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

==== 5.4.3.1 Advances in the characterisation of the effects of elevated atmospheric CO 2 ====

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Elevated CO 2 concentrations stimulate photosynthesis rates and biomass accumulation of C 3 crops, and enhance crop water use efficiency of various crop species, including C 4 crops ( ''high confidence'' ) ( [[#Kimball--2016|Kimball, 2016]] ; [[#Toreti--2020|Toreti et al., 2020]] ). Perennial crops and root crops may have a greater capacity for enhanced biomass under elevated CO 2 concentrations, although this does not always result in higher yields ( [[#Glenn--2013|Glenn et al., 2013]] ; [[#Kimball--2016|Kimball, 2016]] ).

Recent FACE studies found that the effects of elevated CO 2 are greater under water-limited conditions ( ''medium confidence'' ) ( [[#Manderscheid--2014|Manderscheid et al., 2014]] ; [[#Fitzgerald--2016|Fitzgerald et al., 2016]] ; [[#Kimball--2016|Kimball, 2016]] ), which was generally reproduced by crop models ( [[#Deryng--2016|Deryng et al., 2016]] ). However, drought sometimes negates the CO 2 effects ( [[#Jin--2018|Jin et al., 2018]] ).

There are significant interactions between CO 2 , temperature, cultivars, nitrogen and phosphorous nutrients ( [[#Kimball--2016|Kimball, 2016]] ; [[#Toreti--2020|Toreti et al., 2020]] ): positive effects of rising CO 2 on yield are significantly reduced by higher temperatures for soybean, wheat and rice ( ''medium confidence'' ) ( [[#Ruiz-Vera--2013|Ruiz-Vera et al., 2013]] ; [[#Cai--2016|Cai et al., 2016]] ; [[#Gray--2016|Gray et al., 2016]] ; [[#Hasegawa--2016|Hasegawa et al., 2016]] ; [[#Obermeier--2016|Obermeier et al., 2016]] ; [[#Purcell--2018|Purcell et al., 2018]] ; [[#Wang--2018|Wang et al., 2018]] ). In above-ground vegetables, elevated CO 2 can in some cases reduce the impact of other climate stressors, while in others the negative impacts of other abiotic factors negate the potential benefit of elevated CO 2 ( [[#Bourgault--2017|Bourgault et al., 2017]] ; [[#Bourgault--2018|Bourgault et al., 2018]] ; [[#Parvin--2018|Parvin et al., 2018]] ; [[#Parvin--2019|Parvin et al., 2019]] ). Significant variation exists among cultivars in yield response to elevated CO 2 , which is positively correlated with yield potential in rice and soybean, suggesting the potential to develop cultivars for enhanced productivity under future elevated [CO 2 ] ( [[#Ainsworth--2021|Ainsworth and Long, 2021]] ).

Elevated CO 2 reduces some important nutrients such as protein, iron, zinc and some grains, fruit or vegetables to varying degrees depending on crop species and cultivars ( ''high confidence'' ) ( [[#Mattos--2014|Mattos et al., 2014]] ; [[#Myers--2014|Myers et al., 2014]] ; [[#Dong--2018|Dong et al., 2018]] ; [[#Scheelbeek--2018|Scheelbeek et al., 2018]] ; [[#Zhu--2018a|Zhu et al., 2018a]] ; [[#Jin--2019|Jin et al., 2019]] ; [[#Ujiie--2019|Ujiie et al., 2019]] ). This is of particular relevance for fruit and vegetable crops given their importance in human nutrition ( ''high confidence'' ) (see [[#5.12.4|Section 5.12.4]] for potential impacts on nutrition; [[#Nelson--2018|Nelson et al., 2018]] ; [[#Springmann--2018|Springmann et al., 2018]] ). Recent experimental studies ( [[#5.3.2|Section 5.3.2]] ), however, show some complex and counteracting interactions between CO 2 and temperature in wheat, soybean and rice; heat stress negates the adverse effect of elevated CO 2 on some nutrient elements ( [[#Macabuhay--2018|Macabuhay et al., 2018]] ; [[#Kohler--2019|Kohler et al., 2019]] ; [[#Wang--2019b|Wang et al., 2019b]] ). The CO 2 by temperature interaction for grain quality needs to be better understood quantitatively to predict food nutritional security in the future.

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