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

==== 2.4.4.4 Observed Terrestrial Ecosystem Carbon ====

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===== 2.4.4.4.1 Observed terrestrial ecosystem carbon globally =====

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Terrestrial ecosystems contain carbon stocks: 450 GtC (range 380–540 GtC) in vegetation, 1700 ± 250 GtC in soils that are not permanently frozen and 1400 ± 200 GtC in permafrost ( [[#Hugelius--2014|Hugelius et al., 2014]] ; [[#Batjes--2016|Batjes, 2016]] ; [[#Jackson--2017|Jackson et al., 2017]] ; [[#Strauss--2017|Strauss et al., 2017]] ; [[#Erb--2018a|Erb et al., 2018a]] ; [[#Xu--2021a|Xu et al., 2021a]] ). Ecosystem carbon stocks, totalling 3000–4000 GtC (from the lowest and highest estimates above), substantially exceed the ~900 GtC carbon in unextracted fossil fuels (see( [[#Canadell--2021|Canadell et al., 2021]] )).

Deforestation, draining of peatlands and the expansion of agricultural fields, livestock pastures and human settlements and other LULCCs emitted carbon at a rate of 1.6 ± 0.7 Gt yr -1 from 2010 to 2019, ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ), of which wildfires and peat burning emitted 0.4 ± 0.2 Gt yr -1 from 1997 to 2016 ( [[#van%20der%20Werf--2017|van der Werf et al., 2017]] ). Anthropogenic climate change has caused some of these emissions through increases in wildfire ( [[#2.4.4.2.1|Section 2.4.4.2.1]] ) and tree mortality ( [[#2.4.4.3.1|Section 2.4.4.3.1]] ), but the fraction of the total remains unquantified. LUC produced ~15% of global anthropogenic emissions, from fossil fuels and land ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). Terrestrial ecosystems removed carbon from the atmosphere through plant growth at a rate of -3.4 ± 0.9 Gt yr -1 from 2010 to 2019 ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ).

Tropical deforestation and the draining and burning of peatlands produce almost all of the carbon emissions from LUC ( [[#Houghton--2017|Houghton and Nassikas, 2017]] ; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ), while forest growth accounts for two-thirds of ecosystem carbon removals from the atmosphere ( [[#Pugh--2019b|Pugh et al., 2019b]] ). Global terrestrial ecosystems comprised a net sink of -1.9 ± 1.1 Gt yr -1 from 2010 to 2019 ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ), mainly due to growth in forests ( [[#Harris--2021|Harris et al., 2021]] ; [[#Xu--2021a|Xu et al., 2021a]] ), mitigating ~31% of global emissions from the burning of fossil fuels and LUC ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ).

In summary, terrestrial ecosystems contain 3000–4000 GtC in vegetation, permafrost and soils, three to five times the amount of carbon in unextracted fossil fuels and 4.4 times the carbon currently in the atmosphere ( ''robust evidence'' , ''high agreement'' ). Tropical deforestation, the draining and burning of peatlands and other LULCCs emit 0.9–2.3 GtC yr -1 , ~15% of the global emissions from fossil fuels and ecosystems ( ''robust evidence'' , ''high agreement'' ). Terrestrial ecosystems currently remove more carbon from the atmosphere (-3.4±0.9 Gt yr -1 ) than they emit (+1.6±0.7 Gt yr -1 ), a net sink of -1.9±1.1 Gt yr -1 ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ) . Thus, tropical rainforests, Arctic permafrost and other ecosystems provide the global ecosystem service of naturally preventing carbon from contributing to climate change ( ''high confidence'' ).

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===== 2.4.4.4.2 Observed stocks in high-carbon terrestrial ecosystems =====

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The ecosystem that attains the highest above-ground carbon density in the world is the coast redwood ( ''Sequoia sempervirens'' ) forest in California, USA, with 2600 ± 100 tonnes ha -1 carbon ( [[#Van%20Pelt--2016|Van Pelt et al., 2016]] ). The ecosystem with the second highest documented carbon density in the world is the mountain ash ( ''Eucalyptus regnans'' ) forest in Victoria, Australia, with ~1900 tonnes ha -1 ( [[#Keith--2009|Keith et al., 2009]] ). In the Tropics, tropical evergreen broadleaf forests (rainforests) in the Amazon, the Congo and Indonesia attain the highest carbon densities, reaching a maximum of 230 tonnes ha -1 in the Amazon ( [[#Mitchard--2014|Mitchard et al., 2014]] ) and the Congo ( [[#Xu--2017|Xu et al., 2017]] ). Temperature increases reduce the tropical rainforest above-ground carbon density 9.1 tonnes ha -1 per degree Celsius, through reduced growth and increased tree mortality ( [[#Sullivan--2020|Sullivan et al., 2020]] ).

