Editing IPCC:AR6/WGI/Chapter-1 (section)

==== 1.4.3.1 Sources of Uncertainty ====

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===== 1.4.3.1.1 Radiative forcing uncertainty =====

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Future radiative forcing is uncertain due to as-yet-unknown societal choices that will determine future anthropogenic emissions; this is considered ‘scenario uncertainty’. The RCP and SSP scenarios, which form the basis for climate projections assessed in this Report, are designed to span a plausible range of future pathways ( [[#1.6|Section 1.6]] ) and can be used to estimate the magnitude of scenario uncertainty, but the real world may also differ from any one of these example pathways.

Uncertainties also exist regarding past emissions and radiative forcings. These are especially important for simulations of paleoclimate time periods, such as the Pliocene, Last Glacial Maximum or the last millennium, but are also relevant for the CMIP historical simulations of the instrumental period since 1850. In particular, historical radiative forcings due to anthropogenic and natural aerosols are less well constrained by observations than the GHG radiative forcings. There is also uncertainty in the size of large volcanic eruptions (and in the location for some that occurred before around 1850), and the amplitude of changes in solar activity, before satellite observations. The role of historical radiative forcing uncertainty was considered previously ( [[#Knutti--2002|Knutti et al., 2002]] ; [[#Forster--2013|Forster et al., 2013]] ) but, since AR5, specific simulations have been performed to examine this issue, particularly for the effects of uncertainty in anthropogenic aerosol radiative forcing (e.g., [[#Jiménez-de-la-Cuesta--2019|Jiménez-de-la-Cuesta and Mauritsen, 2019]] ; [[#Dittus--2020|Dittus et al., 2020]] ).

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===== 1.4.3.1.2 Climate response uncertainty =====

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Under any particular scenario ( [[#1.6.1|Section 1.6.1]] ), there is uncertainty in how the climate will respond to the specified emissions or radiative forcing combinations. A range of climate models is often used to estimate the range of uncertainty in our understanding of the key physical processes and to define the ‘model response uncertainty’ (Sections [[#1.5.4|1.5.4]] and [[IPCC:Wg1:Chapter:Chapter-4#4.2.5|4.2.5]] ). However, this range does not necessarily represent the full ‘climate response uncertainty ''’'' in how the climate may respond to a particular radiative forcing or emissions scenario. This is because, for example, the climate models used in CMIP experiments have structural uncertainties not explored in a typical multi-model exercise (e.g., [[#Murphy--2004|Murphy et al., 2004]] ) and are not entirely independent of each other ( [[#1.5.4.8|Section 1.5.4.8]] ; [[#Masson--2011|Masson and Knutti, 2011]] ; [[#Abramowitz--2019|Abramowitz et al., 2019]] ); there are small spatial-scale features which cannot be resolved; and long time-scale processes or tipping points are not fully represented. [[#1.4.4|Section 1.4.4]] discusses how some of these issues can still be considered in a risk assessment context. For some metrics, such as equilibrium climate sensitivity (ECS), the CMIP6 model range is found to be broader than the ''very likely'' range assessed by combining multiple lines of evidence (Sections 4.3.4 and 7.5.6).

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===== 1.4.3.1.3 Natural and internal climate variations =====

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Even without any anthropogenic radiative forcing, there would still be uncertainty in projecting future climate because of unpredictable natural factors such as variations in solar activity and volcanic eruptions. For projections of future climate, such as those presented in Chapter 4, the uncertainty in these factors is not normally considered. However, the potential effects on the climate of large volcanic eruptions (Cross-Chapter Box 4.1; [[#Zanchettin--2016|Zanchettin et al., 2016]] ; [[#Bethke--2017|Bethke et al., 2017]] ) and large solar variations ( [[#Feulner--2010|Feulner and Rahmstorf, 2010]] ; [[#Maycock--2015|Maycock et al., 2015]] ) are studied. On longer time scales, orbital effects and plate tectonics also play a role.

Further, even in the absence of any anthropogenic or natural changes in radiative forcing, Earth’s climate fluctuates on time scales from days to decades or longer. These ‘internal’ variations, such as those associated with modes of variability (e.g., ENSO, Pacific Decadal Variability (PDV), or Atlantic Multi-decadal Variability (AMV); Annex IV) are unpredictable on time scales longer than a few years ahead and are a source of uncertainty for understanding how the climate might become in a particular decade, especially regionally. The increased use of ‘large ensembles’ of complex climate model simulations to sample this component of uncertainty is discussed above in [[#1.4.2.1|Section 1.4.2.1]] and further in Chapter 4.

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===== 1.4.3.1.4 Interactions between variability and radiative forcings =====

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It is plausible that there are interactions between radiative forcings and climate variations, such as influences on the phasing or amplitude of internal or natural climate variability ( [[#Zanchettin--2017|Zanchettin, 2017]] ). For example, the timing of volcanic eruptions may influence Atlantic Multi-decadal Variability (e.g., [[#Otterå--2010|Otterå et al., 2010]] ; [[#Birkel--2018|Birkel et al., 2018]] ) or ENSO (e.g., [[#Maher--2015|Maher et al., 2015]] ; [[#Khodri--2017|Khodri et al., 2017]] ; [[#Zuo--2018|Zuo et al., 2018]] ), and anthropogenic aerosols may influence decadal modes of variability in the Pacific (e.g., [[#Smith--2016|Smith et al., 2016]] ). In addition, melting of glaciers and ice caps due to anthropogenic influences has been speculated to increase volcanic activity (e.g., a specific example for Iceland is discussed in [[#Swindles--2018|Swindles et al., 2018]] ).

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