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

=== 5.4.7 Climate Feedbacks from CH <sub>4</sub> and N <sub>2</sub> O ===

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Sources and sinks of CH <sub>4</sub> and N <sub>2</sub> O respond both directly and indirectly to atmospheric CO <sub>2</sub> concentration and climate change, and thereby give rise to additional biogeochemical feedbacks in the climate system, which may amplify or attenuate climate–carbon cycle feedbacks ( [[#Gasser--2017|Gasser et al., 2017]] ; [[#Lade--2018|Lade et al., 2018]] ; [[#Denisov--2019|Denisov et al., 2019]] ). Many of these of feedbacks are only partially understood, and thus were only partially addressed in AR5 (WGI, Sections 6.3.3, 6.3.4 and 6.4.7). Since AR5, a growing body of estimates from ESMs, as well as independent modelling and observation-based studies, enable improved estimates of the associated feedbacks.

The goal of this section is to assess the climate feedback parameters α , as it is defined in [[IPCC:Wg1:Chapter:Chapter-7#7.4.1.1|Section 7.4.1.1]] , for CH <sub>4</sub> and N <sub>2</sub> O biogeochemical feedbacks. The strength of the feedbacks is estimated in a linear framework ( [[#Gregory--2009|Gregory et al., 2009]] ), using the radiative forcing equations for CO <sub>2</sub> , CH <sub>4</sub> and N <sub>2</sub> O ( [[#Etminan--2016|Etminan et al., 2016]] ). In addition to estimates from ESMs, the feedback parameter α is estimated from independent estimates of surface emission climate sensitivities and atmospheric box models, following ( [[#Arneth--2010|Arneth et al., 2010]] ; [[#Thornhill--2021|Thornhill et al., 2021]] ). These assessed feedback parameters are used in [[IPCC:Wg1:Chapter:Chapter-7#7.4.2.5|Section 7.4.2.5]] .

The CH <sub>4</sub> feedbacks may arise from changing wetland emissions (including rice farming) and from sources that are expected to grow under climate change (e.g., related to permafrost thaw, fires, and freshwater bodies). CH <sub>4</sub> emissions from wetlands and landfills generally increase with warming due to enhanced decomposition with higher temperatures, thereby potentially providing a positive CH <sub>4</sub> feedback on climate ( [[#Dean--2018|Dean et al., 2018]] ). The contribution of wetlands to interannual variability of atmospheric CH <sub>4</sub> is shaped by the different impacts of temperature and precipitation anomalies on wetland emissions (e.g., during El Niño episodes) and therefore the relationship between climate anomalies and the wetland contribution to the CH <sub>4</sub> growth rate is complex ( [[#Pison--2013|Pison et al., 2013]] ; [[#Nisbet--2016|Nisbet et al., 2016]] ; X. [[#Zhang--2020|]] [[#Zhang--2020|]] [[#Zhang--2020|Zhang et al., 2020]] ). As assessed by SROCC ( [[#IPCC--2019b|IPCC, 2019b]] ), there is ''high agreement'' across model simulations that wetlands CH <sub>4</sub> emissions will increase in the 21 <sup>st</sup> century, but ''low agreement'' in the magnitude of the change ( [[#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]] ; [[#Koffi--2020|Koffi et al., 2020]] ). Climate change increases wetland emissions ( [[#Gedney--2004|Gedney et al., 2004]] , 2019; [[#Volodin--2008|Volodin, 2008]] ; [[#Ringeval--2011|Ringeval et al., 2011]] ; [[#Denisov--2013|Denisov et al., 2013]] ; [[#Shindell--2013|Shindell et al., 2013]] ) and gives rise to an estimated wetland CH <sub>4</sub> –climate feedback of 0.03 ± 0.01 W m <sup>–2</sup> °C <sup>–1</sup> (mean ± 1 standard deviation; ''limited evidence'' , ''high agreement'' ) ( [[#Arneth--2010|Arneth et al., 2010]] ; [[#Shindell--2013|Shindell et al., 2013]] ; B.D. [[#Stocker--2013|]] [[#Stocker--2013|Stocker et al., 2013]] ; [[#Zhang--2017|Zhang et al., 2017]] ). The effect of rising CO <sub>2</sub> on productivity, and therefore on the substrate for methanogenesis, can further increase the projected increase in wetland CH <sub>4</sub> emissions ( [[#Ringeval--2011|Ringeval et al., 2011]] ; [[#Melton--2013|Melton et al., 2013]] ). Model projections accounting for the combined effects of CO <sub>2</sub> and climate change suggest a potentially larger climate feedback (0.01–0.16 W m <sup>–2</sup> °C <sup>–1</sup> ) ( ''limited evidence'' , ''low agreement'' ) ( [[#Gedney--2019|Gedney et al., 2019]] ; [[#Thornhill--2021|Thornhill et al., 2021]] ). Methane release from wetlands depends on the nutrient availability for methanogenic and methanotrophic microorganisms that can further modify this feedback ( [[#Stepanenko--2016|Stepanenko et al., 2016]] ; [[#Donis--2017|Donis et al., 2017]] ; [[#Beaulieu--2019|Beaulieu et al., 2019]] ). Methane emissions from thermokarst ponds and wetlands resulting from permafrost thaw are estimated to contribute an additional CH <sub>4</sub> -climate feedback of 0.01 [0.003 to 0.04, 5–95% range] W m <sup>–2</sup> °C <sup>–1</sup> ( ''limited evidence, l'' ''ow agreement'' ).

