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

== Cross-Chapter Box 8: 1.5°C Warmer Worlds ==

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====== Lead Authors ======

* Sonia I. Seneviratne (Switzerland)
* Joeri Rogelj (Austria, Belgium)
* Roland Séférian (France)
* Myles R. Allen (United Kingdom)
* Marcos Buckeridge (Brazil)
* Kristie L. Ebi (United States)
* Ove Hoegh-Guldberg (Australia)
* Richard J. Millar (United Kingdom)
* Antony J. Payne (United Kingdom)
* Petra Tschakert (Australia, Austria)
* Rachel Warren (United Kingdom)

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====== Contributing Authors ======

* Neville Ellis (Australia)
* Richard Wartenburger (Switzerland, Germany)

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'''Introduction'''

The Paris Agreement includes goals of stabilizing global mean surface temperature (GMST) well below 2°C and 1.5°C above pre-industrial levels in the longer term. There are several aspects, however, that remain open regarding what a ‘1.5°C warmer world’ could be like, in terms of mitigation (Chapter 2) and adaptation (Chapter 4), as well as in terms of projected warming and associated regional climate change (Chapter 3), which are overlaid on anticipated and differential vulnerabilities (Chapter 5). '''Alternative ‘1.5°C warmer worlds’ resulting from mitigation and adaptation choices, as well as from climate variability (climate ‘noise’), can be vastly different,''' as highlighted in this Cross-Chapter Box. In addition, the range of models underlying 1.5°C projections can be substantial and needs to be considered.

'''Key questions''' <sup>[[#fn:10|10]]</sup> ''':'''

* '''What is a 1.5°C global mean warming, how is it measured, and what temperature increase does it imply for single locations and at specific times?''' Global mean surface temperature (GMST) corresponds to the globally averaged temperature of Earth derived from point-scale ground observations or computed in climate models (Chapters 1 and 3). Global mean surface temperature is additionally defined over a given time frame, for example averaged over a month, a year, or multiple decades. Because of climate variability, a climate-based GMST typically needs to be defined over several decades (typically 20 or 30 years; Chapter 3, Section 3.2). Hence, whether or when global warming reaches 1.5°C depends to some extent on the choice of pre-industrial reference period, whether 1.5°C refers to total or human-induced warming, and which variables and coverage are used to define GMST change (Chapter 1). By definition, because GMST is an average in time and space, there will be locations and time periods in which 1.5°C of warming is exceeded, even if the global mean warming is at 1.5°C. In some locations, these differences can be particularly large (Cross-Chapter Box 8, Figure 1).
* '''What is the impact of different climate models for projected changes in climate at 1.5°C of global warming?''' The range between single model simulations of projected regional changes at 1.5°C GMST increase can be substantial for regional responses (Chapter 3, Section 3.3). For instance, for the warming of cold extremes in a 1.5°C warmer world, some model simulations project a 3°C warming while others project more than 6°C of warming in the Arctic land areas (Cross-Chapter Box 8, Figure 2). For hot temperature extremes in the contiguous United States, the range of model simulations includes temperatures lower than pre-industrial values (–0.3°C) and a warming of 3.5°C (Cross-Chapter Box 8, Figure 2). Some regions display an even larger range (e.g., 1°C–6°C regional warming in hot extremes in central Europe at 1.5°C of warming; Chapter 3, Sections 3.3.1 and 3.3.2). This large spread is due to both modelling uncertainty and internal climate variability. While the range is large, it also highlights risks that can be avoided with near certainty in a 1.5°C warmer world compared to worlds at higher levels of warming (e.g., an 8°C warming of cold extremes in the Arctic is not reached at 1.5°C of global warming in the multimodel ensemble but could happen at 2°C of global warming; Cross-Chapter Box 8, Figure 2). Inferred projected ranges of regional responses (mean value, minimum and maximum) for different mitigation scenarios from Chapter 2 are displayed in Cross-Chapter Box 8, Table 1.

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'''Cross-Chapter Box 8, Figure 1'''

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'''Range of projected realized temperatures at 1.5°C of global warming (due to stochastic noise and model-based spread).'''
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[[File:293e5020e8a314dd251bb1f6511f02cf CC-box-8-figure-1-976x1024.jpg]]

Temperatures with a 25% chance of occurrence at any location within a 10-year time frame are shown, corresponding to GMST anomalies of 1.5°C (Coupled Model Intercomparison Project Phase 5 (CMIP5) multimodel ensemble). The plots display the 25th percentile (Q25, left) and 75th percentile (Q75, right) values of mean temperature (Tmean), yearly maximum daytime temperature (TXx) and yearly minimum night-time temperature (TNn), sampled from all time frames with GMST anomalies of 1.5°C in Representative Concentration Pathway (RCP)8.5 model simulations of the CMIP5 ensemble. From Seneviratne et al. (2018b).

