among the most abundant species in terms of mass and are consequently an essential component of polar foodwebs (Atkinson et al., 2009) <sup>[[#fn:r667|667]]</sup> . The last group, fin fishes, is vitally important components of ocean foodwebs, contribute to the income of coastal communities, industries and nations, and are important to the foodsecurity and livelihood of hundreds of millions of people globally (FAO, 2016) <sup>[[#fn:r668|668]]</sup> . Further background for this section is provided in Supplementary Material 3.SM.3.2.

There is a moderate risk to ocean foodwebs under present-day conditions ( ''medium to high confidence'' ) (Figure 3.18). Changing water chemistry and temperature are already affecting the ability of pteropods to produce their shells, swim and survive (Bednaršek et al., 2016) <sup>[[#fn:r669|669]]</sup> . Shell dissolution, for example, has increased by 19–26% in both nearshore and offshore populations since the pre-industrial period (Feely et al., 2016) <sup>[[#fn:r670|670]]</sup> . There is considerable concern as to whether these organisms are declining further, especially given the central importance in ocean foodwebs (David et al., 2017) <sup>[[#fn:r671|671]]</sup> . Reviewing the literature reveals that pteropods are projected to face high risks of impact at average global temperatures 1.5°C above pre-industrial levels and increasing risks of impacts at 2°C ( ''medium confidence'' ).

As GMST increases by 1.5°C and more, the risk of impacts from ocean warming and acidification are expected to be moderate to high, except in the case of bivalves (mid-latitudes) where the risks of impacts are projected to be high to very high (Figure 3.18). Ocean warming and acidification are already affecting the life history stages of bivalve molluscs (e.g., Asplund et al., 2014; Mackenzie et al., 2014; Waldbusser et al., 2014; Zittier et al., 2015; Shi et al., 2016; Velez et al., 2016; Q. Wang et al., 2016; Castillo et al., 2017; Lemasson et al., 2017; Ong et al., 2017; X. Zhao et al., 2017) <sup>[[#fn:r672|672]]</sup> . Impacts on adult bivalves include decreased growth, increased respiration and reduced calcification, whereas larval stages tend to show greater developmental abnormalities and increased mortality after exposure to these conditions ( ''medium to high confidence'' ) (Q. Wang et al., 2016; Lemasson et al., 2017; Ong et al., 2017; X. Zhao et al., 2017) <sup>[[#fn:r673|673]]</sup> . Risks are expected to accumulate at higher temperatures for bivalve molluscs, with very high risks expected at 1.8°C of warming or more. This general pattern applies to low-latitude fin fish, which are expected to experience moderate to high risks of impact at 1.3°C of global warming ( ''medium confidence'' ), and very high risks at 1.8°C at low latitudes ( ''medium confidence'' ) (Figure 3.18).

Large-scale changes to foodweb structure are occurring in all oceans. For example, record levels of sea ice loss in the Antarctic (Notz and Stroeve, 2016; Turner et al., 2017b) <sup>[[#fn:r674|674]]</sup> translate into a loss of habitat and hence reduced abundance of krill (Piñones and Fedorov, 2016) <sup>[[#fn:r675|675]]</sup> , with negative ramifications for the seabirds and whales which feed on krill (Croxall, 1992; Trathan and Hill, 2016) <sup>[[#fn:r676|676]]</sup> ( ''low-medium confidence'' ). Other influences, such as high rates of ocean acidification coupled with shoaling of the aragonite saturation horizon, are ''likely'' to also play key roles (Kawaguchi et al., 2013; Piñones and Fedorov, 2016) <sup>[[#fn:r677|677]]</sup> . As with many risks associated with impacts at the ecosystem scale, most adaptation options focus on the management of stresses unrelated to climate change but resulting from human activities, such as pollution and habitat destruction. Reducing these stresses will be important in efforts to maintain important foodweb components. Fisheries management at local to regional scales will be important in reducing stress on foodweb organisms, such as those discussed here, and in helping communities and industries adapt to changing foodweb structures and resources (see further discussion of fisheries ''per se'' below; Section 3.4.6.3). One strategy is to maintain larger population levels of fished species in order to provide more resilient stocks in the face of challenges that are increasingly driven by climate change (Green et al., 2014; Bell and Taylor, 2015) <sup>[[#fn:r678|678]]</sup> .

<div id="section-3-4-4-12"></div>

<span id="key-ecosystem-services-e.g.-carbon-uptake-coastal-protection-and-tropical-coral-reef-recreation"></span>
==== 3.4.4.12 Key ecosystem services (e.g., carbon uptake, coastal protection, and tropical coral reef recreation) ====

