Editing IPCC:AR6/SR15/Chapter-2 (section)

==== 2.2.1.2 Geophysical uncertainties: climate and Earth system feedbacks ====

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Climate sensitivity uncertainty impacts future projections as well as carbon-budget estimates (Schneider et al., 2017) <sup>[[#fn:r57|57]]</sup> . AR5 assessed the equilibrium climate sensitivity (ECS) to be ''likely'' in the 1.5°–4.5°C range, ''extremely unlikely'' less than 1°C and ''very unlikely'' greater than 6°C. The lower bound of this estimate is lower than the range of CMIP5 models (Collins et al., 2013) <sup>[[#fn:r58|58]]</sup> . The evidence for the 1.5°C lower bound on ECS in AR5 was based on analysis of energy-budget changes over the historical period. Work since AR5 has suggested that the climate sensitivity inferred from such changes has been lower than the 2 × CO <sub>2</sub> climate sensitivity for known reasons (Forster, 2016; Gregory and Andrews, 2016; Rugenstein et al., 2016; Armour, 2017; Ceppi and Gregory, 2017; Knutti et al., 2017; Proistosescu and Huybers, 2017) <sup>[[#fn:r59|59]]</sup> . Both a revised interpretation of historical estimates and other lines of evidence based on analysis of climate models with the best representation of today’s climate (Sherwood et al., 2014; Zhai et al., 2015; Tan et al., 2016; Brown and Caldeira, 2017; Knutti et al., 2017) <sup>[[#fn:r60|60]]</sup> suggest that the lower bound of ECS could be revised upwards, which would decrease the chances of limiting warming below 1.5°C in assessed pathways. However, such a reassessment has been challenged (Lewis and Curry, 2018) <sup>[[#fn:r61|61]]</sup> , albeit from a single line of evidence. Nevertheless, it is premature to make a major revision to the lower bound. The evidence for a possible revision of the upper bound on ECS is less clear, with cases argued from different lines of evidence for both decreasing (Lewis and Curry, 2015, 2018; Cox et al., 2018) <sup>[[#fn:r62|62]]</sup> and increasing (Brown and Caldeira, 2017) <sup>[[#fn:r63|63]]</sup> the bound presented in the literature. The tools used in this chapter employ ECS ranges consistent with the AR5 assessment. The MAGICC ECS distribution has not been selected to explicitly reflect this but is nevertheless consistent (Rogelj et al., 2014a) <sup>[[#fn:r64|64]]</sup> . The FAIR model used here to estimate carbon budgets explicitly constructs log-normal distributions of ECS and transient climate response based on a multi-parameter fit to the AR5 assessed ranges of climate sensitivity and individual historic effective radiative forcings (Smith et al., 2018) <sup>[[#fn:r65|65]]</sup> (Supplementary Material 2.SM.1.1.1).

Several feedbacks of the Earth system, involving the carbon cycle, non-CO <sub>2</sub> GHGs and/or aerosols, may also impact the future dynamics of the coupled carbon–climate system’s response to anthropogenic emissions. These feedbacks are caused by the effects of nutrient limitation (Duce et al., 2008; Mahowald et al., 2017) <sup>[[#fn:r66|66]]</sup> , ozone exposure (de Vries et al., 2017) <sup>[[#fn:r67|67]]</sup> , fire emissions (Narayan et al., 2007) <sup>[[#fn:r68|68]]</sup> and changes associated with natural aerosols (Cadule et al., 2009 <sup>[[#fn:r69|69]]</sup> ; Scott et al., 2018) <sup>[[#fn:r70|70]]</sup> . Among these Earth system feedbacks, the importance of the permafrost feedback’s influence has been highlighted in recent studies. Combined evidence from both models (MacDougall et al., 2015; Burke et al., 2017; Lowe and Bernie, 2018) <sup>[[#fn:r71|71]]</sup> and field studies (like Schädel et al., 2014; Schuur et al., 2015) <sup>[[#fn:r72|72]]</sup> shows ''high agreement'' that permafrost thawing will release both CO <sub>2</sub> and CH <sub>4</sub> as the Earth warms, amplifying global warming. This thawing could also release N <sub>2</sub> O (Voigt et al., 2017a, b) <sup>[[#fn:r73|73]]</sup> . Field, laboratory and modelling studies estimate that the vulnerable fraction in permafrost is about 5–15% of the permafrost soil carbon (~5300–5600 GtCO <sub>2</sub> in Schuur et al., 2015) <sup>[[#fn:r74|74]]</sup> and that carbon emissions are expected to occur beyond 2100 because of system inertia and the large proportion of slowly decomposing carbon in permafrost (Schädel et al., 2014) <sup>[[#fn:r75|75]]</sup> . Published model studies suggest that a large part of the carbon release to the atmosphere is in the form of CO <sub>2</sub> (Schädel et al., 2016) <sup>[[#fn:r76|76]]</sup> , while the amount of CH <sub>4</sub> released by permafrost thawing is estimated to be much smaller than that CO <sub>2</sub> . Cumulative CH <sub>4</sub> release by 2100 under RCP2.6 ranges from 0.13 to 0.45 Gt of methane (Burke et al., 2012; Schneider von Deimling et al., 2012, 2015) <sup>[[#fn:r77|77]]</sup> , with fluxes being the highest in the middle of the century because of maximum thermokarst lake extent by mid-century (Schneider von Deimling et al., 2015) <sup>[[#fn:r78|78]]</sup> .

The reduced complexity climate models employed in this assessment do not take into account permafrost or non-CO <sub>2</sub> Earth system feedbacks, although the MAGICC model has a permafrost module that can be enabled. Taking the current climate and Earth system feedbacks understanding together, there is a possibility that these models would underestimate the longer-term future temperature response to stringent emission pathways (Section 2.2.2).

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