Editing IPCC:AR6/SRCCL/Chapter-2 (section)

== B 2.3 CO <sub>2</sub> fertilisation and enhanced terrestrial uptake of carbon ==

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All DGVMs and ESMs represent the CO <sub>2</sub> fertilisation effect (Le Quéré et al. 2017 <sup>[[#fn:r741|741]]</sup> ; Hoffman et al. 2014 <sup>[[#fn:r742|742]]</sup> ). There is ''high confidence'' that elevated CO <sub>2</sub> results in increased short-term CO <sub>2</sub> uptake per unit leaf area (Swann et al. 2016 <sup>[[#fn:r743|743]]</sup> ; Field et al. 1995 <sup>[[#fn:r744|744]]</sup> ; Donohue et al. 2013 <sup>[[#fn:r745|745]]</sup> ), however, whether this increased CO <sub>2</sub> uptake at the leaf level translates into increased growth for the whole plant differs among plant species and environments, because growth is constrained by whole-plant resource allocation and nutrient limitation (e.g., nitrogen, phosphorus, potassium and soil water and light limitations (Körner 2006 <sup>[[#fn:r746|746]]</sup> ; Peñuelas et al. 2017 <sup>[[#fn:r747|747]]</sup> ; Friend et al. 2014a <sup>[[#fn:r748|748]]</sup> )). Interactions between plants and soil microbes further modulate the degree of nutrient limitation on CO <sub>2</sub> fertilisation (Terrer et al. 2017 <sup>[[#fn:r749|749]]</sup> ).

At the ecosystems level, enhanced CO <sub>2</sub> uptake at decadal or longer timescales depends on changes in plant community composition and ecosystem respiration, as well disturbance and natural plant mortality (De Kauwe et al. <sup>[[#fn:r750|750]]</sup> , 2016; Farrior et al., 2015 <sup>[[#fn:r751|751]]</sup> ; Keenan et al., 2017 <sup>[[#fn:r752|752]]</sup> ; Sulman et al, 2019 <sup>[[#fn:r753|753]]</sup> ). The results of free-air carbon dioxide enrichment (FACE) experiments over two decades are highly variable because of these factors (Norby et al. 2010 <sup>[[#fn:r754|754]]</sup> ; Körner 2015 <sup>[[#fn:r755|755]]</sup> ; Feng et al. 2015 <sup>[[#fn:r756|756]]</sup> ; Paschalis et al. 2017 <sup>[[#fn:r757|757]]</sup> ; Terrer et al. 2017 <sup>[[#fn:r758|758]]</sup> ; Du et al. 2019 <sup>[[#fn:r759|759]]</sup> ). Under higher atmospheric CO <sub>2</sub> concentrations, the ratio of CO <sub>2</sub> uptake to water loss (water use efficiency (WUE)), increases and enhances drought tolerance of plants ( ''high confidence'' ) (Berry et al., 2010 <sup>[[#fn:r760|760]]</sup> ; Ainsworth and Rogers 2007 <sup>[[#fn:r761|761]]</sup> ).

Long-term CO <sub>2</sub> and water vapour flux measurements show that WUE in temperate and boreal forests of the northern hemisphere has increased more than predicted by photosynthetic theory and models over the past two decades ( ''high confidence'' ) (Keenan et al. 2013 <sup>[[#fn:r762|762]]</sup> ; Laguë and Swann 2016 <sup>[[#fn:r763|763]]</sup> ). New theories have emerged on how CO <sub>2</sub> uptake by trees is related to water loss and to the risk of damaging xylem (water conducting tissues) in the trunk and branches (Wolf et al. 2016a <sup>[[#fn:r764|764]]</sup> ; Anderegg et al. 2018a <sup>[[#fn:r765|765]]</sup> ). Tree ring studies of stable carbon and oxygen isotopes also detected increased WUE in recent decades (Battipaglia et al. 2013 <sup>[[#fn:r766|766]]</sup> ; Silva and Anand 2013 <sup>[[#fn:r767|767]]</sup> ; van der Sleen et al. 2014 <sup>[[#fn:r768|768]]</sup> ). Yet, tree ring studies often fail to show acceleration of tree growth rates in support of CO <sub>2</sub> fertilisation, even when they show increased WUE (van der Sleen et al. 2014 <sup>[[#fn:r769|769]]</sup> ). The International Tree Ring Data Bank (ITRDB) indicated that only about 20% of the sites in the database showed increasing trends in tree growth that cannot be explained by climate variability, nitrogen deposition, elevation or latitude. Thus there is l ''imited evidence'' ( ''low agreement'' ) among observations of enhanced tree growth due to CO <sub>2</sub> fertilisation of forests during the 20th century (Gedalof and Berg 2010 <sup>[[#fn:r770|770]]</sup> ).

