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

==== 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. ====

[[File:d7f48faae0db0e7f745a792421b3e326 FINAL_CCB8_Fig2-1024x498.jpg|thumb|400x300px]]

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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<span id="cross-chapter-box-8-table-1"></span>
====== 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).

{| 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)
|}

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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<span id="cross-chapter-box-8-table-2"></span>
====== Cross-Chapter Box 8, Table 2 ======

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