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

==== 2.4.3.8 Observed Changes in Peatlands ====

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Globally, peatland ecosystems store approximately 25% (600 ± 100 GtC) of the world’s soil organic carbon ( [[#Yu--2010|Yu et al., 2010]] ; [[#Page--2011|Page et al., 2011]] ; [[#Hugelius--2020|Hugelius et al., 2020]] ) and 10% of the world’s freshwater resources ( [[#Joosten--2002|Joosten and Clarke, 2002]] ), despite only occupying 3% of the global land area ( [[#Xu--2018a|Xu et al., 2018a]] ). The long-term role of northern peatlands in the carbon cycle was mentioned for the first time in IPCC AR4 ( [[#IPCC--2007|IPCC, 2007]] ), while SR1.5 briefly mentioned the combined effects of changes in climate and land use on peatlands ( [[#IPCC--2018b|IPCC, 2018b]] ). New evidence confirms that climate change, including extreme weather events (e.g., droughts; [[IPCC:Wg2:Chapter:Chapter-8#8.3.1|Section 8.3.1.6]] ), permafrost degradation ( [[#2.3|Section 2.3.2.5]] ), SLR ( [[#2.3.3.3|Section 2.3.3.3]] ) and fire ( [[IPCC:Wg2:Chapter:Chapter-5#5.4.3.2|Section 5.4.3.2]] ) ( [[#Henman--2008|Henman and Poulter, 2008]] ; [[#Kirwan--2012|Kirwan and Mudd, 2012]] ; [[#Turetsky--2015|Turetsky et al., 2015]] ; [[#Page--2016|Page and Hooijer, 2016]] ; [[#Swindles--2019|Swindles et al., 2019]] ; [[#Hoyt--2020|Hoyt et al., 2020]] ; [[#Hugelius--2020|Hugelius et al., 2020]] ; [[#Jovani-Sancho--2021|Jovani-Sancho et al., 2021]] ; [[#Veraverbeke--2021|Veraverbeke et al., 2021]] ), superimposed on anthropogenic disturbances (e.g., draining for agriculture or mining; [[IPCC:Wg2:Chapter:Chapter-5#5.2.1|Section 5.2.1.1]] ), has led to rapid losses of peatland carbon across the world ( ''robust evidence'' , ''high agreement'' ) ( [[#Page--2011|Page et al., 2011]] ; [[#Leifeld--2019|Leifeld et al., 2019]] ; [[#Hoyt--2020|Hoyt et al., 2020]] ; [[#Turetsky--2020|Turetsky et al., 2020]] ; [[#Loisel--2021|Loisel et al., 2021]] ). Other essential peatland ecosystem services, such as water storage and biodiversity, are also being lost worldwide ( ''robust evidence'' , ''high agreement'' ) ( [[#Bonn--2014|Bonn et al., 2014]] ; [[#Martin-Ortega--2014|Martin-Ortega et al., 2014]] ; [[#Tiemeyer--2017|Tiemeyer et al., 2017]] ).

