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

== 3.4 Observed Impacts and Projected Risks in Natural and Human Systems ==

<span id="introduction"></span>
=== 3.4.1 Introduction ===

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In Section 3.4, new literature is explored and the assessment of impacts and projected risks is updated for a large number of natural and human systems. This section also includes an exploration of adaptation opportunities that could be important steps towards reducing climate change, thereby laying the ground for later discussions on opportunities to tackle both mitigation and adaptation while at the same time recognising the importance of sustainable development and reducing the inequities among people and societies facing climate change.

Working Group II (WGII) of the IPCC Fifth Assessment Report (AR5) provided an assessment of the literature on the climate risk for natural and human systems across a wide range of environments, sectors and greenhouse gas scenarios, as well as for particular geographic regions (IPCC, 2014a, b) <sup>[[#fn:r369|369]]</sup> . The comprehensive assessment undertaken by AR5 evaluated the evidence of changes to natural systems, and the impact on human communities and industry. While impacts varied substantially among systems, sectors and regions, many changes over the past 50 years could be attributed to human driven climate change and its impacts. In particular, AR5 attributed observed impacts in natural ecosystems to anthropogenic climate change, including changes in phenology, geographic and altitudinal range shifts in flora and fauna, regime shifts and increased tree mortality, all of which can reduce ecosystem functioning and services thereby impacting people. AR5 also reported increasing evidence of changing patterns of disease and invasive species, as well as growing risks for communities and industry, which are especially important with respect to sea level rise and human vulnerability.

One of the important themes that emerged from AR5 is that previous assessments may have under-estimated the sensitivity of natural and human systems to climate change. A more recent analysis of attribution to greenhouse gas forcing at the global scale (Hansen and Stone, 2016) <sup>[[#fn:r370|370]]</sup> confirmed that many impacts related to changes in regional atmospheric and ocean temperature can be confidently attributed to anthropogenic forcing, while attribution to anthropogenic forcing of changes related to precipitation are by comparison less clear. Moreover, there is no strong direct relationship between the robustness of climate attribution and that of impact attribution (Hansen and Stone, 2016) <sup>[[#fn:r371|371]]</sup> . The observed changes in human systems are amplified by the loss of ecosystem services (e.g., reduced access to safe water) that are supported by biodiversity (Oppenheimer et al., 2014) <sup>[[#fn:r372|372]]</sup> . Limited research on the risks of warming of 1.5°C and 2°C was conducted following AR5 for most key economic sectors and services, for livelihoods and poverty, and for rural areas. For these systems, climate is one of many drivers that result in adverse outcomes. Other factors include patterns of demographic change, socio-economic development, trade and tourism. Further, consequences of climate change for infrastructure, tourism, migration, crop yields and other impacts interact with underlying vulnerabilities, such as for individuals and communities engaged in pastoralism, mountain farming and artisanal fisheries, to affect livelihoods and poverty (Dasgupta et al., 2014) <sup>[[#fn:r373|373]]</sup> .

Incomplete data and understanding of these lower-end climate scenarios have increased the need for more data and an improved understanding of the projected risks of warming of 1.5°C and 2°C for reference. In this section, the available literature on the projected risks, impacts and adaptation options is explored, supported by additional information and background provided in Supplementary Material 3.SM.3.1, 3.SM.3.2, 3.SM.3.4, and 3.SM.3.5. A description of the main assessment methods of this chapter is given in Section 3.2.2.

<span id="freshwater-resources-quantity-and-quality"></span>
=== 3.4.2 Freshwater Resources (Quantity and Quality) ===

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<span id="water-availability"></span>
==== 3.4.2.1 Water availability ====

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Working Group II of AR5 concluded that about 80% of the world’s population already suffers from serious threats to its water security, as measured by indicators including water availability, water demand and pollution (Jiménez Cisneros et al., 2014) <sup>[[#fn:r374|374]]</sup> . UNESCO (2011) <sup>[[#fn:r375|375]]</sup> concluded that climate change can alter the availability of water and threaten water security.

Although physical changes in streamflow and continental runoff that are consistent with climate change have been identified (Section 3.3.5), water scarcity in the past is still less well understood because the scarcity assessment needs to take into account various factors, such as the operations of water supply infrastructure and human water use behaviour (Mehran et al., 2017) <sup>[[#fn:r376|376]]</sup> , as well as green water, water quality and environmental flow requirements (J. Liu et al., 2017) <sup>[[#fn:r377|377]]</sup> . Over the past century, substantial growth in populations, industrial and agricultural activities, and living standards have exacerbated water stress in many parts of the world, especially in semi-arid and arid regions such as California in the USA (AghaKouchak et al., 2015 <sup>[[#fn:r378|378]]</sup> ; Mehran et al., 2015) <sup>[[#fn:r379|379]]</sup> . Owing to changes in climate and water consumption behaviour, and particularly effects of the spatial distribution of population growth relative to water resources, the population under water scarcity increased from 0.24 billion (14% of the global population) in the 1900s to 3.8 billion (58%) in the 2000s. In that last period (2000s), 1.1 billion people (17% of the global population) who mostly live in South and East Asia, North Africa and the Middle East faced serious water shortage and high water stress (Kummu et al., 2016) <sup>[[#fn:r380|380]]</sup> .

Over the next few decades, and for increases in global mean temperature less than about 2°C, AR5 concluded that changes in population will generally have a greater effect on water resource availability than changes in climate. Climate change, however, will regionally exacerbate or offset the effects of population pressure (Jiménez Cisneros et al., 2014) <sup>[[#fn:r381|381]]</sup> .

The differences in projected changes to levels of runoff under 1.5°C and 2°C of global warming, particularly those that are regional, are described in Section 3.3.5. Constraining warming to 1.5°C instead of 2°C might mitigate the risks for water availability, although socio-economic drivers could affect water availability more than the risks posed by variation in warming levels, while the risks are not homogeneous among regions ( ''medium confidence'' ) (Gerten et al., 2013; Hanasaki et al., 2013; Arnell and Lloyd-Hughes, 2014; Schewe et al., 2014; Karnauskas et al., 2018) <sup>[[#fn:r382|382]]</sup> . Assuming a constant population in the models used in his study, Gerten et al. (2013) <sup>[[#fn:r383|383]]</sup> determined that an additional 8% of the world population in 2000 would be exposed to new or aggravated water scarcity at 2°C of global warming. This value was almost halved – with 50% greater reliability – when warming was constrained to 1.5°C. People inhabiting river basins, particularly in the Middle East and Near East, are projected to become newly exposed to chronic water scarcity even if global warming is constrained to less than 2°C. Many regions, especially those in Europe, Australia and southern Africa, appear to be affected at 1.5°C if the reduction in water availability is computed for non-water-scarce basins as well as for water-scarce regions. Out of a contemporary population of approximately 1.3 billion exposed to water scarcity, about 3% (North America) to 9% (Europe) are expected to be prone to aggravated scarcity at 2°C of global warming (Gerten et al., 2013) <sup>[[#fn:r384|384]]</sup> . Under the Shared Socio-Economic Pathway (SSP)2 population scenario, about 8% of the global population is projected to experience a severe reduction in water resources under warming of 1.7°C in 2021–2040, increasing to 14% of the population under 2.7°C in 2043–2071, based on the criteria of discharge reduction of either &gt;20% or &gt;1 standard deviation (Schewe et al., 2014) <sup>[[#fn:r385|385]]</sup> . Depending on the scenarios of SSP1–5, exposure to the increase in water scarcity in 2050 will be globally reduced by 184–270 million people at about 1.5°C of warming compared to the impacts at about 2°C. However, the variation between socio-economic levels is larger than the variation between warming levels (Arnell and Lloyd-Hughes, 2014) <sup>[[#fn:r386|386]]</sup> .

