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

== 3.5 Avoided Impacts and Reduced Risks at 1.5°C Compared with 2°C of Global Warming ==

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=== 3.5.1 Introduction ===

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Oppenheimer et al. (2014, AR5 WGII Chapter 19) <sup>[[#fn:r1118|1118]]</sup> provided a framework that aggregates projected risks from global mean temperature change into five categories identified as ‘Reasons for Concern’. Risks are classified as moderate, high or very high and coloured yellow, red or purple, respectively, in Figure 19.4 of that chapter (AR5 WGII Chapter 19 for details and findings). The framework’s conceptual basis and the risk judgements made by Oppenheimer et al. (2014) <sup>[[#fn:r1119|1119]]</sup> were recently reviewed, and most judgements were confirmed in the light of more recent literature (O’Neill et al., 2017) <sup>[[#fn:r1120|1120]]</sup> . The approach of Oppenheimer et al. (2014) <sup>[[#fn:r1121|1121]]</sup> was adopted, with updates to the aggregation of risk informed by the most recent literature, for the analysis of avoided impacts at 1.5°C compared to 2°C of global warming presented in this section.

The regional economic benefits that could be obtained by limiting the global temperature increase to 1.5°C of warming, rather than 2°C or higher levels, are discussed in Section 3.5.3 in the light of the five RFCs explored in Section 3.5.2. Climate change hotspots that could be avoided or reduced by achieving the 1.5°C target are summarized in Section 3.5.4. The section concludes with a discussion of regional tipping points that could be avoided at 1.5°C compared to higher degrees of global warming (Section 3.5.5).

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=== 3.5.2 Aggregated Avoided Impacts and Reduced Risks at 1.5°C versus 2°C of Global Warming ===

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A brief summary of the accrual of RFCs with global warming, as assessed in WGII AR5, is provided in the following sections, which leads into an update of relevant literature published since AR5. The new literature is used to confirm the levels of global warming at which risks are considered to increase from undetectable to moderate, from moderate to high, and from high to very high. Figure 3.21 modifies Figure 19.4 from AR5 WGII, and the following text in this subsection provides justification for the modifications. O’Neill et al. (2017) <sup>[[#fn:r1122|1122]]</sup> presented a very similar assessment to that of WGII AR5, but with further discussion of the potential to create ‘embers’ specific to socio-economic scenarios in the future. There is insufficient literature to do this at present, so the original, simple approach has been used here. As the focus of the present assessment is on the consequences of global warming of 1.5°C–2°C above the pre-industrial period, no assessment for global warming of 3°C or more is included in the figure (i.e., analysis is discontinued at 2.5°C).

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'''Figure 3.21'''

<span id="the-dependence-of-risks-andor-impacts-associated-with-the-reasons-for-concern-rfcs-on-the-level-of-climate-change-updated-and-adapted-from-wgii-ar5-ch-19-figure-19.4-and-highlighting-the-nature-of-this-dependence-between-0c-and-2c-warming-above-pre-industrial-levels."></span>
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'''The dependence of risks and/or impacts associated with the Reasons for Concern (RFCs) on the level of climate change, updated and adapted from WGII AR5 Ch 19, Figure 19.4 and highlighting the nature of this dependence between 0°C and 2°C warming above pre-industrial levels.'''
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[[File:076a10356b0cf2399270b9a97dbb86ce figure_3.21-1024x497.jpg]]

As in the AR5, literature was used to make expert judgements to assess the levels of global warming at which levels of impact and/or risk are undetectable (white), moderate (yellow), high (red) or very high (purple). The colour scheme thus indicates the additional risks due to climate change. The transition from red to purple, introduced for the first time in AR4, is defined by very high risk of severe impacts and the presence of significant irreversibility, or persistence of climate-related hazards combined with a limited ability to adapt due to the nature of the hazard or impact. Comparison of the increase of risk across RFCs indicates the relative sensitivity of RFCs to increases in GMST. As was done previously, this assessment takes autonomous adaptation into account, as well as limits to adaptation (RFC 1, 3, 5) independently of development pathway. The rate and timing of impacts were taken into account in assessing RFC 1 and 5. The levels of risk illustrated reflect the judgements of the Ch 3 authors. RFC1 Unique and threatened systems: ecological and human systems that have restricted geographic ranges constrained by climate related conditions and have high endemism or other distinctive properties. Examples include coral reefs, the Arctic and its indigenous people, mountain glaciers and biodiversity hotspots. RFC2 Extreme weather events: risks/impacts to human health, livelihoods, assets and ecosystems from extreme weather events such as heat waves, heavy rain, drought and associated wildfires, and coastal flooding. RFC3 Distribution of impacts: risks/impacts that disproportionately affect particular groups due to uneven distribution of physical climate change hazards, exposure or vulnerability. RFC4 Global aggregate impacts: global monetary damage, global scale degradation and loss of ecosystems and biodiversity. RFC5 Large-scale singular events: are relatively large, abrupt and sometimes irreversible changes in systems that are caused by global warming. Examples include disintegration of the Greenland and Antarctic ice sheets. The grey bar represents the range of GMST for the most recent decade: 2006–2015.

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==== 3.5.2.1 RFC 1 – Unique and threatened systems ====

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WGII AR5 Chapter 19 found that some unique and threatened systems are at risk from climate change at current temperatures, with increasing numbers of systems at potential risk of severe consequences at global warming of 1.6°C above pre-industrial levels. It was also observed that many species and ecosystems have a limited ability to adapt to the very large risks associated with warming of 2.6°C or more, particularly Arctic sea ice and coral reef systems ( ''high confidence'' ). In the AR5 analysis, a transition from white to yellow indicated that the onset of moderate risk was located below present-day global temperatures ( ''medium confidence'' ); a transition from yellow to red indicated that the onset of high risk was located at 1.6°C, and a transition from red to purple indicated that the onset of very high risk was located at about 2.6°C. This WGII AR5 analysis already implied that there would be a significant reduction in risks to unique and threatened systems if warming were limited to 1.5°C compared with 2°C. Since AR5, evidence of present-day impacts in these systems has continued to grow (Sections 3.4.2, 3.4.4 and 3.4. 5), whilst new evidence has also accumulated for reduced risks at 1.5°C compared to 2°C of warming in Arctic ecosystems (Section 3.3.9), coral reefs (Section 3.4.4) and some other unique ecosystems (Section 3.4.3), as well as for biodiversity.

New literature since AR5 has provided a closer focus on the comparative levels of risk to coral reefs at 1.5°C versus 2°C of global warming. As assessed in Section 3.4.4 and Box 3.4, reaching 2°C will increase the frequency of mass coral bleaching and mortality to a point at which it will result in the total loss of coral reefs from the world’s tropical and subtropical regions. Restricting overall warming to 1.5°C will still see a downward trend in average coral cover (70–90% decline by mid-century) but will prevent the total loss of coral reefs projected with warming of 2°C (Frieler et al., 2013) <sup>[[#fn:r1123|1123]]</sup> . The remaining reefs at 1.5°C will also benefit from increasingly stable ocean conditions by the mid-to-late 21st century. Limiting global warming to 1.5°C during the course of the century may, therefore, open the window for many ecosystems to adapt or reassort geographically. This indicates a transition in risk in this system from high to very high ( ''high confidence'' ) at 1.5°C of warming and contributes to a lowering of the transition from high to very high (Figure 3.21) in this RFC1 compared to in AR5. Further details of risk transitions for ocean systems are described in Figure 3.18.

Substantial losses of Arctic Ocean summer ice were projected in WGI AR5 for global warming of 1.6°C, with a nearly ice-free Arctic Ocean being projected for global warming of more than 2.6°C. Since AR5, the importance of a threshold between 1°C and 2°C has been further emphasized in the literature, with sea ice projected to persist throughout the year for a global warming of less than 1.5°C, yet chances of an ice-free Arctic during summer being high at 2°C of warming (Section 3.3.8). Less of the permafrost in the Arctic is projected to thaw under 1.5°C of warming (17–44%) compared with under 2°C (28–53%) (Section 3.3.5.2; Chadburn et al., 2017) <sup>[[#fn:r1124|1124]]</sup> , which is expected to reduce risks to both social and ecological systems in the Arctic. This indicates a transition in the risk in this system from high to very high between 1.5°C and 2°C of warming and contributes to a lowering of the transition from high to very high in this RFC1 compared to in AR5.

AR5 identified a large number of threatened systems, including mountain ecosystems, highly biodiverse tropical wet and dry forests, deserts, freshwater systems and dune systems. 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, impacts accrue with greater warming and impacts at 2°C are expected to be greater than those at 1.5°C ( ''medium confidence'' ). One study since AR5 has shown that constraining global warming to 1.5°C would maintain the functioning of prairie pothole ecosystems in North America in terms of their productivity and biodiversity, whilst warming of 2°C would not do so (Johnson et al., 2016) <sup>[[#fn:r1125|1125]]</sup> . The large proportion of insects projected to lose over half their range at 2°C of warming (25%) compared to at 1.5°C (9%) also suggests a significant loss of functionality in these threatened systems at 2°C of warming, owing to the critical role of insects in nutrient cycling, pollination, detritivory and other important ecosystem processes (Section 3.4.3).

Unique and threatened systems in small island states and in systems fed by glacier meltwater were also considered to contribute to this RFC in AR5, but there is little new information about these systems that pertains to 1.5°C or 2°C of global warming. Taken together, the evidence suggests that the transition from high to very high risk in unique and threatened systems occurs at a lower level of warming, between 1.5°C and 2°C ( ''high confidence'' ), than in AR5, where this transition was located at 2.6°C. The transition from moderate to high risk relocates very slightly from 1.6°C to 1.5°C ( ''high confidence'' ). There is also ''high confidence'' in the location of the transition from low to moderate risk below present-day global temperatures.

