Editing IPCC:AR6/SROCC/Chapter-5 (section)

=== 5.2.5 Risk Assessment of Open Ocean Ecosystems ===

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This section synthesises the assessment of climate impacts on open ocean and deep seafloor ecosystem structure and functioning and the levels of risk under future conditions of global warming (see SM5.2). The format for Figure 5.16 matches that of Figure 19.4 of AR5 (Pörtner et al., 2014) and Figure 3.20 of SR15 (Hoegh-Guldberg et al., 2018), indicating the levels of additional risk as colours (white, yellow, red and purple). Each column in Figure 5.16 indicates how risks increase with ocean warming, acidification (OA), deoxygenation, and POC flux with a focus on present day conditions (2000s) and future conditions by the year 2100 under low (RCP2.6) and high (RCP8.5) CO 2 emission scenarios. The transition between the levels of risk to each type of ecosystem is estimated from key evidence assessed in earlier parts of this chapter (Sections 5.2.2, 5.2.3, 5.2.4). SST is chosen to provide an indication of the changes in all these variables because it is closely related to cumulative carbon emission (Gattuso et al., 2015) which is the main climatic driver of the hazards. SST scales with Global Mean Surface Temperature (GMST) by a factor of 1.44 according to changes in an ensemble of RCP8.5 simulations; with an uncertainty of about 4 % in this scaling factor based on differences between the RCP2.6 and RCP8.5 scenarios. The transition values may have an error of ±0.3°C depending on the consensus of expert judgment. The deep seafloor embers are generated based on earth system model projection of climate variables to the seafloor under RCP2.6 and RCP8.5 scenarios, and then translated to RCP associated change in SST. The assessed confidence in assigning the levels of risk at present day and future scenarios are ''low, medium, high'' and ''very high'' levels of confidence. A detailed account of the procedures involved in the ember for each type of ecosystem, such as their exposure to climate hazards, sensitivity of key biotic and abiotic components, natural adaptive capacity, observed impacts and projected risks, and regional hotspots of vulnerability is provided in the SM5.2 and Table 5.5. The risk assessment for cold water corals is in agreement with the conclusions in AR5 Chapter 6.3.1.4.1, although more recent literature is assessed in Box 5.2 and Table SM5.5.

Overall, the upper ocean (0−700 m) and 700−2000 m layers have both warmed from 2004 to 2016 ( ''virtually certain'' ) and the abyssal ocean continues to warm in the Southern Hemisphere ( ''high confidence'' ). The ocean is stratifying; observed warming and high-latitude freshening are both surface intensified trends making the surface ocean lighter at a faster rate than deeper in the ocean ( ''high confidence'' ) (Section 5.2.2.2). It is  ''very likely''  that stratification in the upper few hundred meters of the ocean will increase significantly in the 21st century. It is ''virtually certain'' that ocean pH is declining by ~0.02 pH units per decade where time series observations exist (Section 5.2.2.3). The anthropogenic pH signal has already emerged over the entire surface ocean ( ''high confidence'' ) and emission scenarios are the most important control of surface ocean pH relative to internal variability for most of the 21st century at both global and local scale ( ''virtually certain'' ). The oxygen content of the global ocean has declined by about 0.5−3.3% in 0−1000 m layer (Section 5.2.2.4). Over the next century oxygen declines of 3.5% by 2100 are predicted by CMIP5 models globally ( ''medium confidence'' ), with ''low confidence'' at regional scales, especially in the tropics. The largest changes in the deep sea will occur after 2100 (Section 5.2.2.3). CMIP5 models project a decrease in global NPP ( ''medium confidence'' ) with increases in high-latitude ( ''low confidence'' ) and decreases in low latitude ( ''medium confidence'' ) (Section 5.2.2.6) in response to changes in ocean nutrient supply (Section 5.2.2.5). These models also project reductions by 8.9−15.8% in the globally integrated POC flux for RCP8.5, with decreases in tropical regions and increases at higher latitudes ''(medium confidence)'' , affecting the organic carbon supply to the deep sea floor ecosystems ( ''high confidence'' ) (Section 5.2.2.6). However, there is ''low confidence'' on the mechanistic understanding of how climatic drivers will affect the different components of the biological pump in the epipelagic ocean (Table 5.4). Therefore, the exposure to hazard for epipelagic ecosystems ranges from moderate (RCP2.6) to high (RCP8.5), with uncertain effects and tolerance of planktonic organisms, fishes and large vertebrates to interactive climate stressors. Major risks are predicted for declining productivity and fish biomass in tropical and subtropical waters (RCP8.5) (SM5.2).