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'''Cross-Chapter Box 8, Figure 2'''

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'''Spread of projected multimodel changes in minimum annual night-time temperature (TNn) in Arctic land (left) and in maximum annual daytime temperature (TXx) in the contiguous United States as a function of mean global warming in climate simulations.'''
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[[File:d7f48faae0db0e7f745a792421b3e326 FINAL_CCB8_Fig2-1024x498.jpg]]

The multimodel range (due to model spread and internal climate variability) is indicated in red shading (minimum and maximum value based on climate model simulations). The multimodel mean value is displayed with solid red and blue lines for two emissions pathways (blue: Representative Concentration Pathway (RCP)4.5; red: RCP8.5). The dashed red line indicates projections for a 1.5°C warmer world. The dashed black line displays the 1:1 line. The figure is based on Figure 3 of Seneviratne et al. (2016) <sup>[[#fn:r1380|1380]]</sup> .

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* '''What is the impact of emissions pathways with, versus without, an overshoot?''' All mitigation pathways projecting less than 1.5°C of global warming over or at the end of the 21st century include some probability of overshooting 1.5°C. These pathways include some periods with warming stronger than 1.5°C in the course of the coming decades and/or some probability of not reaching 1.5°C (Chapter 2, Section 2.2). This is inherent to the difficulty of limiting global warming to 1.5°C, given that we are already very close to this warming level. The implications of overshooting are large for risks to natural and human systems, especially if the temperature at peak warming is high, because some risks may be long lasting and irreversible, such as the loss of some ecosystems (Chapter 3, Box 3.4). The chronology of emissions pathways and their implied warming is also important for the more slowly evolving parts of the Earth system, such as those associated with sea level rise. In addition, for several types of risks the rate of change may be most relevant (Loarie et al., 2009; LoPresti et al., 2015) <sup>[[#fn:r1381|1381]]</sup> , with potentially large risks occurring in the case of a rapid rise to overshooting temperatures, even if a decrease to 1.5°C may be achieved at the end of the 21st century or later. On the other hand, if overshoot is to be minimized, the remaining equivalent CO <sub>2</sub> budget available for emissions has to be very small, which implies that large, immediate and unprecedented global efforts to mitigate GHGs are required (Cross-Chapter Box 8, Table 1; Chapter 4).
* ''' ''' '''What is the probability of reaching 1.5°C of global warming if emissions compatible with 1.5°C pathways are followed?''' Emissions pathways in a ‘prospective scenario’ (see Chapter 1, Section 1.2.3, and Cross-Chapter Box 1 in Chapter 1 on ‘Scenarios and pathways’) compatible with 1.5°C of global warming are determined based on their probability of reaching 1.5°C by 2100 (Chapter 2, Section 2.1), given current knowledge of the climate system response. These probabilities cannot be quantified precisely but are typically 50–66% in 1.5°C-consistent pathways (Section 1.2.3). This implies a one-in-two to one-in-three probability that global warming would exceed 1.5°C even under a 1.5°C-consistent pathway, including some possibility that global warming would be substantially over this value (generally about 5–10% probability; see Cross-Chapter Box 8, Table 1 and Seneviratne et al., 2018b) <sup>[[#fn:r1382|1382]]</sup> . These alternative outcomes need to be factored into the decision-making process. To address this issue, ‘adaptive’ mitigation scenarios have been proposed in which emissions are continually adjusted to achieve a temperature goal (Millar et al., 2017) <sup>[[#fn:r1383|1383]]</sup> . The set of dimensions involved in mitigation options (Chapter 4) is complex and need system-wide approaches to be successful. Adaptive scenarios could be facilitated by the global stocktake mechanism established in the Paris Agreement, and thereby transfer the risk of higher-than-expected warming to a risk of faster-than-expected mitigation efforts. However, there are some limits to the feasibility of such approaches because some investments, for example in infrastructure, are long term and also because the actual departure from an aimed pathway will need to be detected against the backdrop of internal climate variability, typically over several decades (Haustein et al., 2017; Seneviratne et al., 2018b) <sup>[[#fn:r1384|1384]]</sup> . Avoiding impacts that depend on atmospheric composition as well as GMST (Baker et al., 2018) <sup>[[#fn:r1385|1385]]</sup> would also require limits on atmospheric CO <sub>2</sub> concentrations in the event of a lower-than-expected GMST response.
* '''How can the transformation towards a 1.5°C warmer world be implemented?''' This can be achieved in a variety of ways, such as decarbonizing the economy with an emphasis on demand reductions and sustainable lifestyles, or, alternatively, with an emphasis on large-scale technological solutions, amongst many other options (Chapter 2, Sections 2.3 and 2.4; Chapter 4, Sections 4.1 and 4.4.4). Different portfolios of mitigation measures come with distinct synergies and trade-offs with respect to other societal objectives. Integrated solutions and approaches are required to achieve multiple societal objectives simultaneously (see Chapter 4, Section 4.5.4 for a set of synergies and trade-offs).
* '''What determines risks and opportunities in a 1.5°C warmer world?''' The risks to natural, managed and human systems in a 1.5°C warmer world will depend not only on uncertainties in the regional climate that results from this level of warming, but also very strongly on the methods that humanity uses to limit global warming to 1.5°C. This is particularly the case for natural ecosystems and agriculture (see Cross-Chapter Box 7 in this chapter and Chapter 4, Section 4.3.2). The risks to human systems will also depend on the magnitude and effectiveness of policies and measures implemented to increase resilience to the risks of climate change and on development choices over coming decades, which will influence the underlying vulnerabilities and capacities of communities and institutions for responding and adapting.
* '''Which aspects are not considered, or only partly considered, in the mitigation scenarios from Chapter 2?''' These include biophysical impacts of land use, water constraints on energy infrastructure, and regional implications of choices of specific scenarios for tropospheric aerosol concentrations or the modulation of concentrations of short-lived climate forcers, that is, greenhouse gases (Chapter 3, Section 3.6.3). Such aspects of development pathways need to be factored into comprehensive assessments of the regional implications of mitigation and adaptation measures. On the other hand, some of these aspects are assessed in Chapter 4 as possible options for mitigation and adaptation to a 1.5°C warmer world.
* '''Are there commonalities to all alternative 1.5°C warmer worlds?''' Human-driven warming linked to CO <sub>2</sub> emissions is nearly irreversible over time frames of 1000 years or more (Matthews and Caldeira, 2008; Solomon et al., 2009) <sup>[[#fn:r1386|1386]]</sup> . The GSMT of the Earth responds to the cumulative amount of CO <sub>2</sub>  emissions. Hence, '''all 1.5°C stabilization scenarios''' '''require both net CO <sub>2</sub> emissions and multi-gas CO <sub>2</sub> -forcing-equivalent emissions to be zero''' at some point (Chapter 2, Section 2.2). This is also the case for stabilization scenarios at higher levels of warming (e.g., at 2°C); the only difference is the projected time at which the net CO <sub>2</sub> budget is zero.

