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

=== 3.5.5 Avoiding Regional Tipping Points by Achieving More Ambitious Global Temperature Goals ===

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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.

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==== 3.5.5.1 Arctic sea ice ====

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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> .

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==== 3.5.5.2 Tundra ====

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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> .

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==== 3.5.5.3 Permafrost ====

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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> .

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==== 3.5.5.4 Asian monsoon ====

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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.

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==== 3.5.5.5 West African monsoon and the Sahel ====

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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> .

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==== 3.5.5.6 Rainforests ====

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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.

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==== 3.5.5.7 Boreal forests ====

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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.

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==== 3.5.5.8 Heatwaves, unprecedented heat and human health ====

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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> .

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==== 3.5.5.9 Agricultural systems: key staple crops ====

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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> .

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==== 3.5.5.10 Agricultural systems: livestock in the tropics and subtropics ====

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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.

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'''Table 3.7'''

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'''Summary of enhanced risks in the exceedance of regional tipping points under different global temperature goals'''

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{| 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''
|}
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