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

=== 5.4.1 Direct CO <sub>2</sub> Effect on Land Carbon Uptake ===

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The AR5 (WGI, Box 6.3) and SRCCL ( [[#IPCC--2019a|IPCC, 2019a]] ) concluded with ''high confidence'' that rising atmospheric CO <sub>2</sub> increases leaf-level photosynthesis. This effect is represented in all ESMs. New studies since AR5 add evidence that the leaf-level CO <sub>2</sub> fertilization is modulated by acclimation of photosynthesis to long-term CO <sub>2</sub> exposure, growth temperature, seasonal drought, and nutrient availability, but these effects are not yet routinely represented in ESMs ( [[#Smith--2013|Smith and Dukes, 2013]] ; [[#Baig--2015|Baig et al., 2015]] ; [[#Kelly--2016|Kelly et al., 2016]] ; [[#Drake--2017|Drake et al., 2017]] ; [[#Jiang--2020a|Jiang et al., 2020a]] ). Cross-Chapter Box 5.1 assesses multiple lines of evidence, which suggest that the ratio of plant CO <sub>2</sub> uptake to water loss – plant water-use efficiency (WUE) – increases in near proportionality to atmospheric CO <sub>2</sub> . Despite advances in the regional coverage of field experiments, observations of the consequences of CO <sub>2</sub> fertilization at ecosystem level are still scarce, in particular from outside the temperate zone ( [[#Song--2019|Song et al., 2019]] ). New syntheses since AR5 corroborate that the effect of elevated CO <sub>2</sub> on plant growth and ecosystem carbon storage is generally positive ( ''high confidence'' ), but is modulated by temperature, water and nutrient availability ( [[#Reich--2014|Reich et al., 2014]] ; [[#Obermeier--2017|Obermeier et al., 2017]] ; [[#Peñuelas--2017|Peñuelas et al., 2017]] ; [[#Hovenden--2019|Hovenden et al., 2019]] ; [[#Song--2019|Song et al., 2019]] ). Plant carbon allocation, changes in plant community composition, disturbance, and natural plant mortality are important processes affecting the magnitude of the response, but are currently poorly represented in models (De Kauwe et al., 2014; [[#Friend--2014|Friend et al., 2014]] ; [[#Reich--2018|Reich et al., 2018]] ; A.P. [[#Walker--2019|]] [[#Walker--2019|Walker et al., 2019]] ; K. [[#Yu--2019|]] [[#Yu--2019|Yu et al., 2019]] ), and thus contribute strongly to uncertainty in ESM projections ( [[#Arora--2020|Arora et al., 2020]] ).

Field studies with elevated CO <sub>2</sub> have demonstrated that the initial stimulation of above-ground growth may decline if insufficient nutrients such as nitrogen or phosphorus are available ( [[#Finzi--2007|Finzi et al., 2007]] ; [[#Norby--2010|Norby et al., 2010]] ; [[#Hungate--2013|Hungate et al., 2013]] ; [[#Reich--2013|Reich and Hobbie, 2013]] ; [[#Talhelm--2014|Talhelm et al., 2014]] ; [[#Terrer--2018|Terrer et al., 2018]] ). Model-data syntheses have demonstrated that capturing the observed long-term effect of elevated CO <sub>2</sub> depends on the ability of models to predict the effect of vegetation on soil biogeochemistry ( [[#Zaehle--2014|Zaehle et al., 2014]] ; [[#Koven--2015b|Koven et al., 2015b]] ; [[#Medlyn--2015|Medlyn et al., 2015]] ; [[#Walker--2015|Walker et al., 2015]] ). Meta-analyses of CO <sub>2</sub> manipulation experiments point to increased soil microbial activity and accelerated turnover of soil organic matter ( [[#van%20Groenigen--2017|van Groenigen et al., 2017]] ) as a result of increased below-ground carbon allocation by plants ( [[#Song--2019|Song et al., 2019]] ), and increased root exudation or mycorrhizal activity due to enhanced plant nutrient requirements under elevated CO <sub>2</sub> ( [[#Drake--2011|Drake et al., 2011]] ; [[#Terrer--2016|Terrer et al., 2016]] ; [[#Meier--2017|Meier et al., 2017]] ). These effects are not considered in most ESMs. One global model that attempts to represent these processes suggests that elevated CO <sub>2</sub> -related carbon accumulation is reduced in soils but increased in vegetation relative to more conventional models ( [[#Sulman--2019|Sulman et al., 2019]] ).