The switch from carbon sink to carbon source in peatlands globally is mainly attributable to changes in the depth of the water table, regardless of management or status ( ''robust evidence'' , ''high agreement'' ) ( [[#Lafleur--2005|Lafleur et al., 2005]] ; [[#Dommain--2011|Dommain et al., 2011]] ; [[#Lund--2012|Lund et al., 2012]] ; [[#Cobb--2017|Cobb et al., 2017]] ; [[#Evans--2021|Evans et al., 2021]] ; [[#Novita--2021|Novita et al., 2021]] ). Across the temperate and tropical biomes, extensive drainage and deforestation have caused widespread water table draw-downs and/or peat subsidence, as well as high CO 2 emissions ( ''medium evidence'' , ''high agreement'' ). Climate change is compounding these impacts ( ''medium evidence'' , ''medium agreement'' ). For example, in Indonesia, the highest emissions from drained tropical peatlands were reported in the extremely dry year of the 1997 El Niño (810–2570 TgC yr -1 ) ( [[#Page--2002|Page et al., 2002]] ) and the 2015 fire season (380 TgC yr -1 ) ( [[#Field--2016|Field et al., 2016]] ). These prolonged dry seasons have also led to tree die-offs and fires, which are relatively new phenomena at these latitudes ( ''medium evidence'' , ''high agreement'' ) ( [[#Cole--2015|Cole et al., 2015]] ; [[#Mezbahuddin--2015|Mezbahuddin et al., 2015]] ; [[#Fanin--2017|Fanin and van der Werf, 2017]] ; [[#Taufik--2017|Taufik et al., 2017]] ; [[#Cole--2019|Cole et al., 2019]] ). Low soil moisture contributes to increased fire propagation ( [[IPCC:Wg2:Chapter:Chapter-12#12.4|Section 12.4.2.2]] ) ( [[#Dadap--2019|Dadap et al., 2019]] ; [[#Canadell--2021|Canadell et al., 2021]] ), causing long-lasting fires responsible for smoke and haze pollution ( ''robust evidence'' , ''high agreement'' ) ( [[#Ballhorn--2009|Ballhorn et al., 2009]] ; [[#Page--2009|Page et al., 2009]] ; [[#Gaveau--2014|Gaveau et al., 2014]] ; [[#Huijnen--2016|Huijnen et al., 2016]] ; [[#Page--2016|Page and Hooijer, 2016]] ; [[#Hu--2018|Hu et al., 2018]] ; [[#Vadrevu--2019|Vadrevu et al., 2019]] ; [[#Niwa--2021|Niwa et al., 2021]] ). Increases in fires and smoke lead to habitat loss and negatively impact regional faunal populations ( ''limited evidence'' , ''high agreement'' ) ( [[#Neoh--2015|Neoh et al., 2015]] ; [[#Erb--2018b|Erb et al., 2018b]] ; [[#Thornton--2018|Thornton et al., 2018]] ).

In large, lowland tropical peatland basins that are less impacted by anthropogenic activities (i.e., the Amazon and Congo river basins), the direct impact of climate change is that of a decreased carbon sink ( ''limited evidence'' , ''medium agreement'' ) ( [[#Roucoux--2013|Roucoux et al., 2013]] ; [[#Gallego-Sala--2018|Gallego-Sala et al., 2018]] ; [[#Wang--2018a|Wang et al., 2018a]] ; [[#Dargie--2019|Dargie et al., 2019]] ; [[#Ribeiro--2021|Ribeiro et al., 2021]] ). As for the temperate and boreal regions, climatic drying also tends to promote peat oxidation and carbon loss to the atmosphere ( ''medium evidence'' , ''medium agreement'' ) ( [[#2.3.1|Section 2.3.1.3.4]] ) ( [[#Helbig--2020|Helbig et al., 2020]] ; [[#Zhang--2020|Zhang et al., 2020]] ). In Europe, increasing mean annual temperatures in the Baltic, Scandinavia, and continental Europe ( [[IPCC:Wg2:Chapter:Chapter-12#12.4|Section 12.4.5.1]] ) have led to widespread lowering of peatland water tables at intact sites ( [[#Swindles--2019|Swindles et al., 2019]] ), desiccation and die-off of sphagnum moss ( [[#Bragazza--2008|Bragazza, 2008]] ; [[#Lees--2019|Lees et al., 2019]] ) and increased intensity and frequency of fires, resulting in a rapid carbon loss ( [[#Davies--2013|Davies et al., 2013]] ; [[#Veraverbeke--2021|Veraverbeke et al., 2021]] ). Nevertheless, longer growing seasons and warmer, wetter climates have increased carbon accumulation and promoted thick deposits regionally, as reported for some North American sites ( ''limited evidence'' , ''medium agreement'' ) ( [[#Cai--2011|Cai and Yu, 2011]] ; [[#Shiller--2014|Shiller et al., 2014]] ; [[#Ott--2016|Ott and Chimner, 2016]] ).