'''Hence,''' '''a transition to decarbonization of energy use is necessary in all scenarios''' . It should be noted that '''all scenarios of Chapter 2 include approaches for carbon dioxide removal (CDR)''' in order to achieve the net zero CO <sub>2</sub> emissions budget. '''Most of these use''' '''carbon capture and storage (CCS)''' in addition to reforestation, although to varying degrees (Chapter 4, Section 4.3.7). Some potential pathways to 1.5°C of warming in 2100 would minimize the need for CDR (Obersteiner et al., 2018; van Vuuren et al., 2018) <sup>[[#fn:r1387|1387]]</sup> . Taking into account the implementation of CDR, the CO <sub>2</sub> -induced warming by 2100 is determined by the difference between the total amount of CO <sub>2</sub> generated (that can be reduced by early decarbonization) and the total amount permanently stored out of the atmosphere, for example by geological sequestration (Chapter 4, Section 4.3.7). ''' '''

* '''What are possible storylines of ‘warmer worlds’ at 1.5°C versus higher levels of global warming?''' Cross-Chapter Box 8, Table 2 features possible storylines based on the scenarios of Chapter 2, the impacts of Chapters 3 and 5, and the options of Chapter 4. These storylines are not intended to be comprehensive of all possible future outcomes. Rather, they are intended as plausible scenarios of alternative warmer worlds, with two storylines that include stabilization at 1.5°C (Scenario 1) or close to 1.5°C (Scenario 2), and one storyline missing this goal and consequently only including reductions of CO <sub>2</sub> emissions and efforts towards stabilization at higher temperatures (Scenario 3).

''' ''' '''Summary:'''

'''There is no single ‘1.5°C warmer world’. Impacts can vary strongly for different worlds characterized by a 1.5°C global warming. Important aspects to consider (besides the changes in global temperature) are the possible occurrence of an overshoot and its associated peak warming and duration, how stabilization of the increase in global surface temperature at 1.5°C could be achieved, how policies might be able to influence the resilience of human and natural systems, and the nature of regional and subregional risks.'''

''' ''' The implications of overshooting are large for risks to natural and human systems, especially if the temperature at peak warming is high, because some risks may be long lasting and irreversible, such as the loss of some ecosystems. In addition, for several types of risks, the rate of change may be most relevant, with potentially large risks occurring in the case of a rapid rise to overshooting temperatures, even if a decrease to 1.5°C may be achieved at the end of the 21st century or later. If overshoot is to be minimized, the remaining equivalent CO <sub>2</sub> budget available for emissions has to be very small, which implies that large, immediate and unprecedented global efforts to mitigate GHGs are required.

