Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.4.9.2 Abrupt Changes Detected in Earth System Model Projections ====

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Projecting abrupt changes is intrinsically difficult, because by definition abrupt changes occur in a small region of the parameter and/or forcing space. At the time of AR5 there was no available systematic study of abrupt changes or tipping points in ESMs. An analysis of ESMs since AR5 has identified a number of abrupt changes in the CMIP5 ensemble ( [[#Drijfhout--2015|Drijfhout et al., 2015]] ; [[#Bathiany--2020|Bathiany et al., 2020]] ). These include abrupt changes in tropical forests and high-latitude greening, permafrost thaw, and vegetation composition change ( [[#Bathiany--2020|Bathiany et al., 2020]] ). Most modelled abrupt changes were detected in boreal and tundra regions, with few models showing Amazon forest dieback ( [[#Bathiany--2020|Bathiany et al., 2020]] ).

Based on the evidence presented in this section, we conclude that abrupt changes and tipping points in the biogeochemical cycles lead to additional uncertainty in 21st century GHG concentrations changes. However, these are ''very likely'' to be small compared to the uncertainty associated with future anthropogenic emissions ( ''hi'' ''gh confidence'' ).

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