In grasslands, although it is possible for CO <sub>2</sub> fertilisation to alleviate the impacts of drought and heat stress on net carbon uptake (Roy et al. 2016 <sup>[[#fn:r771|771]]</sup> ), there is ''low confidence'' about its projected magnitude. Because of its effect on water use efficiency, CO <sub>2</sub> fertilisation is expected to be pronounced in semi-arid habitats; and because of different metabolic pathways, C3 plants are expected to be more sensitive to elevated CO <sub>2</sub> concentrations than C4 grasses (Donohue et al. 2013 <sup>[[#fn:r772|772]]</sup> ; Morgan et al. 2011 <sup>[[#fn:r773|773]]</sup> ; Derner et al. 2003 <sup>[[#fn:r774|774]]</sup> ). Neither of these expectations was observed over a 12-year study of elevated CO <sub>2</sub> in a grassland system: enhanced growth was not observed during dry summers and growth of C4 grasses was unexpectedly stimulated, while growth of C3 grasses was not (Reich et al. 2014 <sup>[[#fn:r775|775]]</sup> , 2018 <sup>[[#fn:r776|776]]</sup> ).

There is medium ''confidence'' that CO <sub>2</sub> fertilisation effects have increased water use efficiency in crops and thus reduced agricultural water use per unit of crop produced (Deryng et al. 2016 <sup>[[#fn:r777|777]]</sup> ; Nazemi and Wheater 2015 <sup>[[#fn:r778|778]]</sup> ; Elliott et al. 2014 <sup>[[#fn:r779|779]]</sup> ). This effect could lead to near-term continued greening of agricultural areas. However, current assessments of these effects are based on limited observations, mostly from the temperate zone (Deryng et al. 2016 <sup>[[#fn:r780|780]]</sup> ).

One line of evidence for CO <sub>2</sub> fertilisation is the increasing land sink (‘the residual land sink’ in AR5) over the last 50 years as the atmospheric CO <sub>2</sub> concentration has increased (Los 2013 <sup>[[#fn:r781|781]]</sup> ; Sitch et al. 2015 <sup>[[#fn:r782|782]]</sup> ; Campbell et al. 2017 <sup>[[#fn:r783|783]]</sup> ; Keenan and Riley 2018 <sup>[[#fn:r784|784]]</sup> ). A combined analysis of atmospheric inverse analyses, ecosystem models and forest inventory data concluded that 60% of the recent terrestrial carbon sink can be directly attributed to increasing atmospheric CO <sub>2</sub> (Schimel et al. 2015 <sup>[[#fn:r785|785]]</sup> ). A global analysis using a ‘reconstructed vegetation index’ (RVI) for the period 1901–2006 from MODIS satellite-derived normalised vegetation difference index (NDVI) showed that CO <sub>2</sub> fertilisation contributed at least 40% of the observed increase in the land carbon sink (Los 2013 <sup>[[#fn:r786|786]]</sup> ). Without CO <sub>2</sub> fertilisation, ESMs are unable to simulate the increasing land sink and the observed atmospheric CO <sub>2</sub> concentration growth rate since the middle of the 20th century (Shevliakova et al. 2013 <sup>[[#fn:r787|787]]</sup> ). There are other mechanisms that could explain enhanced land carbon uptake such as increased regional forest and shrub cover (Chen et al. 2019 <sup>[[#fn:r788|788]]</sup> ) (Cross-Chapter Box 2 and Chapter 1), and, at higher latitudes, increasing temperatures and longer growing seasons (Zhu et al. 2016 <sup>[[#fn:r789|789]]</sup> ).

In summary, there is ''low confidence'' about the magnitude of the CO <sub>2</sub> effect and other factors that may explain at least a portion of the land sink (e.g., nitrogen deposition, increased growing season, reduced burning, erosion and re-deposition or organic sediments, aerosol-induced cooling). Increases in atmospheric CO <sub>2</sub> result in increased water use efficiency and increase leaf-level photosynthesis ( ''high confidence'' ). The extent to which CO <sub>2</sub> fertilisation results in plant- or ecosystem-level carbon accumulation is highly variable and affected by other environmental constraints ( ''high confidence'' ). Even in ecosystems where CO <sub>2</sub> fertilisation has been detected in recent decades, those effects are found to weaken as a result of physiological acclimation, soil nutrient limitation and other constraints on growth (Friend et al., 2014 <sup>[[#fn:r790|790]]</sup> ; Körner, 2006 <sup>[[#fn:r791|791]]</sup> ; Peñuelas et al., 2017 <sup>[[#fn:r792|792]]</sup> ).

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