The time frame for initiating major mitigation measures is essential in order to reach a 1.5°C (or even a 2°C) global stabilization of climate warming (see consistent cumulative CO <sub>2</sub> emissions up to peak warming in Cross-Chapter Box 8, Table 1). If mitigation pathways are not rapidly activated, much more expensive and complex adaptation measures will have to be taken to avoid the impacts of higher levels of global warming on the Earth system.

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'''Cross-Chapter Box 8, Table 1'''

Different worlds resulting from 1.5°C and 2°C mitigation (prospective) pathways, including 66% (probable) best-case outcome, and 5% worst-case outcome, based on Chapter 2 scenarios and Chapter 3 assessments of changes in regional climate. Note that the pathway characteristics estimates are based on computations with the MAGICC model (Meinshausen et al., 2011) consistent with the set-up used in AR5 WGIII (Clarke et al., 2014), but are uncertain and will be subject to updates and adjust-ments (see Chapter 2 for details). Updated from (Seneviratne et al. (2018b).

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{| class="wikitable"
|-
!
! B1.5_LOS (below 1.5°C with low overshoot) with 2/3 ´probable best-case outcome´ <sup>a</sup>
! B1.5_LOS (below 1.5°C<br />
with low overshoot)<br />
with 1/20 ´worst-case<br />
outcome´ <sup>b</sup>
! L20 (lower than 2°C) with 2/3 ´probable best-case outcome´ <sup>a</sup>
! L20 (lower than 2°C)<br />
with 1/20 ´worst-case<br />
outcome´ <sup>b</sup>
|-
| rowspan="6"| General characteristics of pathway
| Overshoot &gt; 1.5°C in 21st century <sup>c</sup>
| Yes (51/51)
| Yes (72/72)
|-
| Overshoot &gt; 2°C in 21st century
| No (0/51)
| Yes (37/51)
| No (72/72)
| Yes (72/72)
|-
| Cumulative CO <sub>2</sub> emissions up to peak<br />
warming (relative to 2016) <sup>d</sup> [GtCO <sub>2</sub> ]
| 610–760
| 590–750
| 1150–1460
| 1130–1470
|-
| Cumulative CO <sub>2</sub> emissions up to 2100 (relative to 2016) <sup>d</sup> [GtCO <sub>2</sub> ]
| colspan="2"| 170–560
| colspan="2"| 1030–1440
|-
| Global GHG emissions in 2030 <sup>d</sup> [GtCO <sub>2</sub> y <sup>-1</sup> ]
| colspan="2"| 19–23
| colspan="2"| 31–38
|-
| Years of global net zero CO <sub>2</sub> emissions <sup>d</sup>
| colspan="2"| 2055–2066
| colspan="2"| 2082–2090
|-
| rowspan="6"| Possible climate range at peak warming (regional+global)
| Global mean temperature anomaly at peak warming
| 1.7°C (1.66°C–1.72°C)
| 2.05°C (2.00°C–2.09°C)
| 2.11°C (2.05°C–2.17°C)
| 2.67°C (2.59°C–2.76°C)
|-
| Warming in the Arctic <sup>e</sup> (TNn <sup>f</sup> )
| 4.93°C (4.36, 5.52)
| 6.02°C (5.12, 6.89)
| 6.24°C (5.39, 7.21)
| 7.69°C (6.69, 8.93)
|-
| Warming in Central North America <sup>e</sup> (TXx <sup>g</sup> )
| 2.65°C (1.92, 3.15)
| 3.11°C (2.37, 3.63)
| 3.18°C (2.50, 3.71)
| 4.06°C (3.35, 4.63)
|-
| Warming in Amazon region <sup>e</sup> (TXx)
| 2.55°C (2.23, 2.83)
| 3.07°C (2.74, 3.46)
| 3.16°C (2.84, 3.57)
| 4.05°C (3.62, 4.46)
|-
| Drying in the Mediterranean region <sup>e,h</sup>
| –1.11 (–2.24, –0.41)
| –1.28 (–2.44, –0.51)
| –1.38 (–2.58, –0.53)
| –1.56 (–3.19, –0.67)
|-
| Increase in heavy precipitation events <sup>e</sup> in Southern Asia <sup>i</sup>
| 9.94% (6.76, 14.00)
| 11.94% (7.52, 18.86)
| 12.68% (7.71, 22.39)
| 19.67% (11.56, 27.24)
|-
| rowspan="6"| Possible climate range in 2100 (regional+global)
| Global mean temperature warming in 2100
| 1.46°C (1.41°C–1.51°C)
| 1.87°C (1.81°C–1.94°C)
| 2.06°C (1.99°C–2.15°C)
| 2.66°C (2.56°C–2.76°C)
|-
| Warming in the Arctic <sup>j</sup> (TNn)
| 4.28°C (3.71, 4.77)
| 5.50°C (4.74, 6.21)
| 6.08°C (5.20, 6.94)
| 7.63°C (6.66, 8.90)
|-
| Warming in Central North America <sup>j</sup> (TXx)
| 2.31°C (1.56, 2.66)
| 2.83°C (2.03, 3.49)
| 3.12°C (2.38, 3.67)
| 4.06°C (3.33, 4.59)
|-
| Warming in Amazon region <sup>j</sup> (TXx)
| 2.22°C (2.00, 2.45)
| 2.76°C (2.50, 3.07)
| 3.10°C (2.75, 3.49)
| 4.03°C (3.62, 4.45)
|-
| Drying in the Mediterranean region <sup>j</sup>
| –0.95 (–1.98, –0.30)
| –1.10 (–2.17, –0.51)
| –1.26 (–2.43, –0.52)
| –1.55 (–3.17, –0.67)
|-
| Increase in heavy precipitation events<br />
in Southern Asia <sup>j</sup>
| 8.38% (4.63, 12.68)
| 10.34% (6.64, 16.07)
| 12.02% (7.41, 19.62)
| 19.72% (11.34, 26.95)
|}
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Notes:

# 66th percentile for global temperature (that is, 66% likelihood of being at or below values)
# 95th percentile for global temperature (that is, 5% likelihood of being at or above values)
# All 1.5°C scenarios include a substantial probability of overshooting above 1.5°C global warming before returning to 1.5°C.
# Interquartile range (25th percentile, q25, and 75th percentile, q75)
# The regional projections in these rows provide the median and the range [q25, q75] associated with the median global temperature outcomes of the considered mitigation scenarios at peak warming.
# TNn: Annual minimum night-time temperature
# TXx: Annual maximum day-time temperature
# Indicates drying of soil moisture expressed in units of standard deviations of pre-industrial climate (1861–1880) variability (where −1 is dry; −2 is severely dry; and −3 is very severely dry);
# Rx5day: the annual maximum consecutive 5-day precipitation.
# As for footnote e, but for the regional responses associated with the median global temperature outcomes of the considered mitigation scenarios in 2100

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'''Cross-Chapter Box 8, Table 2'''

<span id="storylines-of-possible-worlds-resulting-from-different-mitigation-options.-the-storylines-build-upon-cross-chapter-box-8-table-1-and-the-assessments-of-chapters-15.-only-a-few-of-the-many-possible-storylines-were-chosen-and-they-are-presented-for-illustrative-purposes."></span>
'''Storylines of possible worlds resulting from different mitigation options. The storylines build upon Cross-Chapter Box 8, Table 1 and the assessments of Chapters 1–5. Only a few of the many possible storylines were chosen and they are presented for illustrative purposes.'''

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{| class="wikitable"
|-
| Scenario 1<br />
[one possible storyline among best-case scenarios]: Mitigation:<br />
early move to decarbonization, decarbonization designed to minimize land footprint, coordination and<br />
rapid action of the world’s nations towards 1.5°C goal by 2100 Internal climate variability:<br />
probable (66%) best-case outcome for global and regional climate responses
| In 2020, strong participation and support for the Paris Agreement and its ambitious goals for reducing CO <sub>2</sub> emissions by an almost unanimous international community led to a time frame for net zero emissions that is compatible with halting global warming at 1.5°C by 2100.
There is strong participation in all major world regions at the national, state and/or city levels. Transport is strongly decarbonized through a shift to electric vehicles, with more cars with electric than combustion engines being sold by 2025 (Chapter 2, Section 2.4.3; Chapter 4, Section 4.3.3). Several industry-sized plants for carbon capture and storage are installed and tested in the 2020s (Chapter 2, Section 2.4.2; Chapter 4, Sections 4.3.4 and 4.3.7). Competition for land between bioenergy cropping, food production, and biodiversity conservation is minimized by sourcing bioenergy for carbon capture and storage from agricultural wastes, algae and kelp farms (Cross-Chapter Box 7 in Chapter 3; Chapter 4, Section 4.3.2). Agriculture is intensified in countries with coordinated planning associated with a drastic decrease in food waste (Chapter 2, Section 2.4.4; Chapter 4, Section 4.3.2). This leaves many natural ecosystems relatively intact, supporting continued provision of most ecosystem services, although relocation of species towards higher latitudes and elevations still results in changes in local biodiversity in many regions, particularly in mountain, tropical, coastal and Arctic ecosystems (Chapter 3, Section 3.4.3). Adaptive measures such as the establishment of corridors for the movement of species and parts of ecosystems become a central practice within conservation management (Chapter 3, Section 3.4.3; Chapter 4, Section 4.3.2). The movement of species presents new challenges for resource management as novel ecosystems, as well as pests and disease, increase (Cross-Chapter Box 6 in Chapter 3). Crops are grown on marginal land, no-till agriculture is deployed, and large areas are reforested with native trees (Chapter 2, Section 2.4.4; Chapter 3, Section 3.6.2; Cross-Chapter Box 7 in Chapter 3; Chapter 4, Section 4.3.2). Societal preference for healthy diets reduces meat consumption and associated GHG emissions (Chapter 2, Section 2.4.4; Chapter 4, Section 4.3.2; Cross-Chapter Box 6 in Chapter 3).