On many small islands (e.g., those constituting SIDS), freshwater stress is expected to occur as a result of projected aridity change. Constraining warming to 1.5°C, however, could avoid a substantial fraction of water stress compared to 2°C, especially across the Caribbean region, particularly on the island of Hispaniola (Dominican Republic and Haiti) (Karnauskas et al., 2018) <sup>[[#fn:r387|387]]</sup> . Hanasaki et al. (2013) <sup>[[#fn:r388|388]]</sup> concluded that the projected range of changes in global irrigation water withdrawal (relative to the baseline of 1971–2000), using human configuration fixing non-meteorological variables for the period around 2000, are 1.1–2.3% and 0.6–2.0% lower at 1.5°C and 2°C, respectively. In the same study, Hanasaki et al. (2013) <sup>[[#fn:r389|389]]</sup> highlighted the importance of water use scenarios in water scarcity assessments, but neither quantitative nor qualitative information regarding water use is available.

When the impacts on hydropower production at 1.5°C and 2°C are compared, it is found that mean gross potential increases in northern, eastern and western Europe, and decreases in southern Europe (Jacob et al., 2018; Tobin et al., 2018) <sup>[[#fn:r390|390]]</sup> . The Baltic and Scandinavian countries are projected to experience the most positive impacts on hydropower production. Greece, Spain and Portugal are expected to be the most negatively impacted countries, although the impacts could be reduced by limiting warming to 1.5°C (Tobin et al., 2018) <sup>[[#fn:r391|391]]</sup> . In Greece, Spain and Portugal, warming of 2°C is projected to decrease hydropower potential below 10%, while limiting global warming to 1.5°C would keep the reduction to 5% or less. There is, however, substantial uncertainty associated with these results due to a large spread between the climate models (Tobin et al., 2018) <sup>[[#fn:r392|392]]</sup> .

Due to a combination of higher water temperatures and reduced summer river flows, the usable capacity of thermoelectric power plants using river water for cooling is expected to reduce in all European countries (Jacob et al., 2018; Tobin et al., 2018) <sup>[[#fn:r393|393]]</sup> , with the magnitude of decreases being about 5% for 1.5°C and 10% for 2°C of global warming for most European countries (Tobin et al., 2018) <sup>[[#fn:r394|394]]</sup> . Greece, Spain and Bulgaria are projected to have the largest reduction at 2°C of warming (Tobin et al., 2018) <sup>[[#fn:r395|395]]</sup> .

Fricko et al. (2016) <sup>[[#fn:r396|396]]</sup> assessed the direct water use of the global energy sector across a broad range of energy system transformation pathways in order to identify the water impacts of a 2°C climate policy. This study revealed that there would be substantial divergence in water withdrawal for thermal power plant cooling under conditions in which the distribution of future cooling technology for energy generation is fixed, whereas adopting alternative cooling technologies and water resources would make the divergence considerably smaller.

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<span id="extreme-hydrological-events-floods-and-droughts"></span>
==== 3.4.2.2 Extreme hydrological events (floods and droughts) ====

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Working Group II of AR5 concluded that socio-economic losses from flooding since the mid-20th century have increased mainly because of greater exposure and vulnerability ( ''high confidence'' ) (Jiménez Cisneros et al., 2014) <sup>[[#fn:r397|397]]</sup> . There was ''low confidence'' due to ''limited evidence'' , however, that anthropogenic climate change has affected the frequency and magnitude of floods. WGII AR5 also concluded that there is no evidence that surface water and groundwater drought frequency has changed over the last few decades, although impacts of drought have increased mostly owing to increased water demand (Jiménez Cisneros et al., 2014) <sup>[[#fn:r398|398]]</sup> .

Since AR5, the number of studies related to fluvial flooding and meteorological drought based on long-term observed data has been gradually increasing. There has also been progress since AR5 in identifying historical changes in streamflow and continental runoff (Section 3.3.5). As a result of population and economic growth, increased exposure of people and assets has caused more damage due to flooding. However, differences in flood risks among regions reflect the balance among the magnitude of the flood, the populations, their vulnerabilities, the value of assets affected by flooding, and the capacity to cope with flood risks, all of which depend on socio-economic development conditions, as well as topography and hydro-climatic conditions (Tanoue et al., 2016) <sup>[[#fn:r399|399]]</sup> . AR5 concluded that there was ''low confidence'' in the attribution of global changes in droughts (Bindoff et al., 2013b) <sup>[[#fn:r400|400]]</sup> . However, recent publications based on observational and modelling evidence assessed that human emissions have substantially increased the probability of drought years in the Mediterranean region (Section 3.3.4).

WGII AR5 assessed that global flood risk will increase in the future, partly owing to climate change ( ''low to medium confidence'' ), with projected changes in the frequency of droughts longer than 12 months being more uncertain because of their dependence on accumulated precipitation over long periods (Jiménez Cisneros et al., 2014) <sup>[[#fn:r401|401]]</sup> .

Increases in the risks associated with runoff at the global scale ( ''medium confidence'' ), and in flood hazard in some regions ( ''medium confidence'' ), can be expected at global warming of 1.5°C, with an overall increase in the area affected by flood hazard at 2°C ( ''medium confidence'' ) (Section 3.3.5). There are studies, however, that indicate that socio-economic conditions will exacerbate flood impacts more than global climate change, and that the magnitude of these impacts could be larger in some regions (Arnell and Lloyd-Hughes, 2014; Winsemius et al., 2016; Alfieri et al., 2017; Arnell et al., 2018; Kinoshita et al., 2018) <sup>[[#fn:r402|402]]</sup> . Assuming constant population sizes, countries representing 73% of the world population will experience increasing flood risk, with an average increase of 580% at 4°C compared to the impact simulated over the baseline period 1976–2005. This impact is projected to be reduced to a 100% increase at 1.5°C and a 170% increase at 2°C (Alfieri et al., 2017) <sup>[[#fn:r403|403]]</sup> . Alfieri et al. (2017) <sup>[[#fn:r404|404]]</sup> additionally concluded that the largest increases in flood risks would be found in the US, Asia, and Europe in general, while decreases would be found in only a few countries in eastern Europe and Africa. Overall, Alfieri et al., (2017) <sup>[[#fn:r405|405]]</sup> reported that the projected changes are not homogeneously distributed across the world land surface. Alfieri et al. (2018) <sup>[[#fn:r406|406]]</sup> studied the population affected by flood events using three case studies in European states, specifically central and western Europe, and found that the population affected could be limited to 86% at 1.5°C of warming compared to 93% at 2°C. Under the SSP2 population scenario, Arnell et al. (2018) <sup>[[#fn:r407|407]]</sup> found that 39% (range 36–46%) of impacts on populations exposed to river flooding globally could be avoided at 1.5°C compared to 2°C of warming.

Under scenarios SSP1–5, Arnell and Lloyd-Hughes (2014) <sup>[[#fn:r408|408]]</sup> found that the number of people exposed to increased flooding in 2050 under warming of about 1.5°C could be reduced by 26–34 million compared to the number exposed to increased flooding associated with 2°C of warming. Variation between socio-economic levels, however, is projected to be larger than variation between the two levels of global warming. Kinoshita et al. (2018) <sup>[[#fn:r409|409]]</sup> found that a serious increase in potential flood fatality (5.7%) is projected without any adaptation if global warming increases from 1.5°C to 2°C, whereas the projected increase in potential economic loss (0.9%) is relatively small. Nevertheless, their study indicates that socio-economic changes make a larger contribution to the potentially increased consequences of future floods, and about half of the increase in potential economic losses could be mitigated by autonomous adaptation.

