Editing IPCC:Wg1:Chapter:Chapter-1-comments (section)

==== 1.4.4.1 Low-Likelihood Outcomes ====

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In the AR6, certain low-likelihood outcomes are described and assessed because they may be associated with high levels of risk, and the greatest risks may not be associated with the most likely outcome. The aim of assessing these possible futures is to better inform risk assessment and decision-making. Two types are considered: (i) low-likelihood high-warming (LLHW) scenarios, which describe the climate in a world with very high climate sensitivity; and (ii) low-likelihood, high-impact outcomes that have a low likelihood of occurring, but would cause large potential impacts on societies or ecosystems.

An illustrative example of how low-likelihood outcomes can produce significant additional risks is shown in Figure 1.16. The Reasons for Concern (RFCs) produced by the IPCC AR5 WGII define the additional risks due to climate change at different global warming levels. These have been combined with [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] assessments of projected global temperature for different emissions scenarios (SSPs; [[#1.6|Section 1.6]] ), and [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] assessments about ECS. For example, even following an intermediate emissions scenario could result in high levels of additional risk if ECS is at the upper end of the ''very likely'' range. However, not all possible low-likelihood outcomes relate to ECS, and AR6 considers these issues in more detail than previous IPCC assessment reports (see Table 1.1 and [[#1.4.4.2|Section 1.4.4.2]] for some examples).

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'''Figure 1.16 |''' '''Illustrating concepts of low-likelihood outcomes.''' '''Left:''' schematic likelihood distribution consistent with the IPCC AR6 assessments that equilibrium climate sensitivity (ECS) is ''likely'' in the range 2.5°C to 4.0°C, and ''very likely'' between 2.0°C and 5.0°C (Chapter 7). ECS values outside the assessed ''very likely'' range are designated low-likelihood outcomes in this example (light grey). '''Middle''' and '''right-hand columns:''' additional risks due to climate change for 2020–2090 using the Reasons For Concern (RFCs, see [[#IPCC--2014b|IPCC, 2014b]] ), specifically RFC1 describing the risks to unique and threatened systems and RFC3 describing risks from the distribution of impacts ( [[#O’Neill--2017b|O’Neill et al., 2017b]] ; [[#Zommers--2020|Zommers et al., 2020]] ). The projected changes of GSAT used are the 95%, median and 5% assessed ranges from [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] for each SSP (top, middle and bottom); these are designated High ECS, Mid-range ECS and Low ECS respectively. The ‘burning-ember’ risk spectrum of graduated colours is usually associated with levels of committed GSAT change; instead, this illustration associates the risk spectrum with the GSAT temperature reached in each year from 2020 to 2090. Note that this illustration does not include the vulnerability aspect of each SSP scenario. Further details on data sources and processing are available in the chapter data table (Table 1.SM.1).
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