Tropical forests contain the largest vegetation carbon stocks in the world, with 180–250 GtC above and below ground ( [[#Saatchi--2011|Saatchi et al., 2011]] ; [[#Baccini--2012|Baccini et al., 2012]] ; [[#Avitabile--2016|Avitabile et al., 2016]] ). The Amazon contains a stock of 45–60 GtC ( [[#Baccini--2012|Baccini et al., 2012]] ; [[#Mitchard--2014|Mitchard et al., 2014]] ; [[#Englund--2017|Englund et al., 2017]] ).

Ecosystems with high soil carbon densities include the peat bogs in Ireland with up to 3000 tonnes ha -1 ( [[#Tomlinson--2005|Tomlinson, 2005]] ), the Cuvette Centrale swamp forest peatlands in Congo with an average of ~2200 tonnes ha -1 ( [[#Dargie--2017|Dargie et al., 2017]] ), the Arctic tundra with an average of ~900 tonnes ha -1 ( [[#Tarnocai--2009|Tarnocai et al., 2009]] ) and the mangrove peatlands in Kalimantan, Indonesia, with an average of 850 ± 320 tonnes ha -1 ( [[#Murdiyarso--2015|Murdiyarso et al., 2015]] ). Arctic permafrost contains 1400 ± 200 GtC to a depth of 3 m, the largest soil carbon stock in the world ( [[#Hugelius--2014|Hugelius et al., 2014]] ). Globally, peatlands contain 470–620 GtC ( [[#Page--2011|Page et al., 2011]] ; [[#Hodgkins--2018|Hodgkins et al., 2018]] ), of which boreal and temperate peatlands contain 415 ± 150 GtC ( [[#Hugelius--2020|Hugelius et al., 2020]] ) and tropical peatlands contain 80–350 GtC ( [[#Page--2011|Page et al., 2011]] ; [[#Dargie--2017|Dargie et al., 2017]] ; [[#Gumbricht--2017|Gumbricht et al., 2017]] ; [[#Ribeiro--2021|Ribeiro et al., 2021]] ). Other analyses increase the upper estimates for boreal and temperate peatlands to 800–1200 GtC ( [[#Nichols--2019|Nichols and Peteet, 2019]] ; [[#Mishra--2021b|Mishra et al., 2021b]] ).

Tropical forests and Arctic permafrost contain the highest ecosystem carbon stocks in above-ground vegetation and soil, respectively, in the world ( ''robust evidence'' , ''high agreement'' ). These ecosystems form natural sinks that prevent the emission to the atmosphere of 1400–1800 GtC that would otherwise increase the magnitude of climate change ( ''high confidence'' ).

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===== 2.4.4.4.3 Biodiversity and observed terrestrial ecosystem carbon =====

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High biodiversity and ecosystem carbon generally occur together, with rainforests in the Amazon, Congo and Indonesia containing the largest above-ground vegetation carbon stocks ( [[#Saatchi--2011|Saatchi et al., 2011]] ; [[#Baccini--2012|Baccini et al., 2012]] ; [[#Avitabile--2016|Avitabile et al., 2016]] ) and the highest vascular plant species richness ( [[#Kreft--2007|Kreft and Jetz, 2007]] ) in the world. Above-ground ecosystem carbon and animal species richness show high correlation but also high spatial variability ( [[#Strassburg--2010|Strassburg et al., 2010]] ). Above-ground carbon is correlated to genus richness globally ( [[#Cavanaugh--2014|Cavanaugh et al., 2014]] ), but to species richness only in local areas ( [[#Poorter--2015|Poorter et al., 2015]] ; [[#Sullivan--2017|Sullivan et al., 2017]] ). Species richness generally increases vegetation productivity in the humid tropics while tree abundance increases productivity in drier conditions ( [[#Madrigal-Gonzalez--2020|Madrigal-Gonzalez et al., 2020]] ). Across the Amazon, ~1% of tree species contain 50% of the above-ground carbon, due to abundance and maximum height ( [[#Fauset--2015|Fauset et al., 2015]] ). Above-ground carbon in tropical forests shows positive correlations to vertebrate species richness (P values not reported) ( [[#Deere--2018|Deere et al., 2018]] ; [[#Di%20Marco--2018|Di Marco et al., 2018]] ). In logged and burned tropical forest in Brazil, species richness of plants, birds and beetles increased with carbon density up to ~100 tonnes ha -1 ( [[#Ferreira--2018|Ferreira et al., 2018]] ).

