Editing IPCC:AR6/WGI/Chapter-6 (section)

=== 6.4.4 Indirect Radiative Forcing Through Effects of SLCFs on the Carbon Cycle ===

<div id="h2-22-siblings" class="h2-siblings"></div>

Deposition of reactive nitrogen (Nr; i.e., NH <sub>3</sub> and NO <sub>x</sub> ) increases the plant productivity and carbon sequestration in N-limited forests and grasslands, and also in open and coastal waters and open ocean. Such inadvertent fertilization of the biosphere can lead to eutrophication and reduction in biodiversity in terrestrial and aquatic ecosystems. The AR5 assessed that it is ''likely'' that Nr deposition over land currently increases natural CO <sub>2</sub> sinks, in particular in forests, but the magnitude of this effect varies between regions ( [[#Ciais--2013|Ciais et al., 2013]] ). Increasing Nr deposition or the synergy between increasing Nr deposition and atmospheric CO <sub>2</sub> concentration could have contributed to the increasing global-net land CO <sub>2</sub> sink ( [[IPCC:Wg1:Chapter:Chapter-5#5.2.1.4.1|Section 5.2.1.4.1]] ).

Ozone uptake itself damages photosynthesis and reduces plant growth with consequences for the carbon and water cycles ( [[#Ainsworth--2012|Ainsworth et al., 2012]] ; [[#Emberson--2018|Emberson et al., 2018]] ). The AR5 concluded there was robust evidence of the effect of ozone on plant physiology and subsequent alteration of the carbon storage, but considered insufficient quantification of and a lack of systematic incorporation of the ozone effect in carbon-cycle models as a limitation to assess the terrestrial carbon balance ( [[#Ciais--2013|Ciais et al., 2013]] ). Since AR5 several more ESMs have incorporated interactive ozone-vegetation damage resulting in an increase in evidence to support the influence of ozone on the land carbon cycle. The new modelling studies tend to focus on ozone effects on plant productivity rather than the land carbon storage and agree that ozone-induced gross-primary productivity (GPP) losses are largest today in the eastern USA, Europe and eastern China, ranging from 5–20% on the regional scale ( ''low confidence'' ) (Yue and [[#Unger--2014|Unger, 2014]] ; Lombardozzi et al. , 2015; Yue et al. , 2017; Oliver et al. , 2018). There is ''medium evidence'' and ''high agreement'' based on observational studies and models that ozone-vegetation interactions further influence the climate system, including water and carbon cycles by affecting stomatal control over plant transpiration of water vapour between the leaf surface and atmosphere (Wittig et al. , 2007; Sun et al. , 2012; VanLoocke et al. , 2012; Lombardozzi et al. , 2013; Hoshika et al. , 2015; Arnold et al. , 2018). While some modelling studies suggest that the unintended Nr deposition fertilization effect in forests may potentially offset the ozone-induced carbon losses ( [[#Felzer--2007|Felzer et al., 2007]] ; [[#de%20Vries--2017|de Vries et al., 2017]] ), complex interactions have been observed between ozone and Nr deposition to ecosystems that have not yet been included in ESMs. For some plants, the effects of increasing ozone on root biomass become more pronounced as Nr deposition increased, and the beneficial effects of Nr on root development were lost at higher ozone treatments ( [[#Mills--2016|Mills et al., 2016]] ). Reducing uncertainties in ozone vegetation damage effects on the carbon cycle requires improved information on the sensitivity of different plant species to ozone, and measurements of ozone dose-response relationships for tropical plants, which are currently lacking. Surface ozone effect on the land carbon sink and indirect CO <sub>2</sub> forcing, therefore, remains uncertain. Collins et al. (2010) showed that adding in the effects of surface ozone on vegetation damage and reduced uptake of CO <sub>2</sub> added about 10% to the methane emissions metrics and could change the sign of the NO <sub>x</sub> metrics. However, this estimate has to be considered as an upper limit due to limitation of the paramaterization used by Sitch et al. (2007) considering more recent knowledge and is thus not included in the current metrics ( [[IPCC:Wg1:Chapter:Chapter-7#7.6.1.3|Section 7.6.1.3]] ).

Tropospheric aerosols influence the land and ocean ecosystem productivity and the carbon cycle through changing physical climate and meteorology ( [[#Jones--2003|Jones, 2003]] ; [[#Cox--2008|Cox et al., 2008]] ; [[#Mahowald--2011|Mahowald, 2011]] ; [[#Unger--2017|Unger et al., 2017]] ) and through changing deposition of nutrients including nitrogen, sulphur, iron and phosphorous ( [[#Mahowald--2017|Mahowald et al., 2017]] ; [[#Kanakidou--2018|Kanakidou et al., 2018]] ). There is ''robust evidence'' and ''high agreement'' from field (Oliveira et al. , 2007; Cirino et al. , 2014; Rap et al. , 2015; X. Wang et al. , 2018) and modelling ( [[#Mercado--2009|Mercado et al., 2009]] ; [[#Strada--2016|Strada and Unger, 2016]] ; [[#Lu--2017|Lu et al., 2017]] ; [[#Yue--2017|Yue et al., 2017]] ) studies that aerosols affect plant productivity through increasing the diffuse fraction of downward shortwave radiation, although the magnitude and importance to the global land carbon sink is controversial. At large scales the dominant effect of aerosols on the carbon cycle is ''likely'' a global cooling effect of the climate ( ''medium confidence'' ) ( [[#Jones--2003|Jones, 2003]] ; [[#Mahowald--2011|Mahowald, 2011]] ; [[#Unger--2017|Unger et al., 2017]] ). We assess that these interactions between aerosols and the carbon cycle are currently too uncertain to constrain quantitatively the indirect CO <sub>2</sub> forcing.

In summary, reactive nitrogen, ozone and aerosols affect terrestrial vegetation and the carbon cycle through deposition and effects on large-scale radiation ( ''high confidence'' ) but the magnitude of these effects on the land carbon sink, ecosystem productivity and indirect CO <sub>2</sub> forcing remain uncertain due to the difficulty in disentangling the complex interactions between the effects. As such, we assess the effects to be of second order in comparison to the direct CO <sub>2</sub> forcing ( ''high confidence'' ) but, at least for ozone, it could add a substantial (positive) forcing compared with its direct forcing ( ''low confidence'' ).

<div id="6.4.5" class="h2-container"></div>

<span id="non-co-2-biogeochemical-feedbacks"></span>