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

=== 5.4.8 Combined Biogeochemical Climate Feedback ===

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This section assesses the magnitude of the combined biogeochemical feedback in the climate system (Figure 5.29) by integrating evidence from: carbon-cycle projections represented in Earth system models ( [[#5.4.5.5|Section 5.4.5.5]] ), independent estimates of CO <sub>2</sub> emissions due to permafrost thaw (Box 5.1) and fire ( [[#5.4.3.2|Section 5.4.3.2]] ), natural CH <sub>4</sub> and N <sub>2</sub> O emissions ( [[#5.4.7|Section 5.4.7]] ), and aerosol and atmospheric chemistry (Section 6.3.6). We derive a physical climate feedback parameter α , as defined in [[IPCC:Wg1:Chapter:Chapter-7#7.4.1.1|Section 7.4.1.1]] , for CO <sub>2</sub> -based feedbacks using the linear framework proposed by [[#Gregory--2009|Gregory et al. (2009)]] , using the radiative forcing equations for CO <sub>2</sub> ( [[#Etminan--2016|Etminan et al., 2016]] ).

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[[File:5351138eee306fa6bb1a9910c191dbd9 IPCC_AR6_WGI_Figure_5_29.png]]

'''Figure 5.29 |''' '''Estimates of the biogeochemical climate feedback parameter''' ( α ''')''' . The parameter α (W m <sup>−2</sup> °C <sup>−1</sup> ) for a feedback variable ''x'' is defined as

<code> α </code> <sub>x</sub> =

δ <code> N </code> d <code> x </code> / δ <code> x </code> d <code> T </code>

where

δ <code> N </code> / δ <code> x </code>

is the change in top-of-atmosphere energy balance in response to a change in ''x'' induced by a change in surface temperature ( ''T'' ), as in [[IPCC:Wg1:Chapter:Chapter-7#7.4.1.1|Section 7.4.1.1]] . '''(a)''' Synthesis of biogeochemical feedbacks from panels (b) and (c). Orange (blue) bars correspond to positive (negative) feedbacks increasing (decreasing) radiative forcing at the top of the atmosphere. Bars denote the mean and the error bar represents the 5–95% range of the estimates; '''(b)''' carbon-cycle feedbacks as estimated by coupled carbon-cycle climate models in the CMIP5 ( [[#Arora--2013|Arora et al., 2013]] ) and CMIP6 ( [[#Arora--2020|Arora et al., 2020]] ) ensembles, where dots represent single model estimates, and filled (open) circles are those estimates which do (not) include the representation of a terrestrial nitrogen cycle; '''(c)''' Estimates of other biogeochemical feedback mechanisms based on various modelling studies. Dots represent single estimates, and coloured bars denote the mean of these estimates with no weighting being made regarding the likelihood of any single estimate, and error bars the 5–95% range derived from these estimates. Results in panel (c) have been compiled from (a) [[#5.4.3.2|Section 5.4.3.2]] ( [[#Eliseev--2014a|Eliseev et al., 2014a]] ; [[#Harrison--2018|Harrison et al., 2018]] ); (b) [[#5.4.3.3|Section 5.4.3.3]] ( [[#Schneider%20von%20Deimling--2012|Schneider von Deimling et al., 2012]] ; [[#Burke--2013|Burke et al., 2013]] , 2017b; [[#Koven--2015a|Koven et al., 2015a]] , c; [[#MacDougall--2016b|MacDougall and Knutti, 2016b]] ; [[#Gasser--2018|Gasser et al., 2018]] ; [[#Kleinen--2018|Kleinen and Brovkin, 2018]] ), where the estimates from [[#Burke--2013|Burke et al., 2013]] have been constrained as assessed in their study (c) [[#5.4.7|Section 5.4.7]] ( [[#Schneider%20von%20Deimling--2012|Schneider von Deimling et al., 2012]] , 2015; [[#Koven--2015c|Koven et al., 2015c]] ; [[#Turetsky--2020|Turetsky et al., 2020]] ); (d) [[#5.4.7|Section 5.4.7]] ( [[#Arneth--2010|Arneth et al., 2010]] ; [[#Denisov--2013|Denisov et al., 2013]] ; [[#Shindell--2013|Shindell et al., 2013]] ; B.D. [[#Stocker--2013|]] [[#Stocker--2013|Stocker et al., 2013]] ; [[#Zhang--2017|Zhang et al., 2017]] ); (f) [[#5.4.7|Section 5.4.7]] ( [[#Xu-Ri--2012|Xu-Ri et al., 2012]] ; B.D. [[#Stocker--2013|]] [[#Stocker--2013|Stocker et al., 2013]] ; [[#Zaehle--2013|Zaehle, 2013]] ; [[#Tian--2019|Tian et al., 2019]] ); (g) [[#5.4.7|Section 5.4.7]] ( [[#Martinez-Rey--2015|Martinez-Rey et al., 2015]] ; [[#Landolfi--2017|Landolfi et al., 2017]] ; [[#Battaglia--2018b|Battaglia and Joos, 2018b]] ). (h) Section 6.3, Table 6.9 mean and the 5–95% range the assessed feedback parameter. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