In high-latitude peatlands, the net effect of climate change on the permafrost peatland carbon sink capacity remains uncertain ( [[#Abbott--2016|Abbott et al., 2016]] ; [[#McGuire--2018b|McGuire et al., 2018b]] ; [[#Laamrani--2020|Laamrani et al., 2020]] ; [[#Loisel--2021|Loisel et al., 2021]] ; [[#Sim--2021|Sim et al., 2021]] ; [[#Väliranta--2021|Väliranta et al., 2021]] ). Increasing air temperatures have been linked to permafrost degradation and altered hydrological regimes (2.3.3.2; Figure 2.4a; 2.4.3.9; Box 5.1), which have led to rapid changes in plant communities and bio-geochemical cycling ( ''robust evidence'' , ''high agreement'' ) ( [[#Liljedahl--2016|Liljedahl et al., 2016]] ; [[#Swindles--2016|Swindles et al., 2016]] ; [[#Voigt--2017|Voigt et al., 2017]] ; [[#Zhang--2017b|Zhang et al., 2017b]] ; [[#Voigt--2020|Voigt et al., 2020]] ; [[#Sim--2021|Sim et al., 2021]] ). In many instances, permafrost degradation triggers thermokarst land subsidence associated with local wetting ( ''robust evidence'' , ''high agreement'' ) ( [[#Jones--2013|Jones et al., 2013]] ; [[#Borge--2017|Borge et al., 2017]] ; [[#Olvmo--2020|Olvmo et al., 2020]] ; [[#Olefeldt--2021|Olefeldt et al., 2021]] ). Permafrost thaw in peatland-rich landscapes can also cause local drying through increased hydrological connectivity and runoff ( [[#Connon--2014|Connon et al., 2014]] ). In the first decades following thaw, increases in methane, CO 2 and nitrous oxide emissions have been recorded from peatland sites, depending on surface moisture conditions ( [[#Schuur--2009|Schuur et al., 2009]] ; [[#O’Donnell--2012|O’Donnell et al., 2012]] ; [[#Elberling--2013|Elberling et al., 2013]] ; [[#Matveev--2016|Matveev et al., 2016]] ; [[#Euskirchen--2020|Euskirchen et al., 2020]] ; [[#Hugelius--2020|Hugelius et al., 2020]] ). Conversely, some evidence suggests increased peat accumulation after thaw ( [[#Jones--2013|Jones et al., 2013]] ; [[#Estop-Aragonés--2018|Estop-Aragonés et al., 2018]] ; [[#Väliranta--2021|Väliranta et al., 2021]] ). There is also a need to consider the impact of wildfire on permafrost thaw, due to its effect on soil temperature regime ( [[#Gibson--2018|Gibson et al., 2018]] ), as fire intensity and frequency have increased across the boreal and Arctic biomes ( ''limited evidence'' , ''high agreement'' ) ( [[#Kasischke--2010|Kasischke et al., 2010]] ; [[#Scholten--2021|Scholten et al., 2021]] ).

The CO 2 emissions from degrading peatlands is contributing to climate change in a positive feedback loop ( ''robust evidence'' , ''high agreement)'' . At mid-latitudes, widespread anthropogenic disturbance led to large historical GHG emissions and current legacy emissions of 0.15 PgC yr -1 between 1990 and 2000 ( ''limited evidence'' , ''high agreement'' ) ( [[#Maljanen--2010|Maljanen et al., 2010]] ; [[#Tiemeyer--2016|Tiemeyer et al., 2016]] ; [[#Drexler--2018|Drexler et al., 2018]] ; [[#Qiu--2021|Qiu et al., 2021]] ). About 80 million hectares of peatland have been converted to agriculture, equivalent to 72 PgC emissions in 850–2010 CE ( [[#Leifeld--2019|Leifeld et al., 2019]] ; [[#Qiu--2021|Qiu et al., 2021]] ). In Southeast Asia (SEA), an estimated 20–25 Mha of peatlands have been converted to agriculture with carbon currently being lost at a rate of ~155 ± 30 MtC yr −1 ( [[#Miettinen--2016|Miettinen et al., 2016]] ; [[#Leifeld--2019|Leifeld et al., 2019]] ; [[#Hoyt--2020|Hoyt et al., 2020]] ). Extensive deforestation and drainage have caused widespread peat subsidence and large CO 2 emissions at a current average of ~10 ± 2 tonnes ha -1 yr -1 , excluding fires ( [[#Hoyt--2020|Hoyt et al., 2020]] ), with values estimated from point subsidence measurements being as high as 30–90 tonnes CO 2 ha −1 yr −1 locally ( ''robust evidence'' , ''high agreement'' ) ( [[#Wösten--1997|Wösten et al., 1997]] ; [[#Matysek--2018|Matysek et al., 2018]] ; [[#Swails--2018|Swails et al., 2018]] ; [[#Evans--2019|Evans et al., 2019]] ; [[#Conchedda--2020|Conchedda and Tubiello, 2020]] ; [[#Anshari--2021|Anshari et al., 2021]] ). On average, at the global scale, increases in GHG emissions from peatlands have primarily come from the compounded effects of LUC, drought and fire, with additional emissions from some thawing-permafrost peatlands ( ''robust evidence'' , ''high agreement'' ).

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