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

===== 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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