The climatic hazards for pelagic organisms from plankton to mammals are driving changes in eco-physiology, biogeography and ecology and biodiversity ( ''high confidence'' ) (Section 5.2.3.1). Observed and projected population declines in the equator-ward range boundary ( ''medium confidence'' ), expansion in the poleward boundary ( ''high confidence'' ), earlier timing of biological events ( ''high confidence'' ), overall shift species composition ( ''high confidence'' ) and decreases in animal biomass ( ''medium confidence'' ), are consistent with expected responses to climate change (Section 5.2.3; Figure 5.13). It is ''likely'' that increased OA has not yet caused sufficient reduction in fitness to decrease abundances of calcifying phytoplankton and zooplankton, but ''is very'' ''likely'' ( ''high confidence'' ) that calcifying planktonic organisms will experience great decreases in abundance and diversity under high emission scenarios by the end of the century. Therefore, impacts to the epipelagic ecosystems are already observed in the present day (Figure 5.16). Based on simulation modelling and experimental findings, the combined effects of warming, ocean deoxygenation, OA and changes in NPP in the 21st century are projected to exacerbate the impacts on the growth, reproduction and mortality of fishes, and consequently increase the risk of population decline ( ''high confidence'' ) (Section 5.2.3.1). There may be some capacity for adjustment and evolutionary adaptation that lowers their sensitivity to warming and decrease in oxygen ( ''low confidence'' ). However, historical responses in abundance and ranges of marine fishes to ocean warming and decrease in oxygen in the past suggest that adaptation is not always sufficient to mitigate the observed impacts ( ''medium confidence'' ) (Section 5.2.3) (SM5.2).

Despite its remoteness, most of the deep seafloor ecosystems already have or are projected to experience rising temperatures and declining oxygen, pH and POC flux beyond natural variability within the next half century (See Section 5.2.4). On slopes, seamounts and canyons these changes are projected to be much larger under RCP8.5 than under RCP2.6 ( ''high confidence'' ), with greatest effects on seafloor community diversity and function from expansion of low oxygen zones and aragonite undersaturation ( ''medium confidence'' ). As critical thresholds of temperature, oxygen and CO 2 are exceeded, coral species will alter their depth distributions, non-living carbonate will experience dissolution and bioerosion, and stress will be exacerbated by lower food supply. These changes are projected to cause loss of cold water coral habitat with highest climate hazard in the Arctic and north Atlantic Ocean ( ''medium confidence'' ), while sponges may be more tolerant (Box 5.2) ( ''low confidence'' ). Projected changes in food supply to the seafloor at abyssal depths combined with warmer temperatures are anticipated to cause reductions in biomass and body size ( ''medium confidence'' ) that could affect the carbon cycle in this century under RCP8.5 ( ''low confidence'' ). Even at hydrothermal vents and methane seeps, some dominant species such as mussels may be vulnerable to reduced photosynthetically-based food supply or have planktonic larvae or oxidising symbionts that are negatively affected by warming, acidification and oxygen loss ( ''low confidence'' ).

Widespread attributes of deep seafloor fauna (e.g., great longevity '','' high levels of habitat specialisation including well-defined physiological tolerances and thresholds, dependence on environmental triggers for reproduction, and highly developed mutualistic interactions) can increase the vulnerability of selected taxa to changing conditions (FAO, 2019) ( ''medium confidence'' ). However, some deep sea taxa (e.g., foraminifera and nematodes) may be more resilient to environmental change than their shallow-water counterparts ( ''low confidence'' ). Observations, experiments and model projections indicate that impacts of climate change have or are expected to take place in this century, indicating a transition from undetectable risk to moderate risk at &lt;1.5 o C warming of sea surface temperature for continental slope, canyon and seamount habitats, and for cold water corals (Figure 5.16). Emergence of risk is expected to occur later at around the mid-21st century under RCP8.5 for abyssal plain and chemosynthetic ecosystems (vents and seeps) (Figure 5.16). All deep seafloor ecosystems are expected to be subject to at least moderate risk under RCP8.5 by the end of the 21st century, with cold water corals experiencing a transition from moderate to high risk below 3 o C (SM5.2).

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