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==== 3.5.2.2 RFC 2 – Extreme weather events ====

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Reduced risks in terms of the likelihood of occurrence of extreme weather events are discussed in this sub-subsection for 1.5°C as compared to 2°C of global warming, for those extreme events where evidence is currently available based on the assessments of Section 3.3. AR5 assigned a moderate level of risk from extreme weather events at recent temperatures (1986–2005) owing to the attribution of heat and precipitation extremes to climate change, and a transition to high risk beginning below 1.6°C of global warming based on the magnitude, likelihood and timing of projected changes in risk associated with extreme events, indicating more severe and widespread impacts. The AR5 analysis already suggested a significant benefit of limiting warming to 1.5°C, as doing so might keep risks closer to the moderate level. New literature since AR5 has provided greater confidence in a reduced level of risks due to extreme weather events at 1.5°C versus 2°C of warming for some types of extremes (Section 3.3 and below; Figure 3.21).

'''Temperature:''' It is expected that further increases in the number of warm days/nights and decreases in the number of cold days/nights, and an increase in the overall temperature of hot and cold extremes would occur under 1.5°C of global warming relative to pre-industrial levels ( ''high confidence'' ) compared to under the present-day climate (1°C of warming), with further changes occurring towards 2°C of global warming (Section 3.3). As assessed in Sections 3.3.1 and 3.3.2, impacts of 0.5°C of global warming can be identified for temperature extremes at global scales, based on observations and the analysis of climate models. At 2°C of global warming, it is ''likely'' that temperature increases of more than 2°C would occur over most land regions in terms of extreme temperatures (up to 4°C–6°C depending on region and considered extreme index) (Section 3.3.2, Table 3.2). Regional increases in temperature extremes can be robustly limited if global warming is constrained to 1.5°C, with regional warmings of up to 3°C–4.5°C (Section 3.3.2, Table 3.2). Benefits obtained from this general reduction in extremes depend to a large extent on whether the lower range of increases in extremes at 1.5°C is sufficient for critical thresholds to be exceeded, within the context of wide-ranging aspects such as crop yields, human health and the sustainability of ecosystems.

'''Heavy precipitation:''' AR5 assessed trends in heavy precipitation for land regions where observational coverage was sufficient for assessment. It concluded with ''medium confidence'' that anthropogenic forcing has contributed to a global-scale intensification of heavy precipitation over the second half of the 20th century, for a global warming of approximately 0.5°C (Section 3.3.3). A recent observation-based study likewise showed that a 0.5°C increase in global mean temperature has had a detectable effect on changes in precipitation extremes at the global scale (Schleussner et al., 2017) <sup>[[#fn:r1126|1126]]</sup> , thus suggesting that there would be detectable differences in heavy precipitation at 1.5°C and 2°C of global warming. These results are consistent with analyses of climate projections, although they also highlight a large amount of regional variation in the sensitivity of changes in heavy precipitation (Section 3.3.3).

'''Droughts:''' When considering the difference between precipitation and evaporation (P–E) as a function of global temperature changes, the subtropics generally display an overall trend towards drying, whilst the northern high latitudes display a robust response towards increased wetting (Section 3.3.4, Figure 3.12). Limiting global mean temperature increase to 1.5°C as opposed to 2°C could substantially reduce the risk of reduced regional water availability in some regions (Section 3.3.4). Regions that are projected to benefit most robustly from restricted warming include the Mediterranean and southern Africa (Section 3.3.4).

'''Fire:''' Increasing evidence that anthropogenic climate change has already caused significant increases in fire area globally (Section 3.4.3) is in line with projected fire risks. These risks are projected to increase further under 1.5°C of global warming relative to the present day (Section 3.4.3). Under 1.2°C of global warming, fire frequency has been estimated to increase by over 37.8% of global land areas, compared to 61.9% of global land areas under 3.5°C of warming. For in-depth discussion and uncertainty estimates, see Meehl et al. (2007), Moritz et al. (2012) and Romero-Lankao et al. (2014) <sup>[[#fn:r1127|1127]]</sup> .

Regarding extreme weather events (RFC2), the transition from moderate to high risk is located between 1°C and 1.5°C of global warming (Figure 3.21), which is very similar to the AR5 assessment but is assessed with greater confidence ( ''medium confidence'' ). The impact literature contains little information about the potential for human society to adapt to extreme weather events, and hence it has not been possible to locate the transition from high to very high risk within the context of assessing impacts at 1.5°C and 2°C of global warming. There is thus ''low confidence'' in the level at which global warming could lead to very high risks associated with extreme weather events in the context of this report.

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==== 3.5.2.3 RFC 3 – Distribution of impacts ====

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Risks due to climatic change are unevenly distributed and are generally greater at lower latitudes and for disadvantaged people and communities in countries at all levels of development. AR5 located the transition from undetectable to moderate risk below recent temperatures, owing to the detection and attribution of regionally differentiated changes in crop yields ( ''medium to high confidence'' ; Figure 3.20), and new literature has continued to confirm this finding. Based on the assessment of risks to regional crop production and water resources, AR5 located the transition from moderate to high risk between 1.6°C and 2.6°C above pre-industrial levels. Cross-Chapter Box 6 in this chapter highlights that at 2°C of warming, new literature shows that risks of food shortage are projected to emerge in the African Sahel, the Mediterranean, central Europe, the Amazon, and western and southern Africa, and that these are much larger than the corresponding risks at 1.5°C. This suggests a transition from moderate to high risk of regionally differentiated impacts between 1.5°C and 2°C above pre-industrial levels for food security ( ''medium confidence'' ) (Figure 3.20). Reduction in the availability of water resources at 2°C is projected to be greater than 1.5°C of global warming, although changes in socio-economics could have a greater influence (Section 3.4.2), with larger risks in the Mediterranean (Box 3.2); estimates of the magnitude of the risks remain similar to those cited in AR5. Globally, millions of people may be at risk from sea level rise (SLR) during the 21st century (Hinkel et al., 2014; Hauer et al., 2016) <sup>[[#fn:r1128|1128]]</sup> , particularly if adaptation is limited. At 2°C of warming, more than 90% of global coastlines are projected to experience SLR greater than 0.2 m, suggesting regional differences in the risks of coastal flooding. Regionally differentiated multi-sector risks are already apparent at 1.5°C of warming, being more prevalent where vulnerable people live, predominantly in South Asia (mostly Pakistan, India and China), but these risks are projected to spread to sub-Saharan Africa, the Middle East and East Asia as temperature rises, with the world’s poorest people disproportionately impacted at 2°C of warming (Byers et al., 2018) <sup>[[#fn:r1129|1129]]</sup> . The hydrological impacts of climate change in Europe are projected to increase in spatial extent and intensity across increasing global warming levels of 1.5°C, 2°C and 3°C (Donnelly et al., 2017) <sup>[[#fn:r1130|1130]]</sup> . Taken together, a transition from moderate to high risk is now located between 1.5°C and 2°C above pre-industrial levels, based on the assessment of risks to food security, water resources, drought, heat exposure and coastal submergence ( ''high confidence;'' Figure 3.21).

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==== 3.5.2.4 RFC 4 – Global aggregate impacts ====

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Oppenheimer et al. (2014) <sup>[[#fn:r1131|1131]]</sup> explained the inclusion of non-economic metrics related to impacts on ecosystems and species at the global level, in addition to economic metrics in global aggregate impacts. The degradation of ecosystem services by climate change and ocean acidification have generally been excluded from previous global aggregate economic analyses.

'''Global economic impacts''' ''':''' WGII AR5 found that overall global aggregate impacts become moderate at 1°C–2°C of warming, and the transition to moderate risk levels was therefore located at 1.6°C above pre-industrial levels. This was based on the assessment of literature using model simulations which indicated that the global aggregate economic impact will become significantly negative between 1°C and 2°C of warming ( ''medium confidence'' ), whilst there will be a further increase in the magnitude and likelihood of aggregate economic risks at 3°C of warming ( ''low confidence'' ).

Since AR5, three studies have emerged using two entirely different approaches which indicate that economic damages are projected to be higher by 2100 if warming reaches 2°C than if it is constrained to 1.5°C. The study by Warren et al. (2018c) <sup>[[#fn:r1132|1132]]</sup> used the integrated assessment model PAGE09 to estimate that avoided global economic damages of 22% (10–26%) accrue from constraining warming to 1.5°C rather than 2°C, 90% (77–93%) from 1.5°C rather than 3.66°C, and 87% (74–91%) from 2°C rather than 3.66°C. In the second study, Pretis et al. (2018) <sup>[[#fn:r1133|1133]]</sup> identified several regions where economic damages are projected to be greater at 2°C compared to 1.5°C of warming, further estimating that projected damages at 1.5°C remain similar to today’s levels of economic damage. The third study, by M. Burke et al. (2018) <sup>[[#fn:r1134|1134]]</sup> used an empirical, statistical approach and found that limiting warming to 1.5°C instead of 2°C would save 1.5–2.0% of the gross world product (GWP) by mid-century and 3.5% of the GWP by end-of-century (see Figure 2A in M. Burke et al., 2018) <sup>[[#fn:r1135|1135]]</sup> . Based on a 3% discount rate, this corresponds to 8.1–11.6 trillion USD and 38.5 trillion USD in avoided damages by mid-and end-of-century, respectively, agreeing closely with the estimate by Warren et al. (2018c) <sup>[[#fn:r1136|1136]]</sup> of 15 trillion USD. Under the no-policy baseline scenario, temperature rises by 3.66°C by 2100, resulting in a global gross domestic product (GDP) loss of 2.6% (5–95% percentile range 0.5–8.2%), compared with 0.3% (0.1–0.5%) by 2100 under the 1.5°C scenario and 0.5% (0.1–1.0%) in the 2°C scenario. Limiting warming to 1.5°C rather than 2°C by 2060 has also been estimated to result in co-benefits of 0.5–0.6% of the world GDP, owing to reductions in air pollution (Shindell et al., 2018) <sup>[[#fn:r1137|1137]]</sup> , which is similar to the avoided damages identified for the USA (Box 3.6).