There is limited information about the global and regional projected risks posed by droughts at 1.5°C and 2°C of global warming. However, hazards by droughts at 1.5°C could be reduced compared to the hazards at 2°C in some regions, in particular in the Mediterranean region and southern Africa (Section 3.3.4). Under constant socio-economic conditions, the population exposed to drought at 2°C of warming is projected to be larger than at 1.5°C ( ''low to medium confidence'' ) (Smirnov et al., 2016; Sun et al., 2017; Arnell et al., 2018; Liu et al., 2018) <sup>[[#fn:r410|410]]</sup> . Under the same scenario, the global mean monthly number of people expected to be exposed to extreme drought at 1.5°C in 2021–2040 is projected to be 114.3 million, compared to 190.4 million at 2°C in 2041–2060 (Smirnov et al., 2016) <sup>[[#fn:r411|411]]</sup> . Under the SSP2 population scenario, Arnell et al. (2018) <sup>[[#fn:r412|412]]</sup> projected that 39% (range 36–51%) of impacts on populations exposed to drought could be globally avoided at 1.5°C compared to 2°C warming.

Liu et al. (2018) <sup>[[#fn:r413|413]]</sup> studied the changes in population exposure to severe droughts in 27 regions around the globe for 1.5°C and 2°C of warming using the SSP1 population scenario compared to the baseline period of 1986–2005 based on the Palmer Drought Severity Index (PDSI). They concluded that the drought exposure of urban populations in most regions would be decreased at 1.5°C (350.2 ± 158.8 million people) compared to 2°C (410.7 ± 213.5 million people). Liu et al. (2018) <sup>[[#fn:r414|414]]</sup> also suggested that more urban populations would be exposed to severe droughts at 1.5ºC in central Europe, southern Europe, the Mediterranean, West Africa, East and West Asia, and Southeast Asia, and that number of affected people would increase further in these regions at 2°C. However, it should be noted that the PDSI is known to have limitations (IPCC SREX, Seneviratne et al., 2012) <sup>[[#fn:r415|415]]</sup> , and drought projections strongly depend on considered indices (Section 3.3.4); thus only ''medium confidence'' is assigned to these projections. In the Haihe River basin in China, a study has suggested that the proportion of the population exposed to droughts is projected to be reduced by 30.4% at 1.5°C but increased by 74.8% at 2°C relative to the baseline value of 339.65 million people in the 1986–2005 period, when assessing changes in droughts using the Standardized Precipitation-Evaporation Index, using a Penman–Monteith estimate of potential evaporation (Sun et al., 2017) <sup>[[#fn:r416|416]]</sup> .

Alfieri et al. (2018) <sup>[[#fn:r417|417]]</sup> estimated damage from flooding in Europe for the baseline period (1976–2005) at 5 billion euro of losses annually, with projections of relative changes in flood impacts that will rise with warming levels, from 116% at 1.5°C to 137% at 2°C.

Kinoshita et al. (2018) <sup>[[#fn:r418|418]]</sup> studied the increase of potential economic loss under SSP3 and projected that the smaller loss at 1.5°C compared to 2°C (0.9%) is marginal, regardless of whether the vulnerability is fixed at the current level or not. By analysing the differences in results with and without flood protection standards, Winsemius et al. (2016) <sup>[[#fn:r419|419]]</sup> showed that adaptation measures have the potential to greatly reduce present-day and future flood damage. They concluded that increases in flood-induced economic impacts (% gross domestic product, GDP) in African countries are mainly driven by climate change and that Africa’s growing assets would become increasingly exposed to floods. Hence, there is an increasing need for long-term and sustainable investments in adaptation in Africa.

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<span id="groundwater"></span>
==== 3.4.2.3 Groundwater ====

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Working Group II of AR5 concluded that the detection of changes in groundwater systems, and attribution of those changes to climatic changes, are rare, owing to a lack of appropriate observation wells and an overall small number of studies (Jiménez Cisneros et al., 2014) <sup>[[#fn:r420|420]]</sup> .

Since AR5, the number of studies based on long-term observed data continues to be limited. The groundwater-fed lakes in northeastern central Europe have been affected by climate and land-use changes, and they showed a predominantly negative lake-level trend in 1999–2008 (Kaiser et al., 2014) <sup>[[#fn:r421|421]]</sup> .

WGII AR5 concluded that climate change is projected to reduce groundwater resources significantly in most dry subtropical regions ( ''high confidence'' ) (Jiménez Cisneros et al., 2014) <sup>[[#fn:r422|422]]</sup> .

In some regions, groundwater is often intensively used to supplement the excess demand, often leading to groundwater depletion. Climate change adds further pressure on water resources and exaggerates human water demands by increasing temperatures over agricultural lands (Wada et al., 2017) <sup>[[#fn:r423|423]]</sup> . Very few studies have projected the risks of groundwater depletion under 1.5°C and 2°C of global warming. Under 2°C of warming, impacts posed on groundwater are projected to be greater than at 1.5°C ( ''low confidence'' ) (Portmann et al., 2013; Salem et al., 2017) <sup>[[#fn:r424|424]]</sup> .

Portmann et al. (2013) <sup>[[#fn:r425|425]]</sup> indicated that 2% (range 1.1–2.6%) of the global land area is projected to suffer from an extreme decrease in renewable groundwater resources of more than 70% at 2°C, with a clear mitigation at 1.5°C. These authors also projected that 20% of the global land surface would be affected by a groundwater reduction of more than 10% at 1.5°C of warming, with the percentage of land impacted increasing at 2°C. In a groundwater-dependent irrigated region in northwest Bangladesh, the average groundwater level during the major irrigation period (January–April) is projected to decrease in accordance with temperature rise (Salem et al., 2017) <sup>[[#fn:r426|426]]</sup> .

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<span id="water-quality"></span>
==== 3.4.2.4 Water quality ====

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Working Group II of AR5 concluded that most observed changes to water quality from climate change are from isolated studies, mostly of rivers or lakes in high-income countries, using a small number of variables (Jiménez Cisneros et al., 2014) <sup>[[#fn:r427|427]]</sup> . AR5 assessed that climate change is projected to reduce raw water quality, posing risks to drinking water quality with conventional treatment ( ''medium to high confidence'' ) (Jiménez Cisneros et al., 2014) <sup>[[#fn:r428|428]]</sup> .

Since AR5, studies have detected climate change impacts on several indices of water quality in lakes, watersheds and regions (e.g., Patiño et al., 2014; Aguilera et al., 2015; Watts et al., 2015; Marszelewski and Pius, 2016; Capo et al., 2017) <sup>[[#fn:r429|429]]</sup> . The number of studies utilising RCP scenarios at the regional or watershed scale have gradually increased since AR5 (e.g., Boehlert et al., 2015; Teshager et al., 2016; Marcinkowski et al., 2017) <sup>[[#fn:r430|430]]</sup> . Few studies, have explored projected impacts on water quality under 1.5°C versus 2°C of warming, however, the differences are unclear ( ''low confidence'' ) (Bonte and Zwolsman, 2010 <sup>[[#fn:r431|431]]</sup> ; Hosseini et al., 2017) <sup>[[#fn:r432|432]]</sup> . The daily probability of exceeding the chloride standard for drinking water taken from Lake IJsselmeer (Andijk, the Netherlands) is projected to increase by a factor of about five at 2°C relative to the present-day warming level of 1°C since 1990 (Bonte and Zwolsman, 2010) <sup>[[#fn:r433|433]]</sup> . Mean monthly dissolved oxygen concentrations and nutrient concentrations in the upper Qu’Appelle River (Canada) in 2050–2055 are projected to decrease less at about 1.5°C of warming (RCP2.6) compared to concentrations at about 2°C (RCP4.5) (Hosseini et al., 2017) <sup>[[#fn:r434|434]]</sup> . In three river basins in Southeast Asia (Sekong, Sesan and Srepok), about 2°C of warming (corresponding to a 1.05°C increase in the 2030s relative to the baseline period 1981–2008, RCP8.5), impacts posed by land-use change on water quality are projected to be greater than at 1.5°C (corresponding to a 0.89°C increase in the 2030s relative to the baseline period 1981–2008, RCP4.5) (Trang et al., 2017) <sup>[[#fn:r435|435]]</sup> . Under the same warming scenarios, Trang et al. (2017) <sup>[[#fn:r436|436]]</sup> projected changes in the annual nitrogen (N) and phosphorus (P) yields in the 2030s, as well as with combinations of two land-use change scenarios: (i) conversion of forest to grassland, and (ii) conversion of forest to agricultural land. The projected changes in N (P) yield are +7.3% (+5.1%) under a 1.5°C scenario and –6.6% (–3.6%) under 2°C, whereas changes under the combination of land-use scenarios are (i) +5.2% (+12.6%) at 1.5°C and +8.8% (+11.7%) at 2°C, and (ii) +7.5% (+14.9%) at 1.5°C and +3.7% (+8.8%) at 2°C (Trang et al., 2017) <sup>[[#fn:r437|437]]</sup> .