National parks and other protected areas which, in June 2021, covered 15.7% of global terrestrial area (UNEP-WCMC et al., 2021) contain ~90 GtC in vegetation and ~150 GtC in soil (one-fifth and one-tenth, respectively, of global stocks) and remove carbon from the atmosphere at a rate of ~0.5 Gt yr -1 (one-sixth of global removals) ( [[#Melillo--2016|Melillo et al., 2016]] ). The most strictly protected areas contain carbon at higher densities, but illegal deforestation and fires in some protected areas emit 38 ± 17 Mt yr -1 globally ( [[#Collins--2017|Collins and Mitchard, 2017]] ). In the Amazon, protected areas store more than half of the above-ground vegetation carbon stocks of the region, but account for only one-tenth of net emissions ( [[#Walker--2020|Walker et al., 2020]] ). Conservation of high biodiversity areas, particularly in protected areas, protects ecosystem carbon, prevents emissions to the atmosphere and reduces the magnitude of climate change ( ''high confidence'' ).

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===== 2.4.4.4.4 Observed emissions and removals from high-carbon terrestrial ecosystems =====

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Most global deforestation is occurring in tropical forests ( [[#Pan--2011|Pan et al., 2011]] ; [[#Liu--2015|Liu et al., 2015]] ; [[#Houghton--2017|Houghton and Nassikas, 2017]] ; [[#Erb--2018a|Erb et al., 2018a]] ; [[#Li--2018|Li et al., 2018]] ; [[#Harris--2021|Harris et al., 2021]] ), primarily as a result of clearing for agricultural land ( [[#Hong--2021|Hong et al., 2021]] ), causing primary tropical forest to comprise a net source of carbon from 2001 to 2019: emissions to the atmosphere 0.6 GtC yr -1 , removals from the atmosphere -0.5 GtC yr -1 and net 0.1 GtC yr -1 ( [[#Harris--2021|Harris et al., 2021]] ). While wildfires emitted an average of 0.4 ± 0.2 GtC yr -1 from 1997 to 2016 ( [[#van%20der%20Werf--2017|van der Werf et al., 2017]] ), individual fire seasons can emit the same magnitude, such as the 0.4 GtC from the Amazon fires of 2007 ( [[#Aragao--2018|Aragao et al., 2018]] ), the 0.5 GtC from the Amazon fires of 2015–2016 ( [[#Berenguer--2021|Berenguer et al., 2021]] ) and the 0.2 Gt from the Australia fires of 2019–2020 ( [[#Shiraishi--2021|Shiraishi and Hirata, 2021]] ). Wildfires thus account for up to one-third of annual average ecosystem carbon emissions, while major fire seasons can emit up to two-thirds of global ecosystem carbon ( ''medium evidence'' , ''medium agreement'' ).

Primary boreal and temperate forests also comprised net sources in the period 2001–2019; however, when including all tree age classes, boreal, temperate and tropical forests were net sinks (boreal -1.6 ± 1.1 Gt yr -1 , temperate -3.6 ± 48 Gt yr -1 ), as growth exceeded permanent forest cover losses ( [[#Harris--2021|Harris et al., 2021]] ), with boreal and temperate forests being much stronger sinks ( [[#Pan--2011|Pan et al., 2011]] ; [[#Liu--2015|Liu et al., 2015]] ; [[#Houghton--2017|Houghton and Nassikas, 2017]] ). Estimates of carbon removals from remote sensing may provide more accurate estimates of boreal forest carbon balances than ESMs which overestimate regrowth after timber harvesting and other disturbance ( [[#Wang--2021a|Wang et al., 2021a]] ). Mortality of the boreal forest in British Columbia from mountain pine beetle infestations converted 374,000 km 2 from a net carbon sink to a net carbon source ( [[#Kurz--2008|Kurz et al., 2008]] ). Modelling suggests that a potential increase in water-use efficiency and regrowth could offset the losses in part of the forest mortality area ( [[#Giles-Hansen--2021|Giles-Hansen et al., 2021]] ).

