Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.4.2.5 Biogeophysical and Non-CO <sub>2</sub> Biogeochemical Feedbacks ====

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The feedbacks presented in the previous sections (Sections 7.4.2.1–7.4.2.4) are directly linked to physical climate variables (for example temperature, water vapour, clouds, or sea ice). The central role of climate feedbacks associated with these variables has been recognized since early studies of climate change. However, in addition to these physical climate feedbacks, the Earth system includes feedbacks for which the effect of global mean surface temperature change on the TOA energy budget is mediated through other mechanisms, such as the chemical composition of the atmosphere, or by vegetation changes. Among these additional feedbacks, the most important is the CO <sub>2</sub> feedback that describes how a change of the global surface temperature affects the atmospheric CO <sub>2</sub> concentration. In ESM simulations in which CO <sub>2</sub> emissions are prescribed, changes in surface carbon fluxes affect the CO <sub>2</sub> concentration in the atmosphere, the TOA radiative energy budget, and eventually the global mean surface temperature. In ESM simulations in which the CO <sub>2</sub> concentration is prescribed, changes in the carbon cycle allow compatible CO <sub>2</sub> emissions to be calculated, that is, the CO <sub>2</sub> emissions that are compatible with both the prescribed CO <sub>2</sub> concentration and the representation of the carbon cycle in the ESM. The CO <sub>2</sub> feedback is assessed in ( [[IPCC:Wg1:Chapter:Chapter-5|Chapter 5]] [[IPCC:Wg1:Chapter:Chapter-5#5.4|Section 5.4]] ). The framework presented in this chapter assumes that the CO <sub>2</sub> concentration is prescribed, and our assessment of the net feedback parameter, α , does not include carbon cycle feedbacks on the atmospheric CO <sub>2</sub> concentration ( [[#7.1|Section 7.1]] and Box 7.1). However, our assessment of α does include non-CO <sub>2</sub> biogeochemical feedbacks (including effects due to changes in atmospheric methane concentration; [[#7.4.2.5.1|Section 7.4.2.5.1]] ) and biogeophysical feedbacks ( [[#7.4.2.5.2|Section 7.4.2.5.2]] ). A synthesis of the combination of biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks is given in [[#7.4.2.5.3|Section 7.4.2.5.3]] .

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===== 7.4.2.5.1 Non-CO <sub>2</sub> biogeochemical feedbacks =====

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The chemical composition of the atmosphere (beyond CO <sub>2</sub> and water vapour changes) is expected to change in response to a warming climate. These changes in greenhouse gases (methane, nitrous oxide and ozone) and aerosol amount (including dust) have the potential to alter the TOA energy budget and are collectively referred to as ‘non-CO <sub>2</sub> biogeochemical feedbacks’. Methane (CH <sub>4</sub> ) and nitrous oxide (N <sub>2</sub> O) feedbacks arise partly from changes in their emissions from natural sources in response to temperature change; these are assessed in ( [[IPCC:Wg1:Chapter:Chapter-5|Chapter 5]] [[IPCC:Wg1:Chapter:Chapter-5#5.4.7|Section 5.4.7]] ; see also Figure 5.29c). Here we exclude the permafrost CH <sub>4</sub> feedback ( [[IPCC:Wg1:Chapter:Chapter-5#5.4.9.1.2|Section 5.4.9.1.2]] ) because, although associated emissions are projected to increase under warming on multi-decadal to centennial time scales, on longer time scales these emissions would eventually substantially decline as the permafrost carbon pools were depleted ( [[#Schneider%20von%20Deimling--2012|Schneider von Deimling et al., 2012]] , 2015). This leaves the wetland CH <sub>4</sub> , land N <sub>2</sub> O, and ocean N <sub>2</sub> O feedbacks, the assessed mean values of which sum to a positive feedback parameter of +0.04 [0.02 to 0.06] W m <sup>–2</sup> °C <sup>–1</sup> [[IPCC:Wg1:Chapter:Chapter-5#5.4.7|Section 5.4.7]] . Other non-CO <sub>2</sub> biogeochemical feedbacks that are relevant to the net feedback parameter are assessed in [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] (Section 6.4.5 and Table 6.8). These feedbacks are associated with sea salt, dimethyl sulphide, dust, ozone, biogenic volatile organic compounds, lightning, and CH <sub>4</sub> lifetime, and sum to a negative feedback parameter of –0.20 [–0.41 to +0.01] W m <sup>–2</sup> °C <sup>–1</sup> . The overall feedback parameter for non-CO <sub>2</sub> biogeochemical feedbacks is obtained by summing the [[IPCC:Wg1:Chapter:Chapter-5|Chapter 5]] and [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] assessments, which gives –0.16 [–0.37 to +0.05] W m <sup>–2</sup> °C <sup>–1</sup> . However, there is ''low confidence'' in the estimates of both the individual non-CO <sub>2</sub> biogeochemical feedbacks as well as their total effect, as evident from the large range in the magnitudes of α from different studies, which can be attributed to diversity in how models account for these feedbacks and limited process-level understanding.