<div id="section-3-4-4-12-block-1"></div>

The ocean provides important services, including the regulation of atmospheric composition via gas exchange across the boundary between ocean and atmosphere, and the storage of carbon in vegetation and soils associated with ecosystems such as mangroves, salt marshes and coastal peatlands. These services involve a series of physicochemical processes which are influenced by ocean chemistry, circulation, biology, temperature and biogeochemical components, as well as by factors other than climate (Boyd, 2015) <sup>[[#fn:r679|679]]</sup> . The ocean is also a net sink for CO <sub>2</sub> (another important service), absorbing approximately 30% of human emissions from the burning of fossil fuels and modification of land use (IPCC, 2013) <sup>[[#fn:r680|680]]</sup> . Carbon uptake by the ocean is decreasing (Iida et al., 2015) <sup>[[#fn:r681|681]]</sup> , and there is increasing concern from observations and models regarding associated changes to ocean circulation (Sections 3.3.7 and 3.4.4., Rahmstorf et al., 2015b) <sup>[[#fn:r682|682]]</sup> ;. Biological components of carbon uptake by the ocean are also changing, with observations of changing net primary productivity (NPP) in equatorial and coastal upwelling systems ( ''medium confidence'' ) (Lluch-Cota et al., 2014; Sydeman et al., 2014; Bakun et al., 2015) <sup>[[#fn:r683|683]]</sup> , as well as subtropical gyre systems ( ''low confidence'' ) (Signorini et al., 2015) <sup>[[#fn:r684|684]]</sup> . There is general agreement that NPP will decline as ocean warming and acidification increase ( ''medium confidence'' ) (Bopp et al., 2013; Boyd et al., 2014; Pörtner et al., 2014; Boyd, 2015) <sup>[[#fn:r685|685]]</sup> .

Projected risks of impacts from reductions in carbon uptake, coastal protection and services contributing to coral reef recreation suggest a transition from moderate to high risks at 1.5°C and higher ( ''low confidence'' ). At 2°C, risks of impacts associated with changes to carbon uptake are high ( ''high confidence'' ), while the risks associated with reduced coastal protection and recreation on tropical coral reefs are high, especially given the vulnerability of this ecosystem type, and others (e.g., seagrass and mangroves), to climate change ( ''medium confidence'' ) (Figure 3.18). Coastal protection is a service provided by natural barriers such as mangroves, seagrass meadows, coral reefs, and other coastal ecosystems, and it is important for protecting human communities and infrastructure against the impacts associated with rising sea levels, larger waves and intensifying storms ( ''high confidence'' ) (Gutiérrez et al., 2012; Kennedy et al., 2013; Ferrario et al., 2014; Barbier, 2015; Cooper et al., 2016; Hauer et al., 2016; Narayan et al., 2016) <sup>[[#fn:r686|686]]</sup> . Both natural and human coastal protection have the potential to reduce these impacts (Fu and Song, 2017) <sup>[[#fn:r687|687]]</sup> . Tropical coral reefs, for example, provide effective protection by dissipating about 97% of wave energy, with 86% of the energy being dissipated by reef crests alone (Ferrario et al., 2014; Narayan et al., 2016) <sup>[[#fn:r688|688]]</sup> . Mangroves similarly play an important role in coastal protection, as well as providing resources for coastal communities, but they are already under moderate risk of not keeping up with sea level rise due to climate change and to contributing factors, such as reduced sediment supply or obstacles to shoreward shifts (Saunders et al., 2014; Lovelock et al., 2015) <sup>[[#fn:r689|689]]</sup> . This implies that coastal areas currently protected by mangroves may experience growing risks over time.

Tourism is one of the largest industries globally (Rosselló-Nadal, 2014; Markham et al., 2016; Spalding et al., 2017) <sup>[[#fn:r690|690]]</sup> . A substantial part of the global tourist industry is associated with tropical coastal regions and islands, where tropical coral reefs and related ecosystems play important roles (Section 3.4.9.1) ( ''medium confidence'' ). Coastal tourism can be a dominant money earner in terms of foreign exchange for many countries, particularly small island developing states (SIDS) (Section 3.4.9.1, Box 3.5; Weatherdon et al., 2016; Spalding et al., 2017) <sup>[[#fn:r691|691]]</sup> . The direct relationship between increasing global temperatures, intensifying storms, elevated thermal stress, and the loss of tropical coral reefs has raised concern about the risks of climate change for local economies and industries based on tropical coral reefs. Risks to coral reef recreational services from climate change are considered here, as well as in Box 3.5, Section 3.4.9 and Supplementary Material 3.SM.3.2.

Adaptations to the broad global changes in carbon uptake by the ocean are limited and are discussed later in this report with respect to changes in NPP and implications for fishing industries. These adaptation options are broad and indirect, and the only other solution at large scale is to reduce the entry of CO <sub>2</sub> into the ocean. Strategies for adapting to reduced coastal protection involve (a) avoidance of vulnerable areas and hazards, (b) managed retreat from threatened locations, and/or (c) accommodation of impacts and loss of services (Bell, 2012; André et al., 2016; Cooper et al., 2016; Mills et al., 2016; Raabe and Stumpf, 2016; Fu and Song, 2017) <sup>[[#fn:r692|692]]</sup> . Within these broad options, there are some strategies that involve direct human intervention, such as coastal hardening and the construction of seawalls and artificial reefs (Rinkevich, 2014, 2015; André et al., 2016; Cooper et al., 2016; Narayan et al., 2016) <sup>[[#fn:r693|693]]</sup> , while others exploit opportunities for increasing coastal protection by involving naturally occurring oyster banks, coral reefs, mangroves, seagrass and other ecosystems (UNEP-WCMC, 2006; Scyphers et al., 2011; Zhang et al., 2012; Ferrario et al., 2014; Cooper et al., 2016) <sup>[[#fn:r694|694]]</sup> . Natural ecosystems, when healthy, also have the ability to repair themselves after being damaged, which sets them apart from coastal hardening and other human structures that require constant maintenance (Barbier, 2015; Elliff and Silva, 2017) <sup>[[#fn:r695|695]]</sup> . In general, recognizing and restoring coastal ecosystems may be more cost-effective than installing human structures, in that creating and maintaining structures is typically expensive (Temmerman et al., 2013; Mycoo, 2017) <sup>[[#fn:r696|696]]</sup> .