Methane release from wildfires may increase by up to a factor of 1.5 during the 21 <sup>st</sup> century ( [[#Eliseev--2014a|Eliseev et al., 2014a]] , b; [[#Kloster--2017|Kloster and Lasslop, 2017]] ). However, given the contemporary estimate for CH <sub>4</sub> from wildfires of no more than 16 TgCH <sub>4</sub> yr <sup>–1</sup> ( [[#van%20der%20Werf--2017|van der Werf et al., 2017]] ; [[#Saunois--2020|Saunois et al., 2020]] ), this feedback is small, adding no more than 40 ppb to the atmospheric CH <sub>44</sub> by the end of the 21 <sup>st</sup> century ( ''medium confidence'' ). Methane emissions from pan-Arctic freshwater bodies is also estimated to increase by 16 TgCH <sub>4</sub> yr <sup>–1</sup> in the 21 <sup>st</sup> century ( [[#Tan--2015|Tan and Zhuang, 2015]] ). Emissions from subsea and permafrost methane hydrates are not expected to change substantially in the 21 <sup>st</sup> century ( [[#5.4.9.1.3|Section 5.4.9.1.3]] ).

Land biosphere models show ''high agreement'' that long-term warming will increase N <sub>2</sub> O release from terrestrial ecosystems ( [[#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]] ). A positive land N <sub>2</sub> O climate feedback is consistent with paleo-evidence based on reconstructed and modelled emissions during the last deglacial period ( [[#Schilt--2014|Schilt et al., 2014]] ; H. [[#Fischer--2019|]] [[#Fischer--2019|Fischer et al., 2019]] ; [[#Joos--2020|Joos et al., 2020]] ). The response of terrestrial N <sub>2</sub> O emissions to atmospheric CO <sub>2</sub> increase and associated warming is dependent on nitrogen availability ( [[#van%20Groenigen--2011|van Groenigen et al., 2011]] ; [[#Butterbach-Bahl--2013|Butterbach-Bahl et al., 2013]] ; [[#Tian--2019|Tian et al., 2019]] ). Model-based estimates do not account for the potentially strong emissions increases in boreal and arctic ecosystems associated with future warming and permafrost thaw ( [[#Elberling--2010|Elberling et al., 2010]] ; [[#Voigt--2017|Voigt et al., 2017]] ). There is ''medium confidence'' that the land N <sub>2</sub> O climate feedback is positive, but ''low confidence'' in the magnitude (0.02 ± 0.01 W m <sup>–2</sup> °C <sup>–1</sup> ).

Climate change will also affect N <sub>2</sub> O production in the ocean ( [[#Codispoti--2010|Codispoti, 2010]] ; [[#Freing--2012|Freing et al., 2012]] ; [[#Bopp--2013|Bopp et al., 2013]] ; [[#Rees--2016|Rees et al., 2016]] ; [[#Breider--2019|Breider et al., 2019]] ). Model projections in the 21 <sup>st</sup> century show a 4–12% decrease in ocean N <sub>2</sub> O emissions under RCP8.5 due to a combination of factors, including increased ocean stratification, decreased ocean productivity, and the impact of increasing atmospheric N <sub>2</sub> O abundance on the air–sea flux, corresponding to an ocean N <sub>2</sub> O climate feedback of –0.008 ± 0.002 W m <sup>–2</sup> °C <sup>–1</sup> ( limited evidence, ''high agreement'' ) ( [[#Martinez-Rey--2015|Martinez-Rey et al., 2015]] ; [[#Landolfi--2017|Landolfi et al., 2017]] ; [[#Battaglia--2018b|Battaglia and Joos, 2018b]] ). On millennial time scales, the ocean N <sub>2</sub> O climate feedback may be positive, owing to ocean deoxygenation and long-term increases in remineralization ( [[#Battaglia--2018b|Battaglia and Joos, 2018b]] ).

Based-on these studies, there is ''medium confidence'' that the combined climate feedback parameter for CH <sub>4</sub> and N <sub>2</sub> O is positive, but there is ''low confidence'' in the magnitude of the estimate (0.05 [0.02 to 0.09] W m <sup>–2</sup> °C <sup>–1</sup> , 5–95% range).

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