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===== 7.4.2.5.2 Biogeophysical feedbacks =====

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Biogeophysical feedbacks are associated with changes in the spatial distribution and/or biophysical properties of vegetation, induced by surface temperature change and attendant hydrological cycle change. These vegetation changes can alter radiative fluxes directly via albedo changes, or via surface momentum or moisture flux changes and hence changes in cloud properties. However, the direct physiological response of vegetation to changes in CO <sub>2</sub> , including changes in stomatal conductance, is considered part of the CO <sub>2</sub> effective radiative forcing rather than a feedback ( [[#7.3.2.1|Section 7.3.2.1]] ). The time scale on which vegetation responds to climate change is relatively uncertain but can be from decades to hundreds of years ( [[#Willeit--2014|Willeit et al., 2014]] ), and could occur abruptly or as a tipping point (Sections 5.4.9.1.1, 8.6.2.1 and 8.6.2.2); equilibrium only occurs when the soil system and associated nutrient and carbon pools equilibrate, which can take millennia ( [[#Brantley--2008|Brantley, 2008]] ; [[#Sitch--2008|Sitch et al., 2008]] ). The overall effects of climate-induced vegetation changes may be comparable in magnitude to those from anthropogenic land-use and land-cover change ( [[#Davies-Barnard--2015|Davies-Barnard et al., 2015]] ). Climate models that include a dynamical representation of vegetation (e.g., [[#Reick--2013|Reick et al., 2013]] ; [[#Harper--2018|Harper et al., 2018]] ) are used to explore the importance of biogeophysical feedbacks ( [[#Notaro--2007|Notaro et al., 2007]] ; [[#Brovkin--2009|Brovkin et al., 2009]] ; [[#O’ishi--2009|O’ishi et al., 2009]] ; [[#Port--2012|Port et al., 2012]] ; [[#Willeit--2014|Willeit et al., 2014]] ; [[#Alo--2017|Alo and Anagnostou, 2017]] ; W. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ; [[#Armstrong--2019|Armstrong et al., 2019]] ). In AR5, it was discussed that such model experiments predicted that expansion of vegetation in the high latitudes of the Northern Hemisphere would enhance warming due to the associated surface-albedo change, and that reduction of tropical forests in response to climate change would lead to regional surface warming, due to reduced evapotranspiration (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ), but there was no assessment of the associated feedback parameter. The SRCCL stated that regional climate change can be dampened or enhanced by changes in local land cover, but that this depends on the location and the season; however, in general the focus was on anthropogenic land-cover change, and no assessment of the biogeophysical feedback parameter was carried out. There are also indications of a marine biogeophysical feedback associated with surface-albedo change due to changes in phytoplankton ( [[#Frouin--2002|Frouin and Iacobellis, 2002]] ; [[#Park--2015|Park et al., 2015]] ), but there is not currently enough evidence to quantitatively assess this feedback.