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<span id="soil-erosion-and-sediment-load"></span>
==== 3.4.2.5 Soil erosion and sediment load ====

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Working Group II of AR5 concluded that there is little or no observational evidence that soil erosion and sediment load have been altered significantly by climate change ( ''low to medium confidence'' ) (Jiménez Cisneros et al., 2014) <sup>[[#fn:r438|438]]</sup> . As the number of studies on climate change impacts on soil erosion has increased where rainfall is an important driver (Lu et al., 2013) <sup>[[#fn:r439|439]]</sup> , studies have increasingly considered other factors, such as rainfall intensity (e.g., Shi and Wang, 2015; Li and Fang, 2016) <sup>[[#fn:r440|440]]</sup> , snow melt, and change in vegetation cover resulting from temperature rise (Potemkina and Potemkin, 2015) <sup>[[#fn:r441|441]]</sup> , as well as crop management practices (Mullan et al., 2012) <sup>[[#fn:r442|442]]</sup> . WGII AR5 concluded that increases in heavy rainfall and temperature are projected to change soil erosion and sediment yield, although the extent of these changes is highly uncertain and depends on rainfall seasonality, land cover, and soil management practices (Jiménez Cisneros et al., 2014) <sup>[[#fn:r443|443]]</sup> .

While the number of published studies of climate change impacts on soil erosion have increased globally since 2000 (Li and Fang, 2016) <sup>[[#fn:r444|444]]</sup> , few articles have addressed impacts at 1.5°C and 2°C of global warming. The existing studies have found few differences in projected risks posed on sediment load under 1.5°C and 2°C ( ''low confidence'' ) (Cousino et al., 2015; Shrestha et al., 2016) <sup>[[#fn:r445|445]]</sup> . The differences between average annual sediment load under 1.5°C and 2°C of warming are not clear, owing to complex interactions among climate change, land cover/surface and soil management (Cousino et al., 2015; Shrestha et al., 2016) <sup>[[#fn:r446|446]]</sup> . Averages of annual sediment loads are projected to be similar under 1.5°C and 2°C of warming, in particular in the Great Lakes region in the USA and in the Lower Mekong region in Southeast Asia (Cross-Chapter Box 6 in this chapter, Cousino et al., 2015; Shrestha et al., 2016) <sup>[[#fn:r447|447]]</sup> .

<span id="terrestrial-and-wetland-ecosystems"></span>
=== 3.4.3 Terrestrial and Wetland Ecosystems ===

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<span id="biome-shifts"></span>
==== 3.4.3.1 Biome shifts ====

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Latitudinal and elevational shifts of biomes (major ecosystem types) in boreal, temperate and tropical regions have been detected (Settele et al., 2014) <sup>[[#fn:r448|448]]</sup> and new studies confirm these changes (e.g., shrub encroachment on tundra; Larsen et al., 2014) <sup>[[#fn:r449|449]]</sup> . Attribution studies indicate that anthropogenic climate change has made a greater contribution to these changes than any other factor ( ''medium confidence'' ) (Settele et al., 2014) <sup>[[#fn:r450|450]]</sup> .

An ensemble of seven Dynamic Vegetation Models driven by projected climates from 19 alternative general circulation models (GCMs) (Warszawski et al., 2013) <sup>[[#fn:r451|451]]</sup> shows 13% (range 8–20%) of biomes transforming at 2°C of global warming, but only 4% (range 2–7%) doing so at 1°C, suggesting that about 6.5% may be transformed at 1.5°C; these estimates indicate a doubling of the areal extent of biome shifts between 1.5°C and 2°C of warming ( ''medium confidence'' ) (Figure 3.16a). A study using the single ecosystem model LPJmL (Gerten et al., 2013) <sup>[[#fn:r452|452]]</sup> illustrated that biome shifts in the Arctic, Tibet, Himalayas, southern Africa and Australia would be avoided by constraining warming to 1.5°C compared with 2°C (Figure 3.16b). Seddon et al. (2016) <sup>[[#fn:r453|453]]</sup> quantitatively identified ecologically sensitive regions to climate change in most of the continents from tundra to tropical rainforest. Biome transformation may in some cases be associated with novel climates and ecological communities (Prober et al., 2012) <sup>[[#fn:r454|454]]</sup> .

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<span id="a-fraction-of-global-natural-vegetation-including-managed-forests-at-risk-of-severe-ecosystem-change-as-a-function-of-global-mean-temperature-change-for-all-ecosystems-models-global-climate-change-models-and-representative-concentration-pathways-rcps."></span>
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'''(a) Fraction of global natural vegetation (including managed forests) at risk of severe ecosystem change as a function of global mean temperature change for all ecosystems, models, global climate change models and Representative Concentration Pathways (RCPs).'''
<!-- IMG FILE -->
[[File:fef2728679f8329baa7d71882cc054c0 Figure-3.16a-1024x736.jpg]]

The colours represent the different ecosystem models, which are also horizontally separated for clarity. Results are collated in unit-degree bins, where the temperature for a given year is the average over a 30-year window centred on that year. The boxes span the 25th and 75th percentiles across the entire ensemble. The short, horizontal stripes represent individual (annual) data points, the curves connect the mean value per ecosystem model in each bin. The solid (dashed) curves are for models with (without) dynamic vegetation composition changes. Source: (Warszawski et al., 2013) <sup>[[#fn:r455|455]]</sup>

<!-- END IMG -->
<div id="section-3-4-3-1-block-3"></div>

<span id="figure-3.16b"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 3.16b'''

<span id="b-threshold-level-of-global-temperature-anomaly-above-pre-industrial-levels-that-leads-to-significant-local-changes-in-terrestrial-ecosystems."></span>
<!-- IMG CAPTION -->
'''(b) Threshold level of global temperature anomaly above pre-industrial levels that leads to significant local changes in terrestrial ecosystems.'''
<!-- IMG FILE -->
[[File:0d97820d01076b5b32d68d0c7864863f figure-3.16b-1024x512.jpg]]

Regions with severe (coloured) or moderate (greyish) ecosystem transformation; delineation refers to the 90 biogeographic regions. All values denote changes found in &gt;50% of the simulations. Source: (Gerten et al., 2013) <sup>[[#fn:r456|456]]</sup> . Regions coloured in dark red are projected to undergo severe transformation under a global warming of 1.5°C while those coloured in light red do so at 2°C; other colours are used when there is no severe transformation unless global warming exceeds 2°C.