Two studies focusing only on the USA found that economic damages are projected to be higher by 2100 if warming reaches 2°C than if it is constrained to 1.5°C. Hsiang et al. (2017) <sup>[[#fn:r1138|1138]]</sup> found a mean difference of 0.35% GDP (range 0.2–0.65%), while Yohe (2017) <sup>[[#fn:r1139|1139]]</sup> identified a GDP loss of 1.2% per degree of warming, hence approximately 0.6% for half a degree. Further, the avoided risks compared to a no-policy baseline are greater in the 1.5°C case (4%, range 2–7%) compared to the 2°C case (3.5%, range 1.8–6.5%). These analyses suggest that the point at which global aggregates of economic impacts become negative is below 2°C ( ''medium confidence'' ), and that there is a possibility that it is below 1.5°C of warming.

Oppenheimer et al. (2014) <sup>[[#fn:r1140|1140]]</sup> noted that the global aggregated damages associated with large-scale singular events has not been explored, and reviews of integrated modelling exercises have indicated a potential underestimation of global aggregate damages due to the lack of consideration of the potential for these events in many studies. Since AR5, further analyses of the potential economic consequences of triggering these large-scale singular events have indicated a two to eight fold larger economic impact associated with warming of 3°C than estimated in most previous analyses, with the extent of increase depending on the number of events incorporated. Lemoine and Traeger (2016) <sup>[[#fn:r1141|1141]]</sup> included only three known singular events whereas Y. Cai et al. (2016) <sup>[[#fn:r1142|1142]]</sup> included five.

'''Biome shifts, species range loss, increased risks of species extinction and risks of loss of ecosystem functioning and services:''' 13% (range 8–20%) of Earth’s land area is projected to undergo biome shifts at 2°C of warming compared to approximately 7% at 1.5°C (medium confidence) (Section 3.4.3; Warszawski et al., 2013) <sup>[[#fn:r1143|1143]]</sup> , implying a halving of biome transformations. Overall levels of species loss at 2°C of warming are similar to values found in previous studies for plants and vertebrates (Warren et al., 2013, 2018a) <sup>[[#fn:r1144|1144]]</sup> , but insects have been found to be more sensitive to climate change, with 18% (6–35%) projected to lose over half their range at 2°C of warming compared to 6% (1–18%) under 1.5°C of warming, corresponding to a difference of 66% (Section 3.4.3). The critical role of insects in ecosystem functioning therefore suggests that there will be impacts on global ecosystem functioning already at 2°C of warming, whilst species that lose large proportions of their range are considered to be at increased risk of extinction (Section 3.4.3.3). Since AR5, new literature has indicated that impacts on marine fish stocks and fisheries are lower under 1.5°C–2°C of global warming relative to pre-industrial levels compared to under higher warming scenarios (Section 3.4.6), especially in tropical and polar systems.

In AR5, the transition from undetectable to moderate impacts was considered to occur between 1.6°C and 2.6°C of global warming reflecting impacts on the economy and on biodiversity globally, whereas high risks were associated with 3.6°C of warming to reflect the high risks to biodiversity and accelerated effects on the global economy. New evidence suggests moderate impacts on the global aggregate economy and global biodiversity by 1.5°C of warming, suggesting a lowering of the temperature level for the transition to moderate risk to 1.5°C (Figure 3.21). Further, recent literature points to higher risks than previously assessed for the global aggregate economy and global biodiversity by 2°C of global warming, suggesting that the transition to a high risk level is located between 1.5°C and 2.5°C of warming (Figure 3.21), as opposed to at 3.6°C as previously assessed ( ''medium confidence'' ).

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==== 3.5.2.5 RFC 5 – Large-scale singular events ====

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Large-scale singular events are components of the global Earth system that are thought to hold the risk of reaching critical tipping points under climate change, and that can result in or be associated with major shifts in the climate system. These components include:

• the cryosphere: West Antarctic ice sheet, Greenland ice sheet<br />
• the thermohaline circulation: slowdown of the Atlantic Meridional Overturning Circulation (AMOC)<br />
• the El Niño–Southern Oscillation (ENSO) as a global mode of climate variability<br />
• role of the Southern Ocean in the global carbon cycle

AR5 assessed that the risks associated with these events become moderate between 0.6°C and 1.6°C above pre-industrial levels, based on early warning signs, and that risk was expected to become high between 1.6°C and 4.6°C based on the potential for commitment to large irreversible sea level rise from the melting of land-based ice sheets ( ''low to medium confidence'' ). The increase in risk between 1.6°C and 2.6°C above pre-industrial levels was assessed to be disproportionately large. New findings since AR5 are described in detail below.

'''Greenland and West Antarctic ice sheets and marine ice sheet instability (MISI):''' Various feedbacks between the Greenland ice sheet and the wider climate system, most notably those related to the dependence of ice melt on albedo and surface elevation, make irreversible loss of the ice sheet a possibility. Church et al. (2013) <sup>[[#fn:r1145|1145]]</sup> assessed this threshold to be at 2°C of warming or higher levels relative to pre-industrial temperature. Robinson et al. (2012) <sup>[[#fn:r1146|1146]]</sup> found a range for this threshold of 0.8°C–3.2°C (95% confidence). The threshold of global temperature increase that may initiate irreversible loss of the West Antarctic ice sheet and marine ice sheet instability (MISI) is estimated to lie be between 1.5°C and 2°C. The time scale for eventual loss of the ice sheets varies between millennia and tens of millennia and assumes constant surface temperature forcing during this period. If temperature were to decline subsequently the ice sheets might regrow, although the amount of cooling required is ''likely'' to be highly dependent on the duration and rate of the previous retreat. The magnitude of global sea level rise that could occur over the next two centuries under 1.5°C–2°C of global warming is estimated to be in the order of several tenths of a metre according to most studies ( ''low confidence'' ) (Schewe et al., 2011; Church et al., 2013; Levermann et al., 2014; Marzeion and Levermann, 2014; Fürst et al., 2015; Golledge et al., 2015) <sup>[[#fn:r1147|1147]]</sup> , although a smaller number of investigations (Joughin et al., 2014; Golledge et al., 2015; DeConto and Pollard, 2016) <sup>[[#fn:r1148|1148]]</sup> project increases of 1–2 m. This body of evidence suggests that the temperature range of 1.5°C–2°C may be regarded as representing moderate risk, in that it may trigger MISI in Antarctica or irreversible loss of the Greenland ice sheet and it may be associated with sea level rise by as much as 1–2 m over a period of two centuries.

'''Thermohaline circulation (slowdown of AMOC):''' It is ''more likely than not'' that the AMOC has been weakening in recent decades, given the detection of cooling of surface waters in the North Atlantic and evidence that the Gulf Stream has slowed since the late 1950s (Rahmstorf et al., 2015b; Srokosz and Bryden, 2015; Caesar et al., 2018) <sup>[[#fn:r1149|1149]]</sup> . There is ''limited evidence'' linking the recent weakening of the AMOC to anthropogenic warming (Caesar et al., 2018) <sup>[[#fn:r1150|1150]]</sup> . It is very ''likely'' that the AMOC will weaken over the 21st century. Best estimates and ranges for the reduction based on CMIP5 simulations are 11% (1–24%) in RCP2.6 and 34% (12–54%) in RCP8.5 (AR5). There is no evidence indicating significantly different amplitudes of AMOC weakening for 1.5°C versus 2°C of global warming, or of a shutdown of the AMOC at these global temperature thresholds. Associated risks are classified as low to moderate.

'''El Niño–Southern Oscillation (ENSO):''' Extreme El Niño events are associated with significant warming of the usually cold eastern Pacific Ocean, and they occur about once every 20 years (Cai et al., 2015) <sup>[[#fn:r1151|1151]]</sup> . Such events reorganize the distribution of regions of organized convection and affect weather patterns across the globe. Recent research indicates that the frequency of extreme El Niño events increases linearly with the global mean temperature, and that the number of such events might double (one event every ten years) under 1.5°C of global warming (G. Wang et al., 2017) <sup>[[#fn:r1152|1152]]</sup> . This pattern is projected to persist for a century after stabilization at 1.5°C, thereby challenging the limits to adaptation, and thus indicates high risk even at the 1.5°C threshold. La Niña event (the opposite or balancing event to El Niño) frequency is projected to remain similar to that of the present day under 1.5°C–2°C of global warming.

'''Role of the Southern Ocean in the global carbon cycle:''' The critical role of the Southern Ocean as a net sink of carbon might decline under global warming, and assessing this effect under 1.5°C compared to 2°C of global warming is a priority. Changes in ocean chemistry (e.g., oxygen content and ocean acidification), especially those associated with the deep sea, are associated concerns (Section 3.3.10).

For large-scale singular events (RFC5), moderate risk is now located at 1°C of warming and high risk is located at 2.5°C (Figure 3.21), as opposed to at 1.6°C (moderate risk) and around 4°C (high risk) in AR5, because of new observations and models of the West Antarctic ice sheet ( ''medium confidence'' ), which suggests that the ice sheet may be in the early stages of marine ice sheet instability (MISI). Very high risk is assessed as lying above 5°C because the growing literature on process-based projections of the West Antarctic ice sheet predominantly supports the AR5 assessment of an MISI contribution of several additional tenths of a metre by 2100.

