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

==== 2.5.3.4 Risk to Terrestrial-Ecosystem Carbon Stocks ====

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Globally, increasing atmospheric CO 2 enhances the terrestrial sink but temperature increases constrain it, reflecting the biological process understanding highlighted in previous IPCC reports ( ''high confidence'' ). Analyses of atmospheric inversion model output and spatial climate data indicate a sensitivity of net ecosystem productivity to CO 2 fertilisation of 3.1 ± 0.1 Gt to 8.1 ± 0.3 Gt per 100 ppm CO 2 (~1°C increase) and a sensitivity to temperature of -0.5 ± 0.2 Gt to -1.1 ± 0.1 Gt per degree Celsius ( [[#Fernandez-Martinez--2019|Fernandez-Martinez et al., 2019]] ). The future of the global land carbon sink ( [[#2.4.4.4|Section 2.4.4.4]] ) nevertheless remains highly uncertain because (i) of regionally complex interactions of climate change and changes in atmospheric CO 2 with vegetation, soil and aquatic processes, (ii) episodic events such as heat waves or droughts (and related impacts through mortality, wildfire or insects, pests and diseases) ( [[#2.5.3.2|Section 2.5.3.2]] , 2.5.3.3) are so far only incompletely captured in carbon cycle models, (iii) the legacy effects from historic LUC and environmental changes are incompletely captured but likely to decline in future and (iv) lateral carbon transport processes such as the export of inland waters and erosion are incompletely understood and modelled ( [[#Pugh--2019a|Pugh et al., 2019a]] ; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ; [[#Krause--2020|Krause et al., 2020]] ; [[#Canadell--2021|Canadell et al., 2021]] ).

Enhanced carbon losses from terrestrial systems further limit the available carbon budget for global warming staying below 1.5°C ( [[#Rogelj--2018|Rogelj et al., 2018]] ). Analyses of satellite remote sensing and ground-based observations have indicated that, between 1982 and 2015, the CO 2 fertilisation effect has already declined, implying a negative climate system feedback ( [[#Wang--2020c|Wang et al., 2020c]] ). Peatlands, permafrost regions and tropical ecosystems are particularly vulnerable due to their large carbon stocks, in combination with over-proportional warming, increases in heat waves and droughts and/or a complex interplay of climate change and increasing atmospheric CO 2 (Sections 2.5.2.8, 2.5.2.9, 2.5.3.2).

Model projections suggest a reduction of permafrost extent and potentially large carbon losses for all warming scenarios ( [[#Canadell--2021|Canadell et al., 2021]] ). Already a mean temperature increase of 2°C could reduce the total permafrost area extent by about 5–20% by 2100 ( [[#Comyn-Platt--2018|Comyn-Platt et al., 2018]] ; [[#Yokohata--2020|Yokohata et al., 2020]] ). Associated CO 2 losses in the order of 15 Gt up to nearly 70 Gt by 2100 have been projected across a number of modelling studies ( [[#Schneider%20von%20Deimling--2015|Schneider von Deimling et al., 2015]] ; [[#Comyn-Platt--2018|Comyn-Platt et al., 2018]] ; [[#Yokohata--2020|Yokohata et al., 2020]] ). Limiting the global temperature increase to 1.5°C versus 2°C could reduce projected permafrost CO 2 losses by 2100 by 24.2 Gt (median, calculated for a 3-m depth) ( [[#Comyn-Platt--2018|Comyn-Platt et al., 2018]] ). Losses are possibly underestimated in the studies that consider only the upper permafrost layers. Likewise, the actual committed carbon loss may well be larger (e.g., eventually a loss of approx. 40% of today’s permafrost area extent if climate is stabilised at 2°C above pre-industrial levels) due to the long time scale of warming in deep permafrost layers ( [[#Chadburn--2017|Chadburn et al., 2017]] ). It is not known at which level of global warming an abrupt permafrost collapse (estimated to enhance CO 2 emissions by 40% in 2300 in a high-emissions scenario) compared to gradual thaw ( [[#Turetsky--2020|Turetsky et al., 2020]] ) would have to be considered an important additional risk. Large uncertainties arise also from interactions with changes in surface hydrology and/or northward migrating woody vegetation as climate warms, which could dampen or even reverse projected net carbon losses in some regions ( [[#McGuire--2018a|McGuire et al., 2018a]] ; [[#Mekonnen--2018|Mekonnen et al., 2018]] ; [[#Pugh--2018|Pugh et al., 2018]] ). Overall, there is ''low confidence'' on how carbon–permafrost interactions will affect future carbon cycle and climate, although net carbon losses and thus positive (amplifying) feedbacks are ''likely'' (Sections 2.5.2.10, 2.5.3.5) ( [[#Shukla--2019|Shukla et al., 2019]] ). See also WGI AR6 ( [[#Canadell--2021|Canadell et al., 2021]] ) for a discussion on impacts of higher-emission and warming scenarios.