<!-- END IMG -->
<div id="section-3-4-3-2"></div>

<span id="changes-in-phenology"></span>
==== 3.4.3.2 Changes in phenology ====

<div id="section-3-4-3-2-block-1"></div>

Advancement in spring phenology of 2.8 ± 0.35 days per decade has been observed in plants and animals in recent decades in most Northern Hemisphere ecosystems (between 30°N and 72°N), and these shifts have been attributed to changes in climate ( ''high confidence'' ) (Settele et al., 2014) <sup>[[#fn:r457|457]]</sup> . The rates of change are particularly high in the Arctic zone owing to the stronger local warming (Oberbauer et al., 2013) <sup>[[#fn:r458|458]]</sup> , whereas phenology in tropical forests appears to be more responsive to moisture stress (Zhou et al., 2014) <sup>[[#fn:r459|459]]</sup> . While a full review cannot be included here, trends consistent with this earlier finding continue to be detected, including in the flowering times of plants (Parmesan and Hanley, 2015) <sup>[[#fn:r460|460]]</sup> , in the dates of egg laying and migration in birds (newly reported in China; Wu and Shi, 2016) <sup>[[#fn:r461|461]]</sup> , in the emergence dates of butterflies (Roy et al., 2015) <sup>[[#fn:r462|462]]</sup> , and in the seasonal greening-up of vegetation as detected by satellites (i.e., in the normalized difference vegetation index, NDVI; Piao et al., 2015) <sup>[[#fn:r463|463]]</sup> .

The potential for decoupling species–species interactions owing to differing phenological responses to climate change is well established (Settele et al., 2014) <sup>[[#fn:r464|464]]</sup> , for example for plants and their insect pollinators (Willmer, 2012; Scaven and Rafferty, 2013) <sup>[[#fn:r465|465]]</sup> . Mid-century projections of plant and animal phenophases in the UK clearly indicate that the timing of phenological events could change more for primary consumers (6.2 days earlier on average) than for higher trophic levels (2.5–2.9 days earlier on average) (Thackeray et al., 2016) <sup>[[#fn:r466|466]]</sup> . This indicates the potential for phenological mismatch and associated risks for ecosystem functionality in the future under global warming of 2.1°C–2.7°C above pre-industrial levels. Further, differing responses could alter community structure in temperate forests (Roberts et al., 2015) <sup>[[#fn:r467|467]]</sup> . Specifically, temperate forest phenology is projected to advance by 14.3 days in the near term (2010–2039) and 24.6 days in the medium term (2040–2069), so as a first approximation the difference between 2°C and 1.5°C of global warming is about 10 days (Roberts et al., 2015) <sup>[[#fn:r468|468]]</sup> . This phenological plasticity is not always adaptive and must be interpreted cautiously (Duputié et al., 2015) <sup>[[#fn:r469|469]]</sup> , and considered in the context of accompanying changes in climate variability (e.g., increased risk of frost damage for plants or earlier emergence of insects resulting in mortality during cold spells). Another adaptive response of some plants is range expansion with increased vigour and altered herbivore resistance in their new range, analogous to invasive plants (Macel et al., 2017) <sup>[[#fn:r470|470]]</sup> .

In summary, limiting warming to 1.5°C compared with 2°C may avoid advance in spring phenology ( ''high confidence'' ) by perhaps a few days ( ''medium confidence'' ) and hence decrease the risks of loss of ecosystem functionality due to phenological mismatch between trophic levels, and also of maladaptation coming from the sensitivity of many species to increased climate variability. Nevertheless, this difference between 1.5°C and 2°C of warming might be limited for plants that are able to expand their range.

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

<span id="changes-in-species-range-abundance-and-extinction"></span>
==== 3.4.3.3 Changes in species range, abundance and extinction ====

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

AR5 (Settele et al., 2014) <sup>[[#fn:r471|471]]</sup> concluded that the geographical ranges of many terrestrial and freshwater plant and animal species have moved over the last several decades in response to warming: approximately 17 km poleward and 11 m up in altitude per decade. Recent trends confirm this finding; for example, the spatial and interspecific variance in bird populations in Europe and North America since 1980 were found to be well predicted by trends in climate suitability (Stephens et al., 2016) <sup>[[#fn:r472|472]]</sup> . Further, a recent meta-analysis of 27 studies concerning a total of 976 species (Wiens, 2016) <sup>[[#fn:r473|473]]</sup> found that 47% of local extinctions (extirpations) reported across the globe during the 20th century could be attributed to climate change, with significantly more extinctions occurring in tropical regions, in freshwater habitats and for animals. IUCN (2018) <sup>[[#fn:r474|474]]</sup> lists 305 terrestrial animal and plant species from Pacific Island developing nations as being threatened by climate change and severe weather. Owing to lags in the responses of some species to climate change, shifts in insect pollinator ranges may result in novel assemblages with unknown implications for biodiversity and ecosystem function (Rafferty, 2017) <sup>[[#fn:r475|475]]</sup> .

Warren et al. (2013) <sup>[[#fn:r476|476]]</sup> simulated climatically determined geographic range loss under 2°C and 4°C of global warming for 50,000 plant and animal species, accounting for uncertainty in climate projections and for the potential ability of species to disperse naturally in an attempt to track their geographically shifting climate envelope. This earlier study has now been updated and expanded to incorporate 105,501 species, including 19,848 insects, and new findings indicate that warming of 2°C by 2100 would lead to projected bioclimatic range losses of &gt;50% in 18% (6–35%) of the 19,848 insects species, 8% (4–16%) of the 12,429 vertebrate species, and 16% (9–28%) of the 73,224 plant species studied (Warren et al., 2018a) <sup>[[#fn:r477|477]]</sup> . At 1.5°C of warming, these values fall to 6% (1–18%) of the insects, 4% (2–9%) of the vertebrates and 8% (4–15%) of the plants studied. Hence, the number of insect species projected to lose over half of their geographic range is reduced by two-thirds when warming is limited to 1.5°C compared with 2°C, while the number of vertebrate and plant species projected to lose over half of their geographic range is halved (Warren et al., 2018a) <sup>[[#fn:r478|478]]</sup> ( ''medium confidence'' ). These findings are consistent with estimates made from an earlier study suggesting that range losses at 1.5°C were significantly lower for plants than those at 2°C of warming (Smith et al., 2018) <sup>[[#fn:r479|479]]</sup> . It should be noted that at 1.5°C of warming, and if species’ ability to disperse naturally to track their preferred climate geographically is inhibited by natural or anthropogenic obstacles, there would still remain 10% of the amphibians, 8% of the reptiles, 6% of the mammals, 5% of the birds, 10% of the insects and 8% of the plants which are projected to lose over half their range, while species on average lose 20–27% of their range (Warren et al., 2018a) <sup>[[#fn:r480|480]]</sup> . Given that bird and mammal species can disperse more easily than amphibians and reptiles, a small proportion can expand their range as climate changes, but even at 1.5°C of warming the total range loss integrated over all birds and mammals greatly exceeds the integrated range gain (Warren et al., 2018a) <sup>[[#fn:r481|481]]</sup> .

A number of caveats are noted for studies projecting changes to climatic range. This approach, for example, does not incorporate the effects of extreme weather events and the role of interactions between species. As well, trophic interactions may locally counteract the range expansion of species towards higher altitudes (Bråthen et al., 2018) <sup>[[#fn:r482|482]]</sup> . There is also the potential for highly invasive species to become established in new areas as the climate changes (Murphy and Romanuk, 2014) <sup>[[#fn:r483|483]]</sup> , but there is no literature that quantifies this possibility for 1.5°C of global warming.