<span id="regional-economic-benefit-analysis-for-the-1.5c-versus-2c-global-goals"></span>
=== 3.5.3 Regional Economic Benefit Analysis for the 1.5°C versus 2°C Global Goals ===

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

This section reviews recent literature that has estimated the economic benefits of constraining global warming to 1.5°C compared to 2°C. The focus here is on evidence pertaining to specific regions, rather than on global aggregated benefits (Section 3.5.2.4). At 2°C of global warming, lower economic growth is projected for many countries than at 1.5C of global warming, with low-income countries projected to experience the greatest losses ( ''low to medium confidence'' ) (M. Burke et al., 2018; Pretis et al., 2018) <sup>[[#fn:r1153|1153]]</sup> . A critical issue for developing countries in particular is that advantages in some sectors are projected to be offset by increasing mitigation costs (Rogelj et al., 2013; M. Burke et al., 2018) <sup>[[#fn:r1154|1154]]</sup> , with food production being a key factor. That is, although restraining the global temperature increase to 2°C is projected to reduce crop losses under climate change relative to higher levels of warming, the associated mitigation costs may increase the risk of hunger in low-income countries ( ''low confidence'' ) (Hasegawa et al., 2016) <sup>[[#fn:r1155|1155]]</sup> . It is ''likely'' that the even more stringent mitigation measures required to restrict global warming to 1.5°C (Rogelj et al., 2013) <sup>[[#fn:r1156|1156]]</sup> will further increase these mitigation costs and impacts. International trade in food might be a key response measure for alleviating hunger in developing countries under 1.5°C and 2°C stabilization scenarios (IFPRI, 2018) <sup>[[#fn:r1157|1157]]</sup> .

Although warming is projected to be the highest in the Northern Hemisphere under 1.5°C or 2°C of global warming, regions in the tropics and Southern Hemisphere subtropics are projected to experience the largest impacts on economic growth ( ''low to medium confidence'' ) (Gallup et al., 1999; M. Burke et al., 2018; Pretis et al., 2018) <sup>[[#fn:r1158|1158]]</sup> . Despite the uncertainties associated with climate change projections and econometrics (e.g., M. Burke et al., 2018) <sup>[[#fn:r1159|1159]]</sup> , it is ''more likely than not'' that there will be large differences in economic growth under 1.5°C and 2°C of global warming for developing versus developed countries (M. Burke et al., 2018; Pretis et al., 2018) <sup>[[#fn:r1160|1160]]</sup> . Statistically significant reductions in gross domestic product (GDP) per capita growth are projected across much of the African continent, Southeast Asia, India, Brazil and Mexico ( ''low to medium confidence'' ). Countries in the western parts of tropical Africa are projected to benefit most from restricting global warming to 1.5°C, as opposed to 2°C, in terms of future economic growth (Pretis et al., 2018) <sup>[[#fn:r1161|1161]]</sup> . An important reason why developed countries in the tropics and subtropics are projected to benefit substantially from restricting global warming to 1.5°C is that present-day temperatures in these regions are above the threshold thought to be optimal for economic production (M. Burke et al., 2015b, 2018) <sup>[[#fn:r1162|1162]]</sup> .

The world’s largest economies are also projected to benefit from restricting warming to 1.5°C as opposed to 2°C ( ''medium confidence'' ), with the likelihood of such benefits being realized estimated at 76%, 85% and 81% for the USA, China and Japan, respectively (M. Burke et al., 2018) <sup>[[#fn:r1163|1163]]</sup> . Two studies focusing only on the USA found that economic damages are projected to be higher by 2100 if warming reaches 2°C than if it is constrained to 1.5°C. Yohe (2017) <sup>[[#fn:r1164|1164]]</sup> found a mean difference of 0.35% GDP (range 0.2–0.65%), while Hsiang et al. (2017) <sup>[[#fn:r1165|1165]]</sup> identified a GDP loss of 1.2% per degree of warming, hence approximately 0.6% for half a degree. Overall, no statistically significant changes in GDP are projected to occur over most of the developed world under 1.5°C of global warming in comparison to present-day conditions, but under 2°C of global warming impacts on GDP are projected to be generally negative ( ''low confidence'' ) (Pretis et al., 2018) <sup>[[#fn:r1166|1166]]</sup> .

A caveat to the analyses of Pretis et al. (2018) <sup>[[#fn:r1167|1167]]</sup> and M. Burke et al. (2018) <sup>[[#fn:r1168|1168]]</sup> is that the effects of sea level rise were not included in the estimations of damages or future economic growth, implying a potential underestimation of the benefits of limiting warming to 1.5°C for the case where significant sea level rise is avoided at 1.5°C but not at 2°C.

<span id="reducing-hotspots-of-change-for-1.5c-and-2c-of-global-warming"></span>
=== 3.5.4 Reducing Hotspots of Change for 1.5°C and 2°C of Global Warming ===

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

This subsection integrates Sections 3.3 and 3.4 in terms of climate-change-induced hotspots that occur through interactions across the physical climate system, ecosystems and socio-economic human systems, with a focus on the extent to which risks can be avoided or reduced by achieving the 1.5°C global warming goal (as opposed to the 2°C goal). Findings are summarized in Table 3.6.

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

<span id="arctic-sea-ice"></span>
==== 3.5.4.1 Arctic sea ice ====

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

Ice-free Arctic Ocean summers are ''very likely'' at levels of global warming higher than 2°C (Notz and Stroeve, 2016; Rosenblum and Eisenman, 2016; Screen and Williamson, 2017; Niederdrenk and Notz, 2018) <sup>[[#fn:r1169|1169]]</sup> . Some studies even indicate that the entire Arctic Ocean summer period will become ice free under 2°C of global warming, whilst others more conservatively estimate this probability to be in the order of 50% (Section 3.3.8; Sanderson et al., 2017) <sup>[[#fn:r1170|1170]]</sup> . The probability of an ice-free Arctic in September at 1.5°C of global warming is low and substantially lower than for the case of 2°C of global warming ( ''high confidence'' ) (Section 3.3.8; Screen and Williamson, 2017; Jahn, 2018; Niederdrenk and Notz, 2018) <sup>[[#fn:r1171|1171]]</sup> . There is, however, a single study that questions the validity of the 1.5°C threshold in terms of maintaining summer Arctic Ocean sea ice (Niederdrenk and Notz, 2018) <sup>[[#fn:r1172|1172]]</sup> . In contrast to summer, little ice is projected to be lost during winter for either 1.5°C or 2°C of global warming ( ''medium confidence'' ) (Niederdrenk and Notz, 2018) <sup>[[#fn:r1173|1173]]</sup> . The losses in sea ice at 1.5°C and 2°C of warming will result in habitat losses for organisms such as seals, polar bears, whales and sea birds (e.g., Larsen et al., 2014) <sup>[[#fn:r1174|1174]]</sup> . There is ''high agreement'' and ''robust evidence'' that photosynthetic species will change because of sea ice retreat and related changes in temperature and radiation (Section 3.4.4.7), and this is ''very likely'' to benefit fisheries productivity in the Northern Hemisphere spring bloom system (Section 3.4.4.7).

<div id="section-3-5-4-2"></div>

<span id="arctic-land-regions"></span>
==== 3.5.4.2 Arctic land regions ====

<div id="section-3-5-4-2-block-1"></div>

In some Arctic land regions, the warming of cold extremes and the increase in annual minimum temperature at 1.5°C are stronger than the global mean temperature increase by a factor of two to three, meaning 3°C–4.5°C of regional warming at 1.5°C of global warming (e.g., northern Europe in Supplementary Material 3.SM, Figure 3.SM.5 see also Section 3.3.2.2 and Seneviratne et al., 2016) <sup>[[#fn:r1175|1175]]</sup> . Moreover, over much of the Arctic, a further increase of 0.5°C in the global surface temperature, from 1.5°C to 2°C, may lead to further temperature increases of 2°C–2.5°C (Figure 3.3). As a consequence, biome (major ecosystem type) shifts are ''likely'' in the Arctic, with increases in fire frequency, degradation of permafrost, and tree cover ''likely'' to occur at 1.5°C of warming and further amplification of these changes expected under 2°C of global warming (e.g., Gerten et al., 2013; Bring et al., 2016) <sup>[[#fn:r1176|1176]]</sup> . Rising temperatures, thawing permafrost and changing weather patterns are projected to increasingly impact people, infrastructure and industries in the Arctic (W.N. Meier et al., 2014) <sup>[[#fn:r1177|1177]]</sup> with these impacts larger at 2°C than at 1.5°C of warming ( ''medium confidence'' ).

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

<span id="alpine-regions"></span>
==== 3.5.4.3 Alpine regions ====

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

Alpine regions are generally regarded as climate change hotspots given that rich biodiversity has evolved in their cold and harsh climate, but with many species consequently being vulnerable to increases in temperature. Under regional warming, alpine species have been found to migrate upwards on mountain slopes (Reasoner and Tinner, 2009) <sup>[[#fn:r1178|1178]]</sup> , an adaptation response that is obviously limited by mountain height and habitability. Moreover, many of the world’s alpine regions are important from a water security perspective through associated glacier melt, snow melt and river flow (see Section 3.3.5.2 for a discussion of these aspects). Projected biome shifts are ''likely'' to be severe in alpine regions already at 1.5°C of warming and to increase further at 2°C (Gerten et al., 2013, Figure 1b <sup>[[#fn:r1179|1179]]</sup> ; B. Chen et al., 2014) <sup>[[#fn:r1180|1180]]</sup> .