The Amazon as a whole was a net carbon emitter in the period 2003–2008 ( [[#Exbrayat--2015|Exbrayat and Williams, 2015]] ; [[#Yang--2018b|Yang et al., 2018b]] ), primarily due to the expansion of agricultural and livestock areas, which caused over two-thirds of deforestation from 1990 to 2005 ( [[#De%20Sy--2015|De Sy et al., 2015]] ; [[#De%20Sy--2019|De Sy et al., 2019]] ). Four sites in the Amazon also showed net carbon emissions in the period 2010–2018, from deforestation and fire ( [[#Gatti--2021|Gatti et al., 2021]] ). In the Amazon, deforestation emitted 0.17 ± 0.05 GtC yr -1 from 2001 to 2015 ( [[#Silva%20Junior--2020|Silva Junior et al., 2020]] ) while fires emitted 0.12 ± 0.14 GtC yr -1 from 2003 to 2015 ( [[#Aragao--2018|Aragao et al., 2018]] ). An analysis of the Amazon carbon loss from deforestation and degradation estimated a loss of 0.5 Gt yr -1 in the period 2010 -2019, with degradation accounting for three-quarters ( [[#Qin--2021|Qin et al., 2021]] ). Intact old-growth Amazon rainforest has been a net carbon sink from 2000 to 2010 (-0.45 Gt yr -1 , min. 0.31, max. 0.57) ( [[#Hubau--2020|Hubau et al., 2020]] ) but may have become a net carbon source in 2010–2019 (0.67 Gt, for the entire period, uncertainty not reported) ( [[#Qin--2021|Qin et al., 2021]] ). These factors combined—recent impacts of climate change on undisturbed forest, coupled with deforestation and agricultural expansion, along with associated intentional burning—have caused Amazon rainforest to become an overall net carbon emitter ''(medium confidence).''

In Indonesia and Malaysia, draining and burning of peat swamp forests for oil palm plantations emitted 60–260 MtC yr -1 from 1990 to 2015, converting peatlands in that period from a carbon sink to a carbon source ( [[#Miettinen--2017|Miettinen et al., 2017]] ; [[#Wijedasa--2018|Wijedasa et al., 2018]] ; [[#Cooper--2020|Cooper et al., 2020]] ). Deforestation of mangrove forests caused 10–30% of deforestation emissions in Indonesia from 1980 to 2005 ( [[#Donato--2011|Donato et al., 2011]] ; [[#Murdiyarso--2015|Murdiyarso et al., 2015]] ), even though mangroves comprised only 3% of Indonesia primary forest area in 2000 ( [[#Margono--2014|Margono et al., 2014]] ; [[#Murdiyarso--2015|Murdiyarso et al., 2015]] ).

In North America, wildfire emitted 0.1 ± 0.02 GtC yr -1 from in the period 1990–2012, but regrowth was slightly greater, producing a net sink ( [[#Chen--2017|Chen et al., 2017]] ). In California, USA, two-thirds of the 70 MtC emitted from natural ecosystems in 2001–2010 came from the 6% of the area that burned ( [[#Gonzalez--2015|Gonzalez et al., 2015]] ). Anthropogenic climate change caused up to half of the burned area ( [[#2.4.4.2.1|Section 2.4.4.2.1]] ).

In the Arctic, anthropogenic climate change has thawed permafrost ( [[#Guo--2020|Guo et al., 2020]] ), leading to emissions of 1.7 ± 0.8 GtC yr -1 in winter in the period 2003–2017 ( [[#Natali--2019|Natali et al., 2019]] ). Wildfires in the Arctic tundra in Alaska from ~1930 to 2010 caused up to a depth of 0.5 m of permafrost thaw ( [[#Brown--2015|Brown et al., 2015]] ), exposing peatland carbon ( [[#Brown--2015|Brown et al., 2015]] ; [[#Gibson--2018|Gibson et al., 2018]] ) including soil carbon deposits up to 1600 years old (Walker et al., 2019).

Tropical deforestation, the draining and burning of peatlands and the thawing of Arctic permafrost due to climate change have caused these ecosystems to emit more carbon to the atmosphere than they naturally remove through vegetation growth ( ''high confidence'' ).

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