Pecl et al. (2017) <sup>[[#fn:r484|484]]</sup> summarized at the global level the consequences of climate-change-induced species redistribution for economic development, livelihoods, food security, human health and culture. These authors concluded that even if anthropogenic greenhouse gas emissions stopped today, the effort for human systems to adapt to the most crucial effects of climate-driven species redistribution will be far-reaching and extensive. For example, key insect crop pollinator families (Apidae, Syrphidae and Calliphoridae; i.e., bees, hoverflies and blowflies) are projected to retain significantly greater geographic ranges under 1.5°C of global warming compared with 2°C (Warren et al., 2018a) <sup>[[#fn:r485|485]]</sup> . In some cases, when species (such as pest and disease species) move into areas which have become climatically suitable they may become invasive or harmful to human or natural systems (Settele et al., 2014) <sup>[[#fn:r486|486]]</sup> . Some studies are beginning to locate ‘refugial’ areas where the climate remains suitable in the future for most of the species currently present. For example, Smith et al., (2018) <sup>[[#fn:r487|487]]</sup> estimated that 5.5–14% more of the globe’s terrestrial land area could act as climatic refugia for plants under 1.5°C of warming compared to 2°C.

There is no literature that directly estimates the proportion of species at increased risk of global (as opposed to local) commitment to extinction as a result of climate change, as this is inherently difficult to quantify. However, it is possible to compare the proportions of species at risk of very high range loss; for example, a discernibly smaller number of terrestrial species are projected to lose over 90% of their range at 1.5°C of global warming compared with 2°C (Figure 2 in Warren et al., 2018a) <sup>[[#fn:r488|488]]</sup> . A link between very high levels of range loss and greatly increased extinction risk may be inferred (Urban, 2015) <sup>[[#fn:r489|489]]</sup> . Hence, limiting global warming to 1.5°C compared with 2°C would be expected to reduce both range losses and associated extinction risks in terrestrial species ( ''high confidence'' ).

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

<span id="changes-in-ecosystem-function-biomass-and-carbon-stocks"></span>
==== 3.4.3.4 Changes in ecosystem function, biomass and carbon stocks ====

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

Working Group II of AR5 (Settele et al., 2014) <sup>[[#fn:r490|490]]</sup> concluded that there is ''high confidence'' that net terrestrial ecosystem productivity at the global scale has increased relative to the pre-industrial era and that rising CO <sub>2</sub> concentrations are contributing to this trend through stimulation of photosynthesis. There is, however, no clear and consistent signal of a climate change contribution. In northern latitudes, the change in productivity has a lower velocity than the warming, possibly because of a lack of resource and vegetation acclimation mechanisms (M. Huang et al., 2017) <sup>[[#fn:r491|491]]</sup> . Biomass and soil carbon stocks in terrestrial ecosystems are currently increasing ( ''high confidence'' ), but they are vulnerable to loss of carbon to the atmosphere as a result of projected increases in the intensity of storms, wildfires, land degradation and pest outbreaks (Settele et al., 2014; Seidl et al., 2017) <sup>[[#fn:r492|492]]</sup> . These losses are expected to contribute to a decrease in the terrestrial carbon sink. Anderegg et al. (2015) <sup>[[#fn:r493|493]]</sup> demonstrated that total ecosystem respiration at the global scale has increased in response to increases in night-time temperature (1 PgC yr <sup>–1</sup> °C <sup>–1</sup> , ''P'' =0.02).

The increase in total ecosystem respiration in spring and autumn, associated with higher temperatures, may convert boreal forests from carbon sinks to carbon sources (Hadden and Grelle, 2016) <sup>[[#fn:r494|494]]</sup> . In boreal peatlands, for example, increased temperature may diminish carbon storage and compromise the stability of the peat (Dieleman et al., 2016) <sup>[[#fn:r495|495]]</sup> . In addition, J. Yang et al. (2015) <sup>[[#fn:r496|496]]</sup> showed that fires reduce the carbon sink of global terrestrial ecosystems by 0.57 PgC yr <sup>–1</sup> in ecosystems with large carbon stores, such as peatlands and tropical forests. Consequently, for adaptation purposes, it is necessary to enhance carbon sinks, especially in forests which are prime regulators within the water, energy and carbon cycles (Ellison et al., 2017) <sup>[[#fn:r497|497]]</sup> . Soil can also be a key compartment for substantial carbon sequestration (Lal, 2014; Minasny et al., 2017) <sup>[[#fn:r498|498]]</sup> , depending on the net biome productivity and the soil quality (Bispo et al., 2017) <sup>[[#fn:r499|499]]</sup> .

AR5 assessed that large uncertainty remains regarding the land carbon cycle behaviour of the future (Ciais et al., 2013) <sup>[[#fn:r500|500]]</sup> , with most, but not all, CMIP5 models simulating continued terrestrial carbon uptake under all four RCP scenarios (Jones et al., 2013) <sup>[[#fn:r501|501]]</sup> . Disagreement between models outweighs differences between scenarios even up to the year 2100 (Hewitt et al., 2016; Lovenduski and Bonan, 2017) <sup>[[#fn:r502|502]]</sup> . Increased atmospheric CO <sub>2</sub> concentrations are expected to drive further increases in the land carbon sink (Ciais et al., 2013; Schimel et al., 2015) <sup>[[#fn:r503|503]]</sup> , which could persist for centuries (Pugh et al., 2016) <sup>[[#fn:r504|504]]</sup> . Nitrogen, phosphorus and other nutrients will limit the terrestrial carbon cycle response to both elevated CO <sub>2</sub> and altered climate (Goll et al., 2012; Yang et al., 2014; Wieder et al., 2015; Zaehle et al., 2015; Ellsworth et al., 2017) <sup>[[#fn:r505|505]]</sup> . Climate change may accelerate plant uptake of carbon (Gang et al., 2015) <sup>[[#fn:r506|506]]</sup> but also increase the rate of decomposition (Todd-Brown et al., 2014; Koven et al., 2015; Crowther et al., 2016) <sup>[[#fn:r507|507]]</sup> . Ahlström et al. (2012) <sup>[[#fn:r508|508]]</sup> found a net loss of carbon in extra-tropical regions and the largest spread across model results in the tropics. The projected net effect of climate change is to reduce the carbon sink expected under CO <sub>2</sub> increase alone (Settele et al., 2014) <sup>[[#fn:r509|509]]</sup> . Friend et al. (2014) <sup>[[#fn:r510|510]]</sup> found substantial uptake of carbon by vegetation under future scenarios when considering the effects of both climate change and elevated CO <sub>2</sub> .

There is limited published literature examining modelled land carbon changes specifically under 1.5°C of warming, but existing CMIP5 models and published data are used in this report to draw some conclusions. For systems with significant inertia, such as vegetation or soil carbon stores, changes in carbon storage will depend on the rate of change of forcing and thus depend on the choice of scenario (Jones et al., 2009; Ciais et al., 2013; Sihi et al., 2017) <sup>[[#fn:r511|511]]</sup> . To avoid legacy effects of the choice of scenario, this report focuses on the response of gross primary productivity (GPP) – the rate of photosynthetic carbon uptake – by the models, rather than by changes in their carbon store.

Figure 3.17 shows different responses of the terrestrial carbon cycle to climate change in different regions. The models show a consistent response of increased GPP in temperate latitudes of approximately 2 GtC yr <sup>–1 °</sup> C <sup>–1</sup> . Similarly, Gang et al. (2015) <sup>[[#fn:r512|512]]</sup> projected a robust increase in the net primary productivity (NPP) of temperate forests. However, Ahlström et al. (2012) <sup>[[#fn:r513|513]]</sup> showed that this effect could be offset or reversed by increases in decomposition. Globally, most models project that GPP will increase or remain approximately unchanged (Hashimoto et al., 2013) <sup>[[#fn:r514|514]]</sup> . This projection is supported by findings by Sakalli et al. (2017) <sup>[[#fn:r515|515]]</sup> for Europe using Euro-CORDEX regional models under a 2°C global warming for the period 2034–2063, which indicated that storage will increase by 5% in soil and by 20% in vegetation. However, using the same models Jacob et al. (2018) <sup>[[#fn:r516|516]]</sup> showed that limiting warming to 1.5°C instead of 2°C avoids an increase in ecosystem vulnerability (compared to a no-climate change scenario) of 40–50%.