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

<span id="southeast-asia"></span>
==== 3.5.4.4 Southeast Asia ====

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

Southeast Asia is a region highly vulnerable to increased flooding in the context of sea level rise (Arnell et al., 2016; Brown et al., 2016, 2018a) <sup>[[#fn:r1181|1181]]</sup> . Risks from increased flooding are projected to rise from 1.5°C to 2°C of warming ( ''medium confidence'' ), with substantial increases projected beyond 2°C (Arnell et al., 2016) <sup>[[#fn:r1182|1182]]</sup> . Southeast Asia displays statistically significant differences in projected changes in heavy precipitation, runoff and high flows at 1.5°C versus 2°C of warming, with stronger increases occurring at 2°C (Section 3.3.3; Wartenburger et al., 2017; Döll et al., 2018; Seneviratne et al., 2018c) <sup>[[#fn:r1183|1183]]</sup> ; thus, this region is considered a hotspot in terms of increases in heavy precipitation between these two global temperature levels ( ''medium confidence'' ) (Schleussner et al., 2016b; Seneviratne et al., 2016) <sup>[[#fn:r1184|1184]]</sup> . For Southeast Asia, 2°C of warming by 2040 could lead to a decline by one-third in per capita crop production associated with general decreases in crop yields (Nelson et al., 2010) <sup>[[#fn:r1185|1185]]</sup> . However, under 1.5°C of warming, significant risks for crop yield reduction in the region are avoided (Schleussner et al., 2016b) <sup>[[#fn:r1186|1186]]</sup> . These changes pose significant risks for poor people in both rural regions and urban areas of Southeast Asia (Section 3.4.10.1), with these risks being larger at 2°C of global warming compared to 1.5°C ( ''medium confidence'' ).

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

<span id="southern-europe-and-the-mediterranean"></span>
==== 3.5.4.5 Southern Europe and the Mediterranean ====

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

The Mediterranean is regarded as a climate change hotspot, both in terms of projected stronger warming of the regional land-based hot extremes compared to the mean global temperature increase (e.g., Seneviratne et al., 2016) <sup>[[#fn:r1187|1187]]</sup> and in terms of of robust increases in the probability of occurrence of extreme droughts at 2°C vs 1.5°C global warming (Section 3.3.4). Low river flows are projected to decrease in the Mediterranean under 1.5°C of global warming (Marx et al., 2018) <sup>[[#fn:r1188|1188]]</sup> , with associated significant decreases in high flows and floods (Thober et al., 2018) <sup>[[#fn:r1189|1189]]</sup> , largely in response to reduced precipitation. The median reduction in annual runoff is projected to almost double from about 9% ( ''likely'' range 4.5–15.5%) at 1.5°C to 17% ( ''likely'' range 8–25%) at 2°C (Schleussner et al., 2016b) <sup>[[#fn:r1190|1190]]</sup> . Similar results were found by Döll et al. (2018) <sup>[[#fn:r1191|1191]]</sup> . Overall, there is ''high confidence'' that strong increases in dryness and decreases in water availability in the Mediterranean and southern Europe would occur from 1.5°C to 2°C of global warming. Sea level rise is expected to be lower for 1.5°C versus 2°C, lowering risks for coastal metropolitan agglomerations. The risks (assuming current adaptation) related to water deficit in the Mediterranean are high for global warming of 2°C but could be substantially reduced if global warming were limited to 1.5°C (Section 3.3.4; Guiot and Cramer, 2016; Schleussner et al., 2016b; Donnelly et al., 2017) <sup>[[#fn:r1192|1192]]</sup> .

<div id="section-3-5-4-6"></div>

<span id="west-africa-and-the-sahel"></span>
==== 3.5.4.6 West Africa and the Sahel ====

<div id="section-3-5-4-6-block-1"></div>

West Africa and the Sahel are ''likely'' to experience increases in the number of hot nights and longer and more frequent heatwaves even if the global temperature increase is constrained to 1.5°C, with further increases expected at 2°C of global warming and beyond (e.g., Weber et al., 2018) <sup>[[#fn:r1193|1193]]</sup> . Moreover, daily rainfall intensity and runoff is expected to increase ( ''low confidence'' ) towards 2°C and higher levels of global warming (Schleussner et al., 2016b; Weber et al., 2018) <sup>[[#fn:r1194|1194]]</sup> , with these changes also being relatively large compared to the projected changes at 1.5°C of warming. Moreover, increased risks are projected in terms of drought, particularly for the pre-monsoon season (Sylla et al., 2015) <sup>[[#fn:r1195|1195]]</sup> , with both rural and urban populations affected, and more so at 2°C of global warming as opposed to 1.5°C (Liu et al., 2018) <sup>[[#fn:r1196|1196]]</sup> . Based on a World Bank (2013) <sup>[[#fn:r1197|1197]]</sup> study for sub-Saharan Africa, a 1.5°C warming by 2030 might reduce the present maize cropping areas by 40%, rendering these areas no longer suitable for current cultivars. Substantial negative impacts are also projected for sorghum suitability in the western Sahel (Läderach et al., 2013; Sultan and Gaetani, 2016) <sup>[[#fn:r1198|1198]]</sup> . An increase in warming to 2°C by 2040 would result in further yield losses and damages to crops (i.e., maize, sorghum, wheat, millet, groundnut and cassava). Schleussner et al. (2016b) <sup>[[#fn:r1199|1199]]</sup> found consistently reduced impacts on crop yield for West Africa under 2°C compared to 1.5°C of global warming. There is ''medium confidence'' that vulnerabilities to water and food security in the African Sahel will be higher at 2°C compared to 1.5°C of global warming (Cheung et al., 2016a; Betts et al., 2018) <sup>[[#fn:r1200|1200]]</sup> , and at 2°C these vulnerabilities are expected to be worse ( ''high evidence'' ) (Sultan and Gaetani, 2016; Lehner et al., 2017; Betts et al., 2018; Byers et al., 2018; Rosenzweig et al., 2018) <sup>[[#fn:r1201|1201]]</sup> ''.'' Under global warming of more than 2°C, the western Sahel might experience the strongest drying and experience serious food security issues (Ahmed et al., 2015; Parkes et al., 2018) <sup>[[#fn:r1202|1202]]</sup> .

<div id="section-3-5-4-7"></div>

<span id="southern-africa"></span>
==== 3.5.4.7 Southern Africa ====

<div id="section-3-5-4-7-block-1"></div>

The southern African region is projected to be a climate change hotspot in terms of both hot extremes (Figures 3.5 and 3.6) and drying (Figure 3.12). Indeed, temperatures have been rising in the subtropical regions of southern Africa at approximately twice the global rate over the last five decades (Engelbrecht et al., 2015) <sup>[[#fn:r1203|1203]]</sup> . Associated elevated warming of the regional land-based hot extremes has occurred (Section 3.3; Seneviratne et al., 2016) <sup>[[#fn:r1204|1204]]</sup> . Increases in the number of hot nights, as well as longer and more frequent heatwaves, are projected even if the global temperature increase is constrained to 1.5°C ( ''high confidence'' ), with further increases expected at 2°C of global warming and beyond ( ''high confidence'' ) (Weber et al., 2018) <sup>[[#fn:r1205|1205]]</sup> .

Moreover, southern Africa is ''likely'' to generally become drier with reduced water availability under low mitigation (Niang et al., 2014; Engelbrecht et al., 2015; Karl et al., 2015; James et al., 2017) <sup>[[#fn:r1206|1206]]</sup> , with this particular risk being prominent under 2°C of global warming and even under 1.5°C (Gerten et al., 2013) <sup>[[#fn:r1207|1207]]</sup> . Risks are significantly reduced, however, under 1.5°C of global warming compared to under higher levels (Schleussner et al., 2016b) <sup>[[#fn:r1208|1208]]</sup> . There are consistent and statistically significant increases in projected risks of increased meteorological drought in southern Africa at 2°C versus 1.5°C of warming ( ''medium confidence'' ). Despite the general rainfall reductions projected for southern Africa, daily rainfall intensities are expected to increase over much of the region ( ''medium confidence'' ), and increasingly so with higher levels of global warming. There is ''medium confidence'' that livestock in southern Africa will experience increased water stress under both 1.5°C and 2°C of global warming, with negative economic consequences (e.g., Boone et al., 2018) <sup>[[#fn:r1209|1209]]</sup> . The region is also projected to experience reduced maize, sorghum and cocoa cropping area suitability, as well as yield losses under 1.5°C of warming, with further decreases occurring towards 2°C of warming (World Bank, 2013) <sup>[[#fn:r1210|1210]]</sup> . Generally, there is ''high confidence'' that vulnerability to decreases in water and food availability is reduced at 1.5°C versus 2°C for southern Africa (Betts et al., 2018) <sup>[[#fn:r1211|1211]]</sup> , whilst at 2°C these are expected to be higher ( ''high confidence'' ) (Lehner et al., 2017; Betts et al., 2018; Byers et al., 2018; Rosenzweig et al., 2018) <sup>[[#fn:r1212|1212]]</sup> .

<div id="section-3-5-4-8"></div>

<span id="tropics"></span>
==== 3.5.4.8 Tropics ====

<div id="section-3-5-4-8-block-1"></div>

Worldwide, the largest increases in the number of hot days are projected to occur in the tropics (Figure 3.7). Moreover, the largest differences in the number of hot days for 1.5°C versus 2°C of global warming are projected to occur in the tropics (Mahlstein et al., 2011) <sup>[[#fn:r1213|1213]]</sup> . In tropical Africa, increases in the number of hot nights, as well as longer and more frequent heatwaves, are projected under 1.5°C of global warming, with further increases expected under 2°C of global warming (Weber et al., 2018) <sup>[[#fn:r1214|1214]]</sup> . Impact studies for major tropical cereals reveal that yields of maize and wheat begin to decline with 1°C to 2°C of local warming in the tropics. Schleussner et al. (2016b) <sup>[[#fn:r1215|1215]]</sup> project that constraining warming to 1.5°C rather than 2°C would avoid significant risks of tropical crop yield declines in West Africa, Southeast Asia, and Central and South America. There is ''limited evidence'' and thus ''low confidence'' that these changes may result in significant population displacement from the tropics to the subtropics (e.g., Hsiang and Sobel, 2016) <sup>[[#fn:r1216|1216]]</sup> .