By 2100, global mean temperature is on average 0.5°C warmer than it was in 2018 (Chapter 1, Section 1.2.1). Only a minor temperature overshoot occurs during the century (Chapter 2, Section 2.2). In mid-latitudes, frequent hot summers and precipitation events tend to be more intense (Chapter 3, Section 3.3). Coastal communities struggle with increased inundation associated with rising sea levels and more frequent and intense heavy rainfall (Chapter 3, Sections 3.3.2 and 3.3.9; Chapter 4, Section 4.3.2; Chapter 5, Box 5.3 and Section 5.3.2; Cross-Chapter Box 12 in Chapter 5), and some respond by moving, in many cases with consequences for urban areas. In the tropics, in particular in megacities, there are frequent deadly heatwaves whose risks are reduced by proactive adaptation (Chapter 3, Sections 3.3.1 and 3.4.8; Chapter 4, Section 4.3.8), overlaid on a suite of development challenges and limits in disaster risk management (Chapter 4, Section 4.3.3; Chapter 5, Sections 5.2.1 and 5.2.2; Cross-Chapter Box 12 in Chapter 5). Glaciers extent decreases in most mountainous areas (Chapter 3, Sections 3.3.5 and 3.5.4). Reduced Arctic sea ice opens up new shipping lanes and commercial corridors (Chapter 3, Section 3.3.8; Chapter 4, Box 4.3). Small island developing states (SIDS), as well as coastal and low-lying areas, have faced significant changes but have largely persisted in most regions (Chapter 3, Sections 3.3.9 and 3.5.4, Box 3.5). The Mediterranean area becomes drier (Chapter 3, Section 3.3.4 and Box 3.2) and irrigation of crops expands, drawing the water table down in many areas (Chapter 3, Section 3.4.6). The Amazon is reasonably well preserved, through avoided risk of droughts (Chapter 3, Sections 3.3.4 and 3.4.3; Chapter 4, Box 4.3) and reduced deforestation (Chapter 2, Section 2.4.4; Cross-Chapter Box 7 in Chapter 3; Chapter 4, Section 4.3.2), and the forest services are working with the pattern observed at the beginning of the 21st century (Chapter 4, Box 4.3). While some climate hazards become more frequent (Chapter 3, Section 3.3), timely adaptation measures help reduce the associated risks for most, although poor and disadvantaged groups continue to experience high climate risks to their livelihoods and well-being (Chapter 5, Section 5.3.1; Cross-Chapter Box 12 in Chapter 5; Chapter 3, Boxes 3.4 and 3.5; Cross-Chapter Box 6 in Chapter 3). Summer sea ice has not completely disappeared from the Arctic (Chapter 3, Section 3.4.4.7) and coral reefs, having been driven to a low level (10–30% of levels in 2018), have partially recovered by 2100 after extensive dieback (Chapter 3, Section 3.4.4.10 and Box 3.4). The Earth system, while warmer, is still recognizable compared to the 2000s, and no major tipping points are reached (Chapter 3, Section 3.5.2.5). Crop yields remain relatively stable (Chapter 3, Section 3.4). Aggregate economic damage of climate change impacts is relatively small, although there are some local losses associated with extreme weather events (Chapter 3, Section 3.5; Chapter 4). Human well-being remains overall similar to that in 2020 (Chapter 5, Section 5.2.2).

|-
| Scenario 2 [one possible storyline among mid-case scenarios]:
Mitigation:<br />
delayed action (ambitious targets reached only after warmer decade in the 2020s due to internal climate variability), overshoot at 2°C, decrease towards 1.5°C afterward, no efforts to minimize the land and water footprints of bioenergy