At the global level, linear scaling is acceptable for net primary production, biomass burning and surface runoff, and impacts on terrestrial carbon storage are projected to be greater at 2°C than at 1.5°C (Tanaka et al., 2017) <sup>[[#fn:r517|517]]</sup> . If global CO <sub>2</sub> concentrations and temperatures stabilize, or peak and decline, then both land and ocean carbon sinks – which are primarily driven by the continued increase in atmospheric CO <sub>2</sub> – will also decline and may even become carbon sources (Jones et al., 2016) <sup>[[#fn:r518|518]]</sup> . Consequently, if a given amount of anthropogenic CO <sub>2</sub> is removed from the atmosphere, an equivalent amount of land and ocean anthropogenic CO <sub>2</sub> will be released to the atmosphere (Cao and Caldeira, 2010) <sup>[[#fn:r519|519]]</sup> .

In conclusion ''',''' ecosystem respiration is expected to increase with increasing temperature, thus reducing soil carbon storage. Soil carbon storage is expected to be larger if global warming is restricted to 1.5°C, although some of the associated changes will be countered by enhanced gross primary production due to elevated CO <sub>2</sub> concentrations (i.e., the ‘fertilization effect’) and higher temperatures, especially at mid-and high latitudes ( ''medium confidence'' ).

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

<span id="figure-3.17"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 3.17'''

<span id="the-response-of-terrestrial-productivity-gross-primary-productivity-gpp-to-climate-change-globally-top-left-and-for-three-latitudinal-regions-30s30n-3060n-and-6090n."></span>
<!-- IMG CAPTION -->
'''The response of terrestrial productivity (gross primary productivity, GPP) to climate change, globally (top left) and for three latitudinal regions: 30°S–30°N; 30–60°N and 60–90°N.'''
<!-- IMG FILE -->
[[File:eed1efa6770815d3006b426ffc7e5322 figure-3.17-1024x755.jpg]]

Data come from the Coupled Model Intercomparison Project Phase 5 (CMIP5) archive (http://cmip-pcmdi.llnl.gov/cmip5/). Seven Earth System Models were used: Norwegian Earth System Model (NorESM-ME, yellow); Community Earth System Model (CESM, red); Institute Pierre Simon Laplace (IPLS)-CM5-LR (dark blue); Geophysical Fluid Dynamics Laboratory (GFDL, pale blue); Max Plank Institute-Earth System Model (MPI-ESM, pink); Hadley Centre New Global Environmental Model 2-Earth System (HadGEM2-ES, orange); and Canadian Earth System Model 2 (CanESM2, green). Differences in GPP between model simulations with (‘1pctCO <sub>2</sub> ’) and without (‘esmfixclim1’) the effects of climate change are shown. Data are plotted against the global mean temperature increase above pre-industrial levels from simulations with a 1% per year increase in CO <sub>2</sub> (‘1pctCO <sub>2</sub> ’).

Original Creation for this Report using CMIP5

<!-- END IMG -->
<div id="section-3-4-3-5"></div>

<span id="regional-and-ecosystem-specific-risks"></span>
==== 3.4.3.5 Regional and ecosystem-specific risks ====

<div id="section-3-4-3-5-block-1"></div>

A large number of threatened systems, including mountain ecosystems, highly biodiverse tropical wet and dry forests, deserts, freshwater systems and dune systems, were assessed in AR5. These include Mediterranean areas in Europe, Siberian, tropical and desert ecosystems in Asia, Australian rainforests, the Fynbos and succulent Karoo areas of South Africa, and wetlands in Ethiopia, Malawi, Zambia and Zimbabwe. In all these systems, it has been shown that impacts accrue with greater warming, and thus impacts at 2°C are expected to be greater than those at 1.5°C ( ''medium confidence'' ).

The High Arctic region, with tundra-dominated landscapes, has warmed more than the global average over the last century (Section 3.3; Settele et al., 2014) <sup>[[#fn:r520|520]]</sup> . The Arctic tundra biome is experiencing increasing fire disturbance and permafrost degradation (Bring et al., 2016; DeBeer et al., 2016; Jiang et al., 2016; Yang et al., 2016) <sup>[[#fn:r521|521]]</sup> . Both of these processes facilitate the establishment of woody species in tundra areas. Arctic terrestrial ecosystems are being disrupted by delays in winter onset and mild winters associated with global warming ( ''high confidence'' ) (Cooper, 2014) <sup>[[#fn:r522|522]]</sup> . Observational constraints suggest that stabilization at 1.5°C of warming would avoid the thawing of approximately 1.5 to 2.5 million km <sup>2</sup> of permafrost ( ''medium confidence'' ) compared with stabilization at 2°C (Chadburn et al., 2017) <sup>[[#fn:r523|523]]</sup> , but the time scale for release of thawed carbon as CO <sub>2</sub> or CH <sub>4</sub> should be many centuries (Burke et al., 2017) <sup>[[#fn:r524|524]]</sup> . In northern Eurasia, the growing season length is projected to increase by about 3–12 days at 1.5°C and 6–16 days at 2°C of warming ( ''medium confidence'' ) (Zhou et al., 2018) <sup>[[#fn:r525|525]]</sup> . Aalto et al. (2017) <sup>[[#fn:r526|526]]</sup> predicted a 72% reduction in cryogenic land surface processes in northern Europe for RCP2.6 in 2040–2069 (corresponding to a global warming of approximately 1.6°C), with only slightly larger losses for RCP4.5 (2°C of global warming).

Projected impacts on forests as climate change occurs include increases in the intensity of storms, wildfires and pest outbreaks (Settele et al., 2014) <sup>[[#fn:r527|527]]</sup> , potentially leading to forest dieback ( ''medium confidence'' ). Warmer and drier conditions in particular facilitate fire, drought and insect disturbances, while warmer and wetter conditions increase disturbances from wind and pathogens (Seidl et al., 2017) <sup>[[#fn:r528|528]]</sup> . Particularly vulnerable regions are Central and South America, Mediterranean Basin, South Africa, South Australia where the drought risk will increase (see Figure 3.12). Including disturbances in simulations may influence productivity changes in European forests in response to climate change (Reyer et al., 2017b) <sup>[[#fn:r529|529]]</sup> . There is additional evidence for the attribution of increased forest fire frequency in North America to anthropogenic climate change during 1984–2015, via the mechanism of increasing fuel aridity almost doubling the western USA forest fire area compared to what would have been expected in the absence of climate change (Abatzoglou and Williams, 2016) <sup>[[#fn:r530|530]]</sup> . This projection is in line with expected fire risks, which indicate that fire frequency could increase over 37.8% of the global land area during 2010–2039 (Moritz et al., 2012) <sup>[[#fn:r531|531]]</sup> , corresponding to a global warming level of approximately 1.2°C, compared with over 61.9% of the global land area in 2070–2099, corresponding to a warming of approximately 3.5°C <sup>[[#fn:9|9]]</sup> .The values in Table 26-1 in a recent paper by Romero-Lankao et al. (2014) <sup>[[#fn:r532|532]]</sup> also indicate significantly lower wildfire risks in North America for near-term warming (2030–2040, considered a proxy for 1.5°C of warming) than at 2°C ( ''high confidence'' ).