The climate feedback parameter for CO <sub>2</sub> (–1.13 ± 0.45 W m <sup>–2</sup> °C <sup>–1</sup> , mean and 1 standard-deviation range) is dominated by the contribution of the CO <sub>2</sub> -induced increase of ocean and land carbon storage (–1.46 ± 0.41 W m <sup>–2</sup> °C <sup>–1</sup> , corresponding to a β <sub>L+O</sub> of 1.66 ± 0.31 PgC ppm <sup>–1</sup> ), with smaller contributions from the carbon cycle’s response to climate (0.24 ± 0.17 W m <sup>–2</sup> °C <sup>–1</sup> , corresponding to γ <sub>L+O</sub> of –50 ± 34 PgC °C <sup>–1</sup> ), and emissions from permafrost thaw (0.09 [0.02 to 0.20] W m <sup>–2</sup> °C <sup>–1</sup> , corresponding to γ of –18 [3 to 41] PgC °C <sup>–1</sup> , mean and 5–95% range) (Figure 5.29a). This estimate does not include an estimate of the fire-related CO <sub>2</sub> feedback (range: 0.01–0.06 W m <sup>–2</sup> °C <sup>–1</sup> ), as only ''limited evidence'' was available to inform its assessment. The sum (mean and 5–95th percentile range) of feedbacks from natural emissions of CH <sub>4</sub> including permafrost thaw, and N <sub>2</sub> O (0.05 [0.02 to 0.09] W m <sup>–2</sup> °C <sup>–1</sup> ), and feedbacks from aerosol and atmospheric chemistry (–0.20 [–0.41 to 0.01] W m <sup>–2</sup> °C <sup>–1</sup> ) leads to an estimate of the non-CO <sub>2</sub> biogeochemical feedback parameter of –0.15 [–0.36 to +0.06] W m <sup>–2</sup> °C <sup>–1</sup> . There is ''low confidence'' in the estimate of the non-CO <sub>2</sub> biogeochemical feedbacks, due to the large range in the estimates of α for some individual feedbacks (Figure 5.29c), which can be attributed to the diversity in how models account for these feedbacks, limited process-level understanding, and the existence of known feedbacks where there is insufficient evidence to assess the feedback strength.

CO <sub>2</sub> and non-CO <sub>2</sub> biogeochemical feedbacks are an important component of the assessment of TCRE and the remaining carbon budget ( [[#5.5|Section 5.5]] ). The feedbacks of the carbon cycle of CO <sub>2</sub> and climate are implicitly taken account in the TCRE assessment, because they are represented in the various underlying lines of evidence. Other feedback contributions, such as the non-CO <sub>2</sub> biogeochemical feedback, can be converted into a carbon-equivalent feedback term ( γ ; [[#5.4.5.5|Section 5.4.5.5]] , 7.6) by reverse application of the linear feedback approximation ( [[#Gregory--2009|Gregory et al., 2009]] ). The contributions of non-CO <sub>2</sub> biogeochemical feedbacks combine to a linear feedback term of 30 ± 27 PgCeq °C <sup>–1</sup> (1 standard deviation range, 111 ± 98 Gt CO <sub>2-</sub> eq °C <sup>–1</sup> ), including a feedback term of –11 [–18 to –5] PgCeq °C <sup>–1</sup> (5–95% range, –40 [–62 to –18] Gt CO <sub>2-</sub> eq °C <sup>–1</sup> ) from natural CH <sub>4</sub> and N <sub>2</sub> O sources. The biogeochemical feedback from permafrost thaw leads to a combined linear feedback term of –21 ± 12 PgCeq °C <sup>–1</sup> (1 standard deviation range –77 ± 44 Gt CO <sub>2-</sub> eq °C <sup>–1</sup> ). For the integration of these feedbacks in the assessment of the remaining carbon budget ( [[#5.5.2|Section 5.5.2]] ), two individual non-CO <sub>2</sub> feedbacks (tropospheric ozone, and methane lifetime) are captured in the AR6-calibrated emulators (Box 7.1). Excluding those two contributions, the resulting combined linear feedback term for application in [[#5.5.2|Section 5.5.2]] is assessed at a reduction of 7 ± 27 PgCeq °C <sup>–1</sup> (1 standard deviation range, –26 ± 97 PgCeq °C <sup>–1</sup> ). For the same reasons as for the feedback terms expressed in W m <sup>–2</sup> °C <sup>–1</sup> (see above), there is overall ''low confidence'' in the magnitude of these feedbacks.

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