<div id="section-3-5-4-9"></div>

<span id="small-islands-1"></span>
==== 3.5.4.9 Small islands ====

<div id="section-3-5-4-9-block-1"></div>

It is widely recognized that small islands are very sensitive to climate change impacts such as sea level rise, oceanic warming, heavy precipitation, cyclones and coral bleaching ( ''high confidence'' ) (Nurse et al., 2014; Ourbak and Magnan, 2017) <sup>[[#fn:r1217|1217]]</sup> ''.'' Even at 1.5°C of global warming, the compounding impacts of changes in rainfall, temperature, tropical cyclones and sea level are ''likely'' to be significant across multiple natural and human systems. There are potential benefits to small island developing states (SIDS) from avoided risks at 1.5°C versus 2°C, especially when coupled with adaptation efforts. In terms of sea level rise, by 2150, roughly 60,000 fewer people living in SIDS will be exposed in a 1.5°C world than in a 2°C world (Rasmussen et al., 2018) <sup>[[#fn:r1218|1218]]</sup> . Constraining global warming to 1.5°C may significantly reduce water stress (by about 25%) compared to the projected water stress at 2°C, for example in the Caribbean region (Karnauskas et al., 2018) <sup>[[#fn:r1219|1219]]</sup> , and may enhance the ability of SIDS to adapt (Benjamin and Thomas, 2016) <sup>[[#fn:r1220|1220]]</sup> . Up to 50% of the year is projected to be very warm in the Caribbean at 1.5°C, with a further increase by up to 70 days at 2°C versus 1.5°C (Taylor et al., 2018) <sup>[[#fn:r1221|1221]]</sup> . By limiting warming to 1.5°C instead of 2°C in 2050, risks of coastal flooding (measured as the flood amplification factors for 100-year flood events) are reduced by 20–80% for SIDS (Rasmussen et al., 2018) <sup>[[#fn:r1222|1222]]</sup> . A case study of Jamaica with lessons for other Caribbean SIDS demonstrated that the difference between 1.5°C and 2°C is ''likely'' to challenge livestock thermoregulation, resulting in persistent heat stress for livestock (Lallo et al., 2018) <sup>[[#fn:r1223|1223]]</sup> .

<div id="section-3-5-4-10"></div>

<span id="fynbos-and-shrub-biomes"></span>
==== 3.5.4.10 Fynbos and shrub biomes ====

<div id="section-3-5-4-10-block-1"></div>

The Fynbos and succulent Karoo biomes of South Africa are threatened systems that were assessed in AR5. Similar shrublands exist in the semi-arid regions of other continents, with the Sonora-Mojave creosotebush-white bursage desert scrub ecosystem in the USA being a prime example. Impacts accrue across these systems with greater warming, with impacts at 2°C ''likely'' to be greater than those at 1.5°C ( ''medium confidence'' ). Under 2°C of global warming, regional warming in drylands is projected to be 3.2°C–4°C, and under 1.5°C of global warming, mean warming in drylands is projected to still be about 3°C. The Fynbos biome in southwestern South Africa is vulnerable to the increasing impact of fires under increasing temperatures and drier winters ( ''high confidence'' ). The Fynbos biome is projected to lose about 20%, 45% and 80% of its current suitable climate area relative to its present-day area under 1°C, 2°C and 3°C of warming, respectively (Engelbrecht and Engelbrecht, 2016) <sup>[[#fn:r1224|1224]]</sup> , demonstrating the value of climate change mitigation in protecting this rich centre of biodiversity.

<div id="section-3-5-4-10-block-2"></div>

<span id="table-3.6"></span>
<!-- START TABLE -->
'''Table 3.6'''

<span id="emergence-and-intensity-of-climate-change-hotspots-under-different-degrees-of-global-warming."></span>
'''Emergence and intensity of climate change hotspots under different degrees of global warming.'''

<!-- TABLE -->
{| class="wikitable"
|-
! Region and/or Phenomenon
! Warming of 1.5°C or less
! Warming of 1.5°C–2°C
! Warming of 2°C–3°C
|-
| Arctic sea ice
| Arctic summer sea ice is ''likely'' to be maintained
Habitat losses for organisms such as polar bears,<br />
whales, seals and sea birds

Benefits for Arctic fisheries

| The risk of an ice-free Arctic in summer is about 50% or higher
Habitat losses for organisms such as polar bears, whales,seals and sea birds may be critical if summers are ice free

Benefits for Arctic fisheries

| The Arctic is ''very likely'' to be ice free in summer
Critical habitat losses for organisms such as polar bears, whales, seals and sea birds

Benefits for Arctic fisheries

|-
| Arctic land regions
| Cold extremes warm by a factor of 2–3, reaching up to 4.5°C ( ''high confidence'' )
Biome shifts in the tundra and permafrost deterioration are ''likely''

| Cold extremes warm by as much as 8°C ( ''high confidence'' )
Larger intrusions of trees and shrubs in the tundra than under 1.5°C of warming are ''likely'' ; larger but constrained losses in permafrost are ''likely''

| Drastic regional warming is ''very likely''
A collapse in permafrost may occur ( ''low confidence'' ); a drastic biome shift from tundra to boreal forest is possible ( ''low confidence'' )

|-
| Alpine regions
| Severe shifts in biomes are ''likely''
| Even more severe shifts are ''likely''
| Critical losses in alpine habitats are ''likely''
|-
| Southeast Asia
| Risks for increased flooding related to sea level rise
Increases in heavy precipitation events

Significant risks of crop yield reductions are avoided

| Higher risks of increased flooding related to sea level rise ( ''medium confidence'' )
Stronger increases in heavy precipitation events ( ''medium confidence'' )

One-third decline in per capita crop production ( ''medium confidence'' )

| Substantial increases in risks related to flooding from sea level rise
Substantial increase in heavy precipitation and high-flow events

Substantial reductions in crop yield

|-
| Mediterranean
| Increase in probability of extreme drought ( ''medium confidence'' )
''Medium confidence'' in reduction in runoff of about 9% ( ''likely'' range 4.5–15.5%)

Risk of water deficit ( ''medium confidence'' )

| Robust increase in probability of extreme drought ( ''medium confidence'' )
''Medium confidence'' in further reductions (about 17%) in runoff ( ''likely'' range 8–28%)

Higher risks of water deficit ( ''medium confidence'' )

| Robust and large increases in extreme drought. Substantial reductions in precipitation and in runoff ( ''medium confidence'' )
Very high risks of water deficit ( ''medium confidence'' )

|-
| West Africa and the Sahel
| Increases in the number of hot nights and longer and more frequent heatwaves are ''likely''
Reduced maize and sorghum production is ''likely'' , with area suitable for maize production reduced by as much as 40%

Increased risks of undernutrition

| Further increases in number of hot nights and longer and more frequent heatwaves are ''likely''
Negative impacts on maize and sorghum production ''likely'' larger than at 1.5°C; ''medium confidence'' that vulnerabilities to food security in the African Sahel will be higher at 2°C compared to 1.5°C

Higher risks of undernutrition

| Substantial increases in the number of hot nights and heatwave duration and frequency ( ''very likely'' )
Negative impacts on crop yield may result in major regional food insecurities ( ''medium confidence'' )

High risks of undernutrition

|-
| Southern Africa
| Reductions in water availability ( ''medium confidence'' )
Increases in number of hot nights and longer and more frequent heatwaves ( ''high confidence'' )

High risks of increased mortality from heatwaves

High risk of undernutrition in communities dependent on dryland agriculture and livestock

| Larger reductions in rainfall and water availability ( ''medium confidence'' )
Further increases in number of hot nights and longer and more frequent heatwaves ( ''high confidence'' ), associated increases in risks of increased mortality from heatwaves compared to 1.5°C warming ( ''high confidence'' )

Higher risks of undernutrition in communities dependent on dryland agriculture and livestock

| Large reductions in rainfall and water availability ( ''medium confidence'' )
Drastic increases in the number of hot nights, hot days and heatwave duration and frequency to impact substantially on agriculture, livestock and human health and mortality ( ''high confidence'' )

Very high risks of undernutrition in communities dependent on dryland agriculture and livestock

|-
| Tropics
| Increases in the number of hot days and hot nights as well as longer and more frequent heatwaves ( ''high confidence'' )
Risks to tropical crop yields in West Africa, Southeast Asia and Central and South America are significantly less than under 2°C of warming

| The largest increase in hot days under 2°C compared to 1.5°C is projected for the tropics.
Risks to tropical crop yields in West Africa, Southeast Asia and Central and South America could be extensive

| Oppressive temperatures and accumulated heatwave duration ''very likely'' to directly impact human health, mortality and productivity
Substantial reductions in crop yield ''very likely''

|-
| Small islands
| Land of 60,000 less people exposed by 2150 on SIDS compared to impacts under 2°C of global warming
Risks for coastal flooding reduced by 20–80% for SIDS compared to 2°C of global warming

Freshwater stress reduced by 25%

Increase in the number of warm days for SIDS in the tropics

Persistent heat stress in cattle avoided

Loss of 70–90% of coral reefs

| Tens of thousands of people displaced owing to<br />
inundation of SIDSHigh risks for coastal floodingFreshwater stress reduced by 25% compared to<br />
2°C of global warming
<br/><br/>
Freshwater stress from projected aridity