Peatland carbon is estimated as about 550–1000 Gt in northern latitudes (many of these peatlands would be found in permafrost regions) ( [[#Turetsky--2015|Turetsky et al., 2015]] ; [[#Nichols--2019|Nichols and Peteet, 2019]] ) and &gt;100 Gt in tropical regions ( [[#Turetsky--2015|Turetsky et al., 2015]] ; [[#Dargie--2017|Dargie et al., 2017]] ). For both northern mid- and high-latitude and tropical peatlands, a shift from contemporary CO 2 sinks to sources were simulated in high-warming scenarios ( [[#Wang--2018a|Wang et al., 2018a]] ; [[#Qiu--2020|Qiu et al., 2020]] ). Due to the lack of large-scale modelling studies, there is ''low confidence'' for climate change impacts on peat carbon uptake and emissions. The largest risk to tropical peatlands is expected to arise from drainage and conversion to forestry or agriculture, which would outpace the impacts of climate change ( [[#Page--2016|Page and Baird, 2016]] ; [[#Leifeld--2019|Leifeld et al., 2019]] ; [[#Cooper--2020|Cooper et al., 2020]] ). The magnitude of possible carbon losses is uncertain, however, and depends strongly on socioeconomic scenarios (Sections 2.4.3.8, 2.4.4.2; 2.4.4.4.2, 2.5.2.8).

For tropical and subtropical regions, the interplay of atmospheric CO 2 with precipitation and temperature becomes of particular importance for future carbon uptake, since in warm and dry environments, elevated CO 2 fosters plants with C3 photosynthesis and enhances their water-use efficiency relative to C4 species ( [[#Moncrieff--2014a|Moncrieff et al., 2014a]] ; [[#Midgley--2015|Midgley and Bond, 2015]] ; [[#Knorr--2016a|Knorr et al., 2016a]] ). As a consequence, enhanced woody cover is expected to occur in the future, especially in mesic savannas, while in xeric savannas an increase in woody cover would occur in regions with enhanced precipitation ( [[#Criado--2020|Criado et al., 2020]] ). Even though semiarid regions have dominated the global trend in land CO 2 uptake in recent decades ( [[#Ahlström--2015|Ahlström et al., 2015]] ), so far, most studies that investigated future climate change impacts on savanna ecosystems have concentrated on changes in the extent of land area affected (2.5.2.5) rather than on carbon cycling, with ''medium confidence'' for increasing woody cover:grass ratios ( [[#Moncrieff--2014a|Moncrieff et al., 2014a]] ; [[#Midgley--2015|Midgley and Bond, 2015]] ; [[#Moncrieff--2016|Moncrieff et al., 2016]] ; [[#Criado--2020|Criado et al., 2020]] ). Increases in woody vegetation in what is now grass-dominated would possibly come with a carbon benefit, for instance, it was found that a broad range of future climate and CO 2 changes would enhance vegetation carbon storage on Australian savannas ( [[#Scheiter--2015|Scheiter et al., 2015]] ). Results from a number of field experiments indicate, however, that impacts on total ecosystem carbon storage may be smaller due to a loss in below-ground carbon ( [[#Coetsee--2013|Coetsee et al., 2013]] ; [[#Wigley--2020|Wigley et al., 2020]] ). [[#Nunez--2021|Nunez et al. (2021)]] critique existing incentives to promote the invasion of non-native trees into treeless areas as a means of carbon sequestration, raising doubts about the effects on fire, albedo, biodiversity and water yield (see Box 2.2).

