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

===== ''1.4.3.1.3 Natural and internal cli'' ''mate 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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