Further increase of about 70 warm days per year

Persistent heat stress in cattle in SIDS

Loss of most coral reefs and weaker remaining structures owing to ocean acidification

| Substantial and widespread impacts through inundation of SIDS, coastal flooding, freshwater stress, persistent heat stress and loss of most coral reefs ( ''very likely'' )
|-
| Fynbos biome
| About 30% of suitable climate area lost<br />
( ''medium confidence'' )
| Increased losses (about 45%) of suitable climate area ( ''medium confidence'' )
| Up to 80% of suitable climate area lost<br />
( ''medium confidence'' )
|}
<!-- END TABLE -->

<span id="avoiding-regional-tipping-points-by-achieving-more-ambitious-global-temperature-goals"></span>
=== 3.5.5 Avoiding Regional Tipping Points by Achieving More Ambitious Global Temperature Goals ===

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

Tipping points refer to critical thresholds in a system that, when exceeded, can lead to a significant change in the state of the system, often with an understanding that the change is irreversible. An understanding of the sensitivities of tipping points in the physical climate system, as well as in ecosystems and human systems, is essential for understanding the risks associated with different degrees of global warming. This subsection reviews tipping points across these three areas within the context of the different sensitivities to 1.5°C versus 2°C of global warming. Sensitivities to less ambitious global temperature goals are also briefly reviewed. Moreover, an analysis is provided of how integrated risks across physical, natural and human systems may accumulate to lead to the exceedance of thresholds for particular systems. The emphasis in this section is on the identification of regional tipping points and their sensitivity to 1.5°C and 2°C of global warming, whereas tipping points in the global climate system, referred to as large-scale singular events, were already discussed in Section 3.5.2. A summary of regional tipping points is provided in Table 3.7.

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

<span id="arctic-sea-ice-1"></span>
==== 3.5.5.1 Arctic sea ice ====

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

Collins et al. (2013) <sup>[[#fn:r1225|1225]]</sup> discussed the loss of Artic sea ice in the context of potential tipping points. Climate models have been used to assess whether a bifurcation exists that would lead to the irreversible loss of Arctic sea ice (Armour et al., 2011; Boucher et al., 2012; Ridley et al., 2012) <sup>[[#fn:r1226|1226]]</sup> and to test whether the summer sea ice extent can recover after it has been lost (Schröder and Connolley, 2007; Sedláček et al., 2011; Tietsche et al., 2011) <sup>[[#fn:r1227|1227]]</sup> . These studies did not find evidence of bifurcation or indicate that sea ice returns within a few years of its loss, leading Collins et al. (2013) <sup>[[#fn:r1228|1228]]</sup> to conclude that there is ''little evidence'' for a tipping point in the transition from perennial to seasonal ice cover. No evidence has been found for irreversibility or tipping points, suggesting that year-round sea ice will return given a suitable climate ( ''medium confidence'' ) (Schröder and Connolley, 2007; Sedláček et al., 2011; Tietsche et al., 2011) <sup>[[#fn:r1229|1229]]</sup> .

<div id="section-3-5-5-2"></div>

<span id="tundra"></span>
==== 3.5.5.2 Tundra ====

<div id="section-3-5-5-2-block-1"></div>

Tree growth in tundra-dominated landscapes is strongly constrained by the number of days with mean air temperature above 0°C. A potential tipping point exists where the number of days below 0°C decreases to the extent that the tree fraction increases significantly. Tundra-dominated landscapes have warmed more than the global average over the last century (Settele et al., 2014) <sup>[[#fn:r1230|1230]]</sup> , with associated increases in fires and permafrost degradation (Bring et al., 2016; DeBeer et al., 2016; Jiang et al., 2016; Yang et al., 2016) <sup>[[#fn:r1231|1231]]</sup> . These processes facilitate conditions for woody species establishment in tundra areas, and for the eventual transition of the tundra to boreal forest. The number of investigations into how the tree fraction may respond in the Arctic to different degrees of global warming is limited, and studies generally indicate that substantial increases will ''likely'' occur gradually (e.g., Lenton et al., 2008) <sup>[[#fn:r1232|1232]]</sup> . Abrupt changes are only plausible at levels of warming significantly higher than 2°C ( ''low confidence'' ) and would occur in conjunction with a collapse in permafrost (Drijfhout et al., 2015) <sup>[[#fn:r1233|1233]]</sup> .

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

<span id="permafrost"></span>
==== 3.5.5.3 Permafrost ====

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

Widespread thawing of permafrost potentially makes a large carbon store (estimated to be twice the size of the atmospheric store; Dolman et al., 2010) <sup>[[#fn:r1234|1234]]</sup> vulnerable to decomposition, which could lead to further increases in atmospheric carbon dioxide and methane and hence to further global warming. This feedback loop between warming and the release of greenhouse gas from thawing tundra represents a potential tipping point. However, the carbon released to the atmosphere from thawing permafrost is projected to be restricted to 0.09–0.19 Gt C yr <sup>–1</sup> at 2°C of global warming and to 0.08–0.16 Gt C yr <sup>–1</sup> at 1.5°C (E.J. Burke et al., 2018) <sup>[[#fn:r1235|1235]]</sup> , which does not indicate a tipping point ( ''medium confidence'' ). At higher degrees of global warming, in the order of 3°C, a different type of tipping point in permafrost may be reached. A single model projection (Drijfhout et al., 2015) <sup>[[#fn:r1236|1236]]</sup> suggested that higher temperatures may induce a smaller ice fraction in soils in the tundra, leading to more rapidly warming soils and a positive feedback mechanism that results in permafrost collapse ( ''low confidence'' ). The disparity between the multi-millennial time scales of soil carbon accumulation and potentially rapid decomposition in a warming climate implies that the loss of this carbon to the atmosphere would be essentially irreversible (Collins et al., 2013) <sup>[[#fn:r1237|1237]]</sup> .

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

<span id="asian-monsoon"></span>
==== 3.5.5.4 Asian monsoon ====

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

At a fundamental level, the pressure gradient between the Indian Ocean and Asian continent determines the strength of the Asian monsoon. As land masses warm faster than the oceans, a general strengthening of this gradient, and hence of monsoons, may be expected under global warming (e.g., Lenton et al., 2008) <sup>[[#fn:r1238|1238]]</sup> . Additional factors such as changes in albedo induced by aerosols and snow-cover change may also affect temperature gradients and consequently pressure gradients and the strength of the monsoon. In fact, it has been estimated that an increase of the regional land mass albedo to 0.5 over India would represent a tipping point resulting in the collapse of the monsoon system (Lenton et al., 2008) <sup>[[#fn:r1239|1239]]</sup> . The overall impacts of the various types of radiative forcing under different emissions scenarios are more subtle, with a weakening of the monsoon north of about 25°N in East Asia but a strengthening south of this latitude projected by Jiang and Tian (2013) <sup>[[#fn:r1240|1240]]</sup> under high and modest emissions scenarios. Increases in the intensity of monsoon precipitation are ''likely'' under low mitigation (AR5). Given that scenarios of 1.5°C or 2°C of global warming would include a substantially smaller radiative forcing than those assessed in the study by Jiang and Tian (2013) <sup>[[#fn:r1241|1241]]</sup> , there is ''low confidence'' regarding changes in monsoons at these low global warming levels, as well as regarding the differences between responses at 1.5°C versus 2°C of warming.

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

<span id="west-african-monsoon-and-the-sahel"></span>
==== 3.5.5.5 West African monsoon and the Sahel ====

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

Earlier work has identified 3°C of global warming as the tipping point leading to a significant strengthening of the West African monsoon and subsequent wettening (and greening) of the Sahel and Sahara (Lenton et al., 2008) <sup>[[#fn:r1242|1242]]</sup> . AR5 (Niang et al., 2014) <sup>[[#fn:r1243|1243]]</sup> , as well as more recent research through the Coordinated Regional Downscaling Experiment for Africa (CORDEX–AFRICA), provides a more uncertain view, however, in terms of the rainfall futures of the Sahel under low mitigation futures. Even if a wetter Sahel should materialize under 3°C of global warming ( ''low confidence'' ), it should be noted that there would be significant offsets in the form of strong regional warming and related adverse impacts on crop yield, livestock mortality and human health under such low mitigation futures (Engelbrecht et al., 2015; Sylla et al., 2016; Weber et al., 2018) <sup>[[#fn:r1244|1244]]</sup> .

<div id="section-3-5-5-6"></div>

<span id="rainforests"></span>
==== 3.5.5.6 Rainforests ====

<div id="section-3-5-5-6-block-1"></div>

A large portion of rainfall over the world’s largest rainforests is recirculated (e.g., Lenton et al., 2008) <sup>[[#fn:r1245|1245]]</sup> , which raises the concern that deforestation may trigger a threshold in reduced forest cover, leading to pronounced forest dieback. For the Amazon, this deforestation threshold has been estimated to be 40% (Nobre et al., 2016) <sup>[[#fn:r1246|1246]]</sup> . Global warming of 3°C–4°C may also, independent of deforestation, represent a tipping point that results in a significant dieback of the Amazon forest, with a key forcing mechanism being stronger El Niño events bringing more frequent droughts to the region (Nobre et al., 2016) <sup>[[#fn:r1247|1247]]</sup> . Increased fire frequencies under global warming may interact with and accelerate deforestation, particularly during periods of El Niño-induced droughts (Lenton et al., 2008; Nobre et al., 2016) <sup>[[#fn:r1248|1248]]</sup> . Global warming of 3°C is projected to reduce the extent of tropical rainforest in Central America, with biomass being reduced by about 40%, which can lead to a large replacement of rainforest by savanna and grassland (Lyra et al., 2017) <sup>[[#fn:r1249|1249]]</sup> . Overall, modelling studies (Huntingford et al., 2013; Nobre et al., 2016) <sup>[[#fn:r1250|1250]]</sup> and observational constraints (Cox et al., 2013) <sup>[[#fn:r1251|1251]]</sup> suggest that pronounced rainforest dieback may only be triggered at 3°C–4°C ( ''medium confidence'' ), although pronounced biomass losses may occur at 1.5°C– 2°C of global warming.