Internal climate variability:<br />
10% worst-case outcome (2020s) followed by normal internal climate variability

| The international community continues to largely support the Paris Agreement and agrees in 2020 on reduction targets for CO <sub>2</sub> emissions and time frames for net zero emissions. However, these targets are not ambitious enough to reach stabilization at 2°C of warming, let alone 1.5°C.
In the 2020s, internal climate variability leads to higher warming than projected, in a reverse development to what happened in the so-called ‘hiatus’ period of the 2000s. Temperatures are regularly above 1.5°C of warming, although radiative forcing is consistent with a warming of 1.2°C or 1.3°C. Deadly heatwaves in major cities (Chicago, Kolkata, Beijing, Karachi, São Paulo), droughts in southern Europe, southern Africa and the Amazon region, and major flooding in Asia, all intensified by the global and regional warming (Chapter 3, Sections 3.3.1, 3.3.2, 3.3.3, 3.3.4 and 3.4.8; Cross-Chapter Box 11 in Chapter 4), lead to increasing levels of public unrest and political destabilization (Chapter 5, Section 5.2.1). An emergency global summit in 2025 moves to much more ambitious climate targets. Costs for rapidly phasing out fossil fuel use and infrastructure, while rapidly expanding renewables to reduce emissions, are much higher than in Scenario 1, owing to a failure to support economic measures to drive the transition (Chapter 4). Disruptive technologies become crucial to face up to the adaptation measures needed (Chapter 4, Section 4.4.4).

Temperature peaks at 2°C of warming by the middle of the century before decreasing again owing to intensive implementation of bioenergy plants with carbon capture and storage (Chapter 2), without efforts to minimize the land and water footprint of bioenergy production (Cross-Chapter Box 7 in Chapter 3). Reaching 2°C of warming for several decades eliminates or severely damages key ecosystems such as coral reefs and tropical forests (Chapter 3, Section 3.4). The elimination of coral reef ecosystems and the deterioration of their calcified frameworks, as well as serious losses of coastal ecosystems such as mangrove forests and seagrass beds (Chapter 3, Boxes 3.4 and 3.5, Sections 3.4.4.10 and 3.4.5), leads to much reduced levels of coastal defence from storms, winds and waves. These changes increase the vulnerability and risks facing communities in tropical and subtropical regions, with consequences for many coastal communities (Cross-Chapter Box 12 in Chapter 5). These impacts are being amplified by steadily rising sea levels (Chapter 3, Section 3.3.9) and intensifying storms (Chapter 3, Section 3.4.4.3). The intensive area required for the production of bioenergy, combined with increasing water stress, puts pressure on food prices (Cross-Chapter Box 6 in Chapter 3), driving elevated rates of food insecurity, hunger and poverty (Chapter 4, Section 4.3.2; Cross-Chapter Box 6 in Chapter 3; Cross-Chapter Box 11 in Chapter 4). Crop yields decline significantly in the tropics, leading to prolonged famines in some African countries (Chapter 3, Section 3.4; Chapter 4, Section 4.3.2). Food trumps environment in terms of importance in most countries, with the result that natural ecosystems decrease in abundance, owing to climate change and land-use change (Cross-Chapter Box 7 in Chapter 3). The ability to implement adaptive action to prevent the loss of ecosystems is hindered under the circumstances and is consequently minimal (Chapter 3, Sections 3.3.6 and 3.4.4.10). Many natural ecosystems, in particular in the Mediterranean, are lost because of the combined effects of climate change and land-use change, and extinction rates increase greatly (Chapter 3, Section 3.4 and Box 3.2).

By 2100, warming has decreased but is still stronger than 1.5°C, and the yields of some tropical crops are recovering (Chapter 3, Section 3.4.3). Several of the remaining natural ecosystems experience irreversible climate change-related damages whilst others have been lost to land-use change, with very rapid increases in the rate of species extinctions (Chapter 3, Section 3.4; Cross-Chapter Box 7 in Chapter 3; Cross-Chapter Box 11 in Chapter 4). Migration, forced displacement, and loss of identity are extensive in some countries, reversing some achievements in sustainable development and human security (Chapter 5, Section 5.3.2). Aggregate economic impacts of climate change damage are small, but the loss in ecosystem services creates large economic losses (Chapter 4, Sections 4.3.2 and 4.3.3). The health and well-being of people generally decrease from 2020, while the levels of poverty and disadvantage increase considerably (Chapter 5, Section 5.2.1).

|-
| Scenario 3 [one possible storyline among worst-case scenarios]:
Mitigation:<br />
uncoordinated action, major actions late in the 21st century, 3°C of warming in 2100

Internal climate variability:<br />
unusual (ca. 10%) best-case scenario for one decade, followed by normal internal climate variability