Our understanding of the effects of phosphorus limitation is less developed than for nitrogen, but a growing body of literature suggests that it is just as important, particularly in regions with highly weathered soils ( [[#Wang--2018|Wang et al., 2018]] ; [[#Terrer--2019|Terrer et al., 2019]] ; [[#Du--2020|Du et al., 2020]] ). CO <sub>2</sub> experiments collectively show that soil phosphorus is an important constraint on the CO <sub>2</sub> fertilization effect on plant biomass ( [[#Terrer--2019|Terrer et al., 2019]] ; [[#Jiang--2020a|Jiang et al., 2020a]] ). For example, despite increases in photosynthesis after four years of CO <sub>2</sub> exposure, a free-air CO <sub>2</sub> enrichment experiment in a phosphorus-limited mature forest ecosystem did not find an increase in biomass production ( [[#Jiang--2020b|Jiang et al., 2020b]] ). The lack of free-air CO <sub>2</sub> enrichment experiments in phosphorus-limited tropical forests limits our understanding of the role of phosphorus availability in constraining the CO <sub>2</sub> fertilization effect globally ( [[#Norby--2016|Norby et al., 2016]] ; [[#Fleischer--2019|Fleischer et al., 2019]] ). Models accounting for the effects of phosphorus availability, in addition to nitrogen, generally show an even stronger reduction of the response of ecosystem carbon storage to elevated CO <sub>2</sub> ( [[#Goll--2012|Goll et al., 2012]] ; [[#Zhang--2014|Zhang et al., 2014]] ; X. [[#Yang--2019|Yang et al., 2019]] ). Insufficient data and uncertainties in the process formulation cause large uncertainty in the magnitude of this effect ( [[#Medlyn--2016|Medlyn et al., 2016]] ; [[#Fleischer--2019|Fleischer et al., 2019]] ).

Consistent with AR5 (WGI, Section 6.4.2), the CO <sub>2</sub> fertilization effect <sub></sub> is the dominant cause for the projected increase in land carbon uptake between 1860 and 2100 in ESMs (Figures 5.26 and 5.27, and Table 5.5; [[#Arora--2020|Arora et al., 2020]] ). In the CMIP6 ensemble, the increase of land carbon storage due to CO <sub>2</sub> fertilization is a global phenomenon but is strongest in the tropics (Figure 5.26). The resulting increase of productivity is a key driver of increases in vegetation and soil carbon storage. However, consistent with earlier findings ( [[#Todd-Brown--2013|Todd-Brown et al., 2013]] ; [[#Friend--2014|Friend et al., 2014]] ; [[#Hajima--2014|Hajima et al., 2014]] ), processes affecting vegetation carbon-use efficiency and turnover, such as allocation changes, mortality, and vegetation structural changes, as well as the pre-industrial soil carbon turnover time, also play an important role ( [[#Arora--2020|Arora et al., 2020]] ).

As a major advance since AR5 (WGI, Section 6.4.2), six out of 11 models in the C4MIP-CMIP6 ensemble account for nitrogen cycle dynamics over land (Table 5.4). On average, these models exhibit a 25–30% lower CO <sub>2</sub> fertilization effect on land carbon storage, compared to models that do not account for nitrogen cycle dynamics (Figure 5.29 and Table 5.5). The only model in the C <sup>4</sup> MIP-CMIP6 ensemble that explicitly represents the effect of P availability on plant growth suggests the lowest carbon storage response to increasing CO <sub>2</sub> ( [[#Arora--2020|Arora et al., 2020]] ). The lower CO <sub>2</sub> effect due to decreased nutrient availability is generally consistent with analyses of the implicit nutrient limitation in CMIP5 simulations ( [[#Wieder--2015|Wieder et al., 2015]] ; [[#Zaehle--2015|Zaehle et al., 2015]] ) and independent assessments by stand-alone land models ( [[#Zaehle--2010|Zaehle et al., 2010]] ; [[#Wårlind--2014|Wårlind et al., 2014]] ; [[#Zhang--2014|Zhang et al., 2014]] ; [[#Goll--2017|Goll et al., 2017]] ; [[#Meyerholt--2020|Meyerholt et al., 2020]] ). The simulated effects are generally consistent with expectations based on independent observations ( [[#Walker--2021|Walker et al., 2021]] ). However, the magnitude of nutrient feedbacks in these models is poorly constrained by observations, owing to the limited geographic distribution of available observations and the uncertain scaling of results obtained from manipulation experiments to transient system dynamics ( [[#Song--2019|Song et al., 2019]] ; [[#Wieder--2019|Wieder et al., 2019]] ; [[#Meyerholt--2020|Meyerholt et al., 2020]] ).

Our understanding of the various biological processes that affect the strength of the CO <sub>2</sub> fertilization effect on photosynthesis and its impact on carbon storage in vegetation and soils, (in particular regarding the limitations imposed by nitrogen and phosphorus availability), has developed since AR5 (WGI, Box 6.2). Based on consistent behaviour across all CMIP6 ESMs, there is ''high confidence'' that CO <sub>2</sub> fertilization of photosynthesis acts as an important negative feedback on anthropogenic climate change, by reducing the rate at which CO <sub>2</sub> accumulates in the atmosphere. Since AR5 (WGI, Box 6.2), an increasing number of CMIP6 ESMs account for nutrient cycles. The consistent results found in their model projections suggests with ''high confidence'' that limited nutrient availability will limit the CO <sub>2</sub> fertilization effect ( [[#Arora--2020|Arora et al., 2020]] ). The magnitude of the direct CO <sub>2</sub> effect on land carbon uptake, and its limitation by nutrients, remains uncertain.

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