<div id="section-3-5-5-7"></div>

<span id="boreal-forests"></span>
==== 3.5.5.7 Boreal forests ====

<div id="section-3-5-5-7-block-1"></div>

Boreal forests are ''likely'' to experience stronger local warming than the global average (WGII AR5; Collins et al., 2013) <sup>[[#fn:r1252|1252]]</sup> . Increased disturbance from fire, pests and heat-related mortality may affect, in particular, the southern boundary of boreal forests ( ''medium confidence'' ) (Gauthier et al., 2015) <sup>[[#fn:r1253|1253]]</sup> , with these impacts accruing with greater warming and thus impacts at 2°C would be expected to be greater than those at 1.5°C ( ''medium confidence'' ). A tipping point for significant dieback of the boreal forests is thought to exist, where increased tree mortality would result in the creation of large regions of open woodlands and grasslands, which would favour further regional warming and increased fire frequencies, thus inducing a powerful positive feedback mechanism (Lenton et al., 2008; Lenton, 2012) <sup>[[#fn:r1254|1254]]</sup> . This tipping point has been estimated to exist between 3°C and 4°C of global warming ( ''low confidence'' ) (Lucht et al., 2006; Kriegler et al., 2009) <sup>[[#fn:r1255|1255]]</sup> , but given the complexities of the various forcing mechanisms and feedback processes involved, this is thought to be an uncertain estimate.

<div id="section-3-5-5-8"></div>

<span id="heatwaves-unprecedented-heat-and-human-health"></span>
==== 3.5.5.8 Heatwaves, unprecedented heat and human health ====

<div id="section-3-5-5-8-block-1"></div>

Increases in ambient temperature are linearly related to hospitalizations and deaths once specific thresholds are exceeded (so there is not a tipping point per se). It is plausible that coping strategies will not be in place for many regions, with potentially significant impacts on communities with low adaptive capacity, effectively representing the occurrence of a local/regional tipping point. In fact, even if global warming is restricted to below 2°C, there could be a substantial increase in the occurrence of deadly heatwaves in cities if urban heat island effects are considered, with impacts being similar at 1.5°C and 2°C but substantially larger than under the present climate (Matthews et al., 2017) <sup>[[#fn:r1256|1256]]</sup> . At 1.5°C of warming, twice as many megacities (such as Lagos, Nigeria, and Shanghai, China) than at present are ''likely'' to become heat stressed, potentially exposing more than 350 million more people to deadly heat stress by 2050. At 2°C of warming, Karachi (Pakistan) and Kolkata (India) could experience conditions equivalent to their deadly 2015 heatwaves on an annual basis ( ''medium confidence'' ). These statistics imply a tipping point in the extent and scale of heatwave impacts. However, these projections do not integrate adaptation to projected warming, for instance cooling that could be achieved with more reflective roofs and urban surfaces in general (Akbari et al., 2009; Oleson et al., 2010) <sup>[[#fn:r1257|1257]]</sup> .

<div id="section-3-5-5-9"></div>

<span id="agricultural-systems-key-staple-crops"></span>
==== 3.5.5.9 Agricultural systems: key staple crops ====

<div id="section-3-5-5-9-block-1"></div>

A large number of studies have consistently indicated that maize crop yield will be negatively affected under increased global warming, with negative impacts being higher at 2°C of warming than at 1.5°C (e.g., Niang et al., 2014; Schleussner et al., 2016b; J. Huang et al., 2017; Iizumi et al., 2017) <sup>[[#fn:r1258|1258]]</sup> . Under 2°C of global warming, losses of 8–14% are projected in global maize production (Bassu et al., 2014) <sup>[[#fn:r1259|1259]]</sup> . Under global warming of more than 2°C, regional losses are projected to be about 20% if they co-occur with reductions in rainfall (Lana et al., 2017) <sup>[[#fn:r1260|1260]]</sup> . These changes may be classified as incremental rather than representing a tipping point. Large-scale reductions in maize crop yield, including the potential collapse of this crop in some regions, may exist under 3°C or more of global warming ( ''low confidence'' ) (e.g., Thornton et al., 2011) <sup>[[#fn:r1261|1261]]</sup> .

<div id="section-3-5-5-10"></div>

<span id="agricultural-systems-livestock-in-the-tropics-and-subtropics"></span>
==== 3.5.5.10 Agricultural systems: livestock in the tropics and subtropics ====

<div id="section-3-5-5-10-block-1"></div>

The potential impacts of climate change on livestock (Section 3.4.6), in particular the direct impacts through increased heat stress, have been less well studied than impacts on crop yield, especially from the perspective of critical thresholds being exceeded. A case study from Jamaica revealed that the difference in heat stress for livestock between 1.5°C and 2°C of warming is ''likely'' to exceed the limits for normal thermoregulation and result in persistent heat stress for these animals (Lallo et al., 2018) <sup>[[#fn:r1262|1262]]</sup> . It is plausible that this finding holds for livestock production in both tropical and subtropical regions more generally ( ''medium confidence'' ) (Section 3.4.6). Under 3°C of global warming, significant reductions in the areas suitable for livestock production could occur ( ''low confidence'' ), owing to strong increases in regional temperatures in the tropics and subtropics ( ''high confidence'' ) ''.'' Thus, regional tipping points in the viability of livestock production may well exist, but ''little evidence'' quantifying such changes exists.

<div id="section-3-5-5-10-block-2"></div>

<span id="table-3.7"></span>
<!-- START TABLE -->
'''Table 3.7'''

<span id="summary-of-enhanced-risks-in-the-exceedance-of-regional-tipping-points-under-different-global-temperature-goals"></span>
'''Summary of enhanced risks in the exceedance of regional tipping points under different global temperature goals'''

<!-- TABLE -->
{| class="wikitable"
|-
! Tipping point
! Warming of 1.5°C or less
! Warming of 1.5°C–2°C
! Warming of up to 3°C
|-
| Arctic sea ice
| Arctic summer sea ice is ''likely'' to be maintained
Sea ice changes reversible under suitable climate<br />
restoration

| The risk of an ice-free Arctic in summer is about 50% or higher
Sea ice changes reversible under suitable climate restoration

| Arctic is ''very likely'' to be ice free in summer
Sea ice changes reversible under suitable climate<br />
restoration

|-
| Tundra
| Decrease in number of growing degree days<br />
below 0°CAbrupt increases in tree cover are ''unlikely''
| Further decreases in number of growing degree days below 0°C
Abrupt increased in tree cover are ''unlikely''

| Potential for an abrupt increase in tree fraction<br />
( ''low confidence'' )
|-
| Permafrost
| 17–44% reduction in permafrost
Approximately 2 million km <sup>2</sup> more permafrost maintained than under 2°C of global warming ( ''medium confidence'' )

Irreversible loss of stored carbon

| 28–53% reduction in permafrost
Irreversible loss of stored carbon

| Potential for permafrost collapse ( ''low confidence'' )
|-
| Asian monsoon
| ''Low confidence'' in projected changes
| Increases in the intensity of monsoon precipitation ''likely''
|-
| West African monsoon and the Sahel
| Uncertain changes; ''unlikely'' that a tipping point is<br />
reached
| Uncertain changes; ''unlikely'' that tipping point is reached
| Strengthening of monsoon with wettening and greening of the Sahel and Sahara ( ''low confidence'' )
Negative associated impacts through increases in extreme temperature events

|-
| Rainforests
| Reduced biomass, deforestation and fire increases pose uncertain risks to forest dieback
| Larger biomass reductions than under 1.5°C of warming; deforestation and fire increases pose uncertain risk to forest dieback
| Reduced extent of tropical rainforest in Central America and large replacement of rainforest by savanna and grassland
Potential tipping point leading to pronounced forest dieback ( ''medium confidence'' )

|-
| Boreal forests
| Increased tree mortality at southern boundary of<br />
boreal forest ( ''medium confidence'' )
| Further increases in tree mortality at southern boundary of boreal forest ( ''medium confidence'' )
| Potential tipping point at 3°C–4°C for significant dieback of boreal forest ( ''low confidence'' )
|-
| Heatwaves, unprecedented heat and human health
| Substantial increase in occurrence of potentially<br />
deadly heatwaves ( ''likely'' )More than 350 million more people exposed to deadly heat by 2050 under a midrange population growth scenario ( ''likely'' )
| Substantial increase in potentially deadly<br />
heatwaves ( ''likely'' )Annual occurrence of heatwaves similar to the deadly 2015 heatwaves in India and Pakistan<br />
( ''medium confidence'' )
| Substantial increase in potentially deadly<br />
heatwaves ''very likely''
|-
| Agricultural systems: key staple crops
| Global maize crop reductions of about 10%
| Larger reductions in maize crop production than<br />
under 1.5°C of about 15%
| Drastic reductions in maize crop globally and in Africa (high confidence)
Potential tipping point for collapse of maize crop in some regions<br />
( ''low confidence'' )

|-
| Livestock in the tropics and subtropics
| Increased heat stress
| Onset of persistent heat stress ( ''medium confidence'' )
| Persistent heat stress ''likely''
|}
<!-- END TABLE -->

<div id="section-3-5-5-10-block-3" class="box"></div>

<span id="box-3.6-economic-damages-from-climate-change"></span>