| In 2020, despite past pledges, the international support for the Paris Agreement starts to wane. In the years that follow, CO <sub>2</sub> emissions are reduced at the local and national level but efforts are limited and not always successful.
Radiative forcing increases and, due to chance, the most extreme events tend to happen in less populated regions and thus do not increase global concerns. Nonetheless, there are more frequent heatwaves in several cities and less snow in mountain resorts in the Alps, Rockies and Andes (Chapter 3, Section 3.3). Global warming of 1.5°C is reached by 2030 but no major changes in policies occur. Starting with an intense El Niño–La Niña phase in the 2030s, several catastrophic years occur while global warming starts to approach 2°C. There are major heatwaves on all continents, with deadly consequences in tropical regions and Asian megacities, especially for those ill-equipped for protecting themselves and their communities from the effects of extreme temperatures (Chapter 3, Sections 3.3.1, 3.3.2 and 3.4.8). Droughts occur in regions bordering the Mediterranean Sea, central North America, the Amazon region and southern Australia, some of which are due to natural variability and others to enhanced greenhouse gas forcing (Chapter 3, Section 3.3.4; Chapter 4, Section 4.3.2; Cross-Chapter Box 11 in Chapter 4). Intense flooding occurs in high-latitude and tropical regions, in particular in Asia, following increases in heavy precipitation events (Chapter 3, Section 3.3.3). Major ecosystems (coral reefs, wetlands, forests) are destroyed over that period (Chapter 3, Section 3.4), with massive disruption to local livelihoods (Chapter 5, Section 5.2.2 and Box 5.3; Cross-Chapter Box 12 in Chapter 5). An unprecedented drought leads to large impacts on the Amazon rainforest (Chapter 3, Sections 3.3.4 and 3.4), which is also affected by deforestation (Chapter 2). A hurricane with intense rainfall and associated with high storm surges (Chapter 3, Section 3.3.6) destroys a large part of Miami. A two-year drought in the Great Plains in the USA and a concomitant drought in eastern Europe and Russia decrease global crop production (Chapter 3, Section 3.3.4), resulting in major increases in food prices and eroding food security. Poverty levels increase to a very large scale, and the risk and incidence of starvation increase considerably as food stores dwindle in most countries; human health suffers (Chapter 3, Section 3.4.6.1; Chapter 4, Sections 4.3.2 and 4.4.3; Chapter 5, Section 5.2.1).

There are high levels of public unrest and political destabilization due to the increasing climatic pressures, resulting in some countries becoming dysfunctional (Chapter 4, Sections 4.4.1 and 4.4.2). The main countries responsible for the CO <sub>2</sub> emissions design rapidly conceived mitigation plans and try to install plants for carbon capture and storage, in some cases without sufficient prior testing (Chapter 4, Section 4.3.6). Massive investments in renewable energy often happen too late and are uncoordinated; energy prices soar as a result of the high demand and lack of infrastructure. In some cases, demand cannot be met, leading to further delays. Some countries propose to consider sulphate-aerosol based Solar Radiation Modification (SRM) (Chapter 4, Section 4.3.8); however, intensive international negotiations on the topic take substantial time and are inconclusive because of overwhelming concerns about potential impacts on monsoon rainfall and risks in case of termination (Cross-Chapter Box 10 in Chapter 5). Global and regional temperatures continue to increase strongly while mitigation solutions are being developed and implemented.

Global mean warming reaches 3°C by 2100 but is not yet stabilized despite major decreases in yearly CO <sub>2</sub> emissions, as a net zero CO <sub>2</sub> emissions budget could not yet be achieved and because of the long lifetime of CO <sub>2</sub> concentrations (Chapters 1, 2 and 3). The world as it was in 2020 is no longer recognizable, with decreasing life expectancy, reduced outdoor labour productivity, and lower quality of life in many regions because of too frequent heatwaves and other climate extremes (Chapter 4, Section 4.3.3). Droughts and stress on water resources renders agriculture economically unviable in some regions (Chapter 3, Section 3.4; Chapter 4, Section 4.3.2) and contributes to increases in poverty (Chapter 5, Section 5.2.1; Cross-Chapter Box 12 in Chapter 5). Progress on the sustainable development goals is largely undone and poverty rates reach new highs (Chapter 5, Section 5.2.3). Major conflicts take place (Chapter 3, Section 3.4.9.6; Chapter 5, Section 5.2.1). Almost all ecosystems experience irreversible impacts, species extinction rates are high in all regions, forest fires escalate, and biodiversity strongly decreases, resulting in extensive losses to ecosystem services. These losses exacerbate poverty and reduce quality of life (Chapter 3, Section 3.4; Chapter 4, Section 4.3.2). Life for many indigenous and rural groups becomes untenable in their ancestral lands (Chapter 4, Box 4.3; Cross-Chapter Box 12 in Chapter 5). The retreat of the West Antarctic ice sheet accelerates (Chapter 3, Sections 3.3 and 3.6), leading to more rapid sea level rise (Chapter 3, Section 3.3.9; Chapter 4, Section 4.3.2). Several small island states give up hope of survival in their locations and look to an increasingly fragmented global community for refuge (Chapter 3, Box 3.5; Cross-Chapter Box 12 in Chapter 5). Aggregate economic damages are substantial, owing to the combined effects of climate changes, political instability, and losses of ecosystem services (Chapter 4, Sections 4.4.1 and 4.4.2; Chapter 3, Box 3.6 and Section 3.5.2.4). The general health and well-being of people is substantially reduced compared to the conditions in 2020 and continues to worsen over the following decades (Chapter 5, Section 5.2.3).

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