Since AR5, several studies have confirmed that a shift from tundra to boreal forests and the associated albedo change leads to increased warming in Northern Hemisphere high latitudes ( ''high confidence'' ) ( [[#Willeit--2014|Willeit et al., 2014]] ; W. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ; [[#Armstrong--2019|Armstrong et al., 2019]] ). However, regional modelling indicates that vegetation feedbacks may act to cool climate in the Mediterranean ( [[#Alo--2017|Alo and Anagnostou, 2017]] ), and in the tropics and subtropics the regional response is in general not consistent across models. On a global scale, several modelling studies have either carried out a feedback analysis ( [[#Stocker--2013|Stocker et al., 2013]] ; [[#Willeit--2014|Willeit et al., 2014]] ) or presented simulations that allow a feedback parameter to be estimated ( [[#O’ishi--2009|O’ishi et al., 2009]] ; [[#Armstrong--2019|Armstrong et al., 2019]] ), in such a way that the physiological response can be accounted for as a forcing rather than a feedback. The central estimates of the biogeophysical feedback parameter from these studies range from close to zero ( [[#Willeit--2014|Willeit et al., 2014]] ) to +0.13 W m <sup>–2</sup> °C <sup>–1</sup> ( [[#Stocker--2013|Stocker et al., 2013]] ). An additional line of evidence comes from the mid-Pliocene warm period (MPWP, Chapter 2, Cross-Chapter Box 2.1), for which paleoclimate proxies provide evidence of vegetation distribution and CO <sub>2</sub> concentrations. Model simulations that include various combinations of modern versus MPWP vegetation and CO <sub>2</sub> allow an associated feedback parameter to be estimated, as long as account is also taken of the orographic forcing ( [[#Lunt--2010|Lunt et al., 2010]] , 2012b). This approach has the advantage over pure modelling studies in that the reconstructed vegetation is based on (paleoclimate) observations, and is in equilibrium with the CO <sub>2</sub> forcing. However, there are uncertainties in the vegetation reconstruction in regions with little or no proxy data, and it is uncertain how much of the vegetation change is associated with the physiological response to CO <sub>2</sub> . This paleoclimate approach gives an estimate for the biogeophysical feedback parameter of +0.3 W m <sup>–2</sup> °C <sup>–1</sup> .

Given the limited number of studies, we take the full range of estimates discussed above for the biogeophysical feedback parameter, and assess the ''very likely'' range to be from 0.0 to +0.3 W m <sup>–2</sup> °C <sup>–1</sup> , with a central estimate of +0.15 W m <sup>–2</sup> °C <sup>–1</sup> ( ''low confidence'' ). Although this assessment is based on evidence from both models and paleoclimate proxies, and the studies above agree on the sign of the change, there is nonetheless ''limited evidence'' . Higher confidence could be obtained if there were more studies that allowed calculation of a biogeophysical feedback parameter (particularly from paleoclimates), and if the partitioning between biogeophysical feedbacks and physiological forcing were clearer for all lines of evidence.

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===== 7.4.2.5.3 Synthesis of biogeophysical and non-CO 2 biogeochemical feedbacks =====

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The non-CO <sub>2</sub> biogeochemical feedbacks are assessed in ( [[#7.4.2.5.1|Section 7.4.2.5.1]] to be –0.16 [–0.37 to +0.05] W m <sup>–</sup> <sup>2</sup> °C <sup>–1</sup> and the biogeophysical feedbacks are assessed in ( [[#7.4.2.5.2|Section 7.4.2.5.2]] to be +0.15 [0.0 to +0.3] W m <sup>–2</sup> °C <sup>–1</sup> . The sum of the biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks is assessed to have a central value of –0.01 W m <sup>–2</sup> °C <sup>–1</sup> and a ''very likely'' range from –0.27 to +0.25 W m <sup>–2</sup> °C <sup>–1</sup> (Table 7.10). Given the relatively long time scales associated with the biological processes that mediate the biogeophysical and many of the non-CO <sub>2</sub> biogeochemical feedbacks, in comparison with the relatively short time scale of many of the underlying model simulations, combined with the small number of studies for some of the feedbacks, and the relatively small signals, this overall assessment has ''low confidence'' .

Some supporting evidence for this overall assessment can be obtained from the CMIP6 ensemble, which provides some pairs of instantaneous 4×CO <sub>2</sub> simulations carried out using related models, with and without biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks. This is not a direct comparison because these pairs of simulations may differ by more than just their inclusion of these additional feedbacks; furthermore, not all biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks are fully represented. However, a comparison of the pairs of simulations does provide a first-order estimate of the magnitude of these additional feedbacks. [[#Séférian--2019|Séférian et al. (2019)]] find a slightly more negative feedback parameter in CNRM-ESM2-1 (with additional feedbacks) then in CNRM-CM6-1 (a decrease of 0.02 W m <sup>–2</sup> °C <sup>–1</sup> , using the linear regression method from years 10–150). [[#Andrews--2019|Andrews et al. (2019)]] also find a slightly more negative feedback parameter when these additional feedbacks are included (a decrease of 0.04 W m <sup>–2</sup> °C <sup>–1</sup> in UKESM1 compared with HadGEM3-GC3.1). Both of these studies suggest a small but slightly negative feedback parameter for the combination of biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks, but with relatively large uncertainty given (i) interannual variability and (ii) that feedbacks associated with natural terrestrial emissions of CH <sub>4</sub> and N <sub>2</sub> O were not represented in either pair.

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