Recent studies have increasingly stressed the need for coastal protection to be considered within the context of coastal land management, including protecting and ensuring that coastal ecosystems are able to undergo shifts in their distribution and abundance as climate change occurs (Clausen and Clausen, 2014; Martínez et al., 2014; Cui et al., 2015; André et al., 2016; Mills et al., 2016) <sup>[[#fn:r697|697]]</sup> . Facilitating these changes will require new tools in terms of legal and financial instruments, as well as integrated planning that involves not only human communities and infrastructure, but also associated ecosystem responses and values (Bell, 2012; Mills et al., 2016) <sup>[[#fn:r698|698]]</sup> . In this regard, the interactions between climate change, sea level rise and coastal disasters are increasingly being informed by models (Bosello and De Cian, 2014) <sup>[[#fn:r699|699]]</sup> with a widening appreciation of the role of natural ecosystems as an alternative to hardened coastal structures (Cooper et al., 2016) <sup>[[#fn:r700|700]]</sup> . Adaptation options for tropical coral reef recreation include: (i) protecting and improving biodiversity and ecological function by minimizing the impact of stresses unrelated to climate change (e.g., pollution and overfishing), (ii) ensuring adequate levels of coastal protection by supporting and repairing ecosystems that protect coastal regions, (iii) ensuring fair and equitable access to the economic opportunities associated with recreational activities, and (iv) seeking and protecting supplies of water for tourism, industry and agriculture alongside community needs.

<div id="section-3-4-4-12-block-2"></div>

<span id="figure-3.18"></span>
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'''Figure 3.18'''

<span id="summary-of-additional-risks-of-impacts-from-ocean-warming-and-associated-climate-change-factors-such-ocean-acidification-for-a-range-of-ocean-organisms-ecosystems-and-sectors-at-1.0c-1.5c-and-2.0c-of-warming-of-the-average-sea-surface-temperature-sst-relative-to-the-pre-industrial-period."></span>
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'''Summary of additional risks of impacts from ocean warming (and associated climate change factors such ocean acidification) for a range of ocean organisms, ecosystems and sectors at 1.0°C, 1.5°C and 2.0°C of warming of the average sea surface temperature (SST) relative to the pre-industrial period.'''
<!-- IMG FILE -->
[[File:e55ba8bf334b2396099150a9dca087ac figure_3.18-1024x913.png]]

The grey bar represents the range of GMST for the most recent decade: 2006–2015. The assessment of changing risk levels and associated confidence were primarily derived from the expert judgement of Gattuso et al. (2015 <sup>[[#fn:r701|701]]</sup> ) and the lead authors and relevant contributing authors of Chapter 3 (SR1.5), while additional input was received from the many reviewers of the ocean systems section of SR1.5. Notes: (i) The analysis shown here is not intended to be comprehensive. The examples of organisms, ecosystems and sectors included here are intended to illustrate the scale, types and projection of risks for representative natural and human ocean systems. (ii) The evaluation of risks by experts did not consider genetic adaptation, acclimatization or human risk reduction strategies (mitigation and societal adaptation). (iii) As discussed elsewhere (Sections 3.3.10 and 3.4.4.5, Box 3.4; Gattuso et al., 2015 <sup>[[#fn:r702|702]]</sup> ), ocean acidification is also having impacts on organisms and ecosystems as carbon dioxide increases in the atmosphere. These changes are part of the responses reported here, although partitioning the effects of the two drivers is difficult at this point in time and hence was not attempted. (iv) Confidence levels for location of transition points between levels of risk (L = low, M = moderate, H = high and VH = very high) are assessed and presented here as in the accompanying study by Gattuso et al. (2015 <sup>[[#fn:r703|703]]</sup> ). Three transitions in risk were possible: W–Y (white to yellow), Y–R (yellow to red), and R–P (red to purple), with the colours corresponding to the level of additional risk posed by climate change. The confidence levels for these transitions were assessed, based on level of agreement and extent of evidence, and appear as letters associated with each transition (see key in diagram).

Original Creation for this Report. Update of Expert assessment by Gattuso et al. (2015).

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<div id="section-3-4-4-12-block-3" class="box"></div>

<span id="box-3.4-warm-water-tropical-coral-reefs-in-a-1.5c-warmer-world"></span>