The Amazon tropical forest has been shown to be close to its climatic limits (Hutyra et al., 2005) <sup>[[#fn:r533|533]]</sup> , but this threshold may move under elevated CO <sub>2</sub> (Good et al., 2011) <sup>[[#fn:r534|534]]</sup> . Future changes in rainfall, especially dry season length, will determine responses of the Amazon forest (Good et al., 2013) <sup>[[#fn:r535|535]]</sup> . The forest may be especially vulnerable to combined pressure from multiple stressors, namely changes in climate and continued anthropogenic disturbance (Borma et al., 2013; Nobre et al., 2016) <sup>[[#fn:r536|536]]</sup> . Modelling (Huntingford et al., 2013) <sup>[[#fn:r537|537]]</sup> and observational constraints (Cox et al., 2013) <sup>[[#fn:r538|538]]</sup> suggest that large-scale forest dieback is less ''likely'' than suggested under early coupled modelling studies (Cox et al., 2000; Jones et al., 2009) <sup>[[#fn:r539|539]]</sup> . Nobre et al. (2016) <sup>[[#fn:r540|540]]</sup> estimated a climatic threshold of 4°C of warming and a deforestation threshold of 40%.

In many places around the world, the savanna boundary is moving into former grasslands. Woody encroachment, including increased tree cover and biomass, has increased over the past century, owing to changes in land management, rising CO <sub>2</sub> levels, and climate variability and change (often in combination) (Settele et al., 2014) <sup>[[#fn:r541|541]]</sup> . For plant species in the Mediterranean region, shifts in phenology, range contraction and health decline have been observed with precipitation decreases and temperature increases ( ''medium confidence'' ) (Settele et al., 2014) <sup>[[#fn:r542|542]]</sup> . Recent studies using independent complementary approaches have shown that there is a regional-scale threshold in the Mediterranean region between 1.5°C and 2°C of warming (Guiot and Cramer, 2016; Schleussner et al., 2016b) <sup>[[#fn:r543|543]]</sup> . Further, Guiot and Cramer (2016) <sup>[[#fn:r544|544]]</sup> concluded that biome shifts unprecedented in the last 10,000 years can only be avoided if global warming is constrained to 1.5°C ( ''medium confidence'' ) – whilst 2°C of warming will result in a decrease of 12–15% of the Mediterranean biome area. The Fynbos biome in southwestern South Africa is vulnerable to the increasing impact of fires under increasing temperatures and drier winters. It is projected to lose about 20%, 45% and 80% of its current suitable climate area under 1°C, 2°C and 3°C of global warming, respectively, compared to 1961–1990 ( ''high confidence'' ) (Engelbrecht and Engelbrecht, 2016) <sup>[[#fn:r545|545]]</sup> . In Australia, an increase in the density of trees and shrubs at the expense of grassland species is occurring across all major ecosystems and is projected to be amplified (NCCARF, 2013) <sup>[[#fn:r546|546]]</sup> . Regarding Central America, Lyra et al. (2017) <sup>[[#fn:r547|547]]</sup> showed that the tropical rainforest biomass would be reduced by about 40% under global warming of 3°C, with considerable replacement by savanna and grassland. With a global warming of close to 1.5°C in 2050, a biomass decrease of 20% is projected for tropical rainforests of Central America (Lyra et al., 2017) <sup>[[#fn:r548|548]]</sup> . If a linear response is assumed, this decrease may reach 30% ( ''medium confidence'' ).

Freshwater ecosystems are considered to be among the most threatened on the planet (Settele et al., 2014) <sup>[[#fn:r549|549]]</sup> . Although peatlands cover only about 3% of the land surface, they hold one-third of the world’s soil carbon stock (400 to 600 Pg) (Settele et al., 2014) <sup>[[#fn:r550|550]]</sup> . When drained, this carbon is released to the atmosphere. At least 15% of peatlands have drained, mostly in Europe and South east Asia, and are responsible for 5% of human derived CO <sub>2</sub> emissions (Green and Page, 2017) <sup>[[#fn:r551|551]]</sup> . Moreover, in the Congo basin (Dargie et al., 2017) <sup>[[#fn:r552|552]]</sup> and in the Amazonian basin (Draper et al., 2014) <sup>[[#fn:r553|553]]</sup> , the peatlands store the equivalent carbon as that of a tropical forest. However, stored carbon is vulnerable to land-use change and future risk of drought, for example in northeast Brazil ( ''high confidence'' ) (Figure 3.12, Section 3.3.4.2). At the global scale, these peatlands are undergoing rapid major transformations through drainage and burning in preparation for oil palm and other crops or through unintentional burning (Magrin et al., 2014) <sup>[[#fn:r554|554]]</sup> . Wetland salinization, a widespread threat to the structure and ecological functioning of inland and coastal wetlands, is occurring at a high rate and large geographic scale (Section 3.3.6; Herbert et al., 2015) <sup>[[#fn:r555|555]]</sup> . Settele et al. (2014) <sup>[[#fn:r556|556]]</sup> found that rising water temperatures are projected to lead to shifts in freshwater species distributions and worsen water quality. Some of these ecosystems respond non-linearly to changes in temperature. For example, Johnson and Poiani (2016) <sup>[[#fn:r557|557]]</sup> found that the wetland function of the Prairie Pothole region in North America is projected to decline at temperatures beyond a local warming of 2°C–3°C above present-day values (1°C local warming, corresponding to 0.6°C of global warming). If the ratio of local to global warming remains similar for these small levels of warming, this would indicate a global temperature threshold of 1.2°C–1.8°C of warming. Hence, constraining global warming to approximately 1.5°C would maintain the functioning of prairie pothole ecosystems in terms of their productivity and biodiversity, although a 20% increase of precipitation could offset 2°C of global warming ( ''high confidence'' ) (Johnson and Poiani, 2016) <sup>[[#fn:r558|558]]</sup> .

<div id="section-3-4-3-6"></div>

<span id="summary-of-implications-for-ecosystem-services"></span>
==== 3.4.3.6 Summary of implications for ecosystem services ====

<div id="section-3-4-3-6-block-1"></div>

In summary, constraining global warming to 1.5°C rather than 2°C has strong benefits for terrestrial and wetland ecosystems and their services ( ''high confidence'' ). These benefits include avoidance or reduction of changes such as biome transformations, species range losses, increased extinction risks (all ''high confidence'' ) and changes in phenology ( ''high confidence'' ), together with projected increases in extreme weather events which are not yet factored into these analyses (Section 3.3). All of these changes contribute to disruption of ecosystem functioning and loss of cultural, provisioning and regulating services provided by these ecosystems to humans. Examples of such services include soil conservation (avoidance of desertification), flood control, water and air purification, pollination, nutrient cycling, sources of food, and recreation.

<span id="ocean-ecosystems"></span>
=== 3.4.4 Ocean Ecosystems ===

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

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.

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

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

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

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> .

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

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

<div id="section-3-4-4-2-block-1"></div>

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> .

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

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

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

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).

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

<span id="ocean-circulation"></span>
==== 3.4.4.4 Ocean circulation ====

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

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).

<div id="section-3-4-4-5"></div>

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

<div id="section-3-4-4-5-block-1"></div>

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> .

<div id="section-3-4-4-6"></div>

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

<div id="section-3-4-4-6-block-1"></div>

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> .

<div id="section-3-4-4-7"></div>

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

<div id="section-3-4-4-7-block-1"></div>

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.

<div id="section-3-4-4-8"></div>

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

<div id="section-3-4-4-8-block-1"></div>

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> .

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

<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) ====

<div id="section-3-4-4-11-block-1"></div>

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.'''
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[[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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