Substantial climate change-driven impacts on tropical tree cover and vegetation type are projected in all studies, irrespective of whether or not the degree amounts to a forest “dieback” (Sections 2.4.3.6, 2.4.4.3, 2.5.2.6, 2.5.3.3) ( [[#Davies-Barnard--2015|Davies-Barnard et al., 2015]] ; [[#Wu--2016a|Wu et al., 2016a]] ; [[#Zemp--2017a|Zemp et al., 2017a]] ; [[#Canadell--2021|Canadell et al., 2021]] ) . Accordingly, models also suggest a continuation of tropical forests acting as carbon sinks ( [[#Huntingford--2013|Huntingford et al., 2013]] ; [[#Mercado--2018|Mercado et al., 2018]] ). A recent study combining field plot data with statistical models ( [[#Hubau--2020|Hubau et al., 2020]] ) indicates that, in the Amazonian and possibly also in the African forest, the carbon sink in above-ground biomass already declined in the three decades up to 2015. This trend is distinct in the Amazon whereas data from Africa suggests a possible decline after 2010. The authors estimate the vegetation carbon sink in 2030–2040 to decline to zero±0.205 PgC yr -1 in the Amazon and to 0.26±0.215 PgC yr -1 in Africa (a loss of 14% compared to the present). Their results suggest that, over time, CO 2 fertilisation is outweighed by the impacts of higher temperatures and drought that enhance tree mortality and diminish growth. The degree of thermal resilience of tropical forests is still uncertain, however ( [[#Sullivan--2020|Sullivan et al., 2020]] ).

The lack of simulation studies that seek to quantify all important interacting factors (CO 2 , drought and fire) for future carbon cycling in savannas and tropical forests and the apparent disagreement between trends projected in models compared to data-driven estimates result in ''low confidence'' regarding the direction or magnitude of carbon flux and pool-size changes. Similar to tropical peatlands, given projected human population growth and socioeconomic changes, the continued conversion of forests and savannas into agricultural or pasture systems ''very likely'' poses a significant risk of rapid carbon loss which will amplify the climate change-induced risks substantially ( ''high confidence'' ) (2.5.2.10, 2.5.3.5) ( [[#Aragao--2014|Aragao et al., 2014]] ; [[#Searchinger--2015|Searchinger et al., 2015]] ; [[#Aleman--2016|Aleman et al., 2016]] ; [[#Nobre--2016|Nobre et al., 2016]] ).

The impacts of climate-induced altered animal composition and trophic cascades on land-ecosystem carbon cycling globally are as yet unquantified ( [[#Schmitz--2018|Schmitz et al., 2018]] ), even though climate change is expected to lead to shifts in consumer–resource interactions that also contribute to losses of top predators or top herbivores (Sections 2.4.2.2, 2.5.1.3, 2.5.4; ( [[#Lurgi--2012|Lurgi et al., 2012]] ; [[#Damien--2019|Damien and Tougeron, 2019]] ). Cascading trophic effects triggered by top predators or the largest herbivores propagate through food webs and reverberate through to the functioning of whole ecosystems, notably altering productivity, carbon and nutrient turnover and net carbon storage ( ''medium confidence'' ) ( [[#Wilmers--2016|Wilmers and Schmitz, 2016]] ; [[#Sobral--2017|Sobral et al., 2017]] ; [[#Stoner--2018|Stoner et al., 2018]] ). Across different field experiments, the ecosystem consequences of the presence or absence of herbivores and carnivores have been found to be quantitatively as large as the effects of other environmental change drivers such as warming, enhanced CO 2 , fire and variable nitrogen deposition ( ''medium confidence'' ) ( [[#Hooper--2012|Hooper et al., 2012]] ; [[#Smith--2015|Smith et al., 2015]] ). Some local and regional modelling experiments have begun to explore animal impacts on vegetation dynamics and carbon and nutrient cycling ( [[#Pachzelt--2015|Pachzelt et al., 2015]] ; [[#Dangal--2017|Dangal et al., 2017]] ; [[#Berzaghi--2019|Berzaghi et al., 2019]] ). Turnover rate is the chief factor that determines future land-ecosystem carbon dynamics and hence carbon–climate feedbacks ( [[#Friend--2014|Friend et al., 2014]] ). To improve projections, it is imperative to better quantify the broader role of carnivores, grazers and browsers and the way these interact in global studies of how ecosystems respond to climate change.

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