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

==== 2.4.3.2 Global Patterns of Observed Biome Shifts Driven by Climate Change ====

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===== 2.4.3.2.1 Observed biome shifts predominantly driven by climate change =====

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AR5 and a meta-analysis found that vegetation at the biome level shifted poleward latitudinally and upward altitudinally due to anthropogenic climate change in at least 19 sites in boreal, temperate and tropical ecosystems from 1700 to 2007 ( [[#Gonzalez--2010|Gonzalez et al., 2010]] ; [[#Settele--2014|Settele et al., 2014]] ). In these areas, temperature increased to 0.4°C–1.6°C above the pre-industrial period ( [[#Gonzalez--2010|Gonzalez et al., 2010]] ; [[#Settele--2014|Settele et al., 2014]] ). Field research since the AR5 detected additional poleward and upslope biome shifts over periods of 24–210 years at numerous sites (described below), but were not directly attributed to anthropogenic climate change as the studies were not designed or conducted properly for full attribution assessment.

Many of the recently detected shifts are nevertheless consistent with climate change-induced temperature increases and observed in areas without agriculture, livestock grazing, timber harvesting and other anthropogenic land uses. For example, in the Andes Mountains in Ecuador, a biome shift was detected by comparing a survey by Alexander von Humboldt in 1802 to a re-survey in 2012, making this the longest time span in the world for this type of data ( [[#Morueta-Holme--2015|Morueta-Holme et al., 2015]] ; [[#Moret--2019|Moret et al., 2019]] ). Over 210 years, temperature increased by 1.7°C ( [[#Morueta-Holme--2015|Morueta-Holme et al., 2015]] ) and the upper edge of alpine grassland shifted 100–450 m upslope ( [[#Moret--2019|Moret et al., 2019]] ).

Other biome shifts consistent with climate change and not substantially affected by local land use include: northward shifts in Canada of deciduous forest into boreal conifer forest, 5 km from 1970–2012 ( [[#Sittaro--2017|Sittaro et al., 2017]] ) and 20 km from 1970–2014 ( [[#Boisvert-Marsh--2019|Boisvert-Marsh et al., 2019]] ) and of temperate conifer into boreal conifer forest, 21 km from 1970–2015 ( [[#Boisvert-Marsh--2021|Boisvert-Marsh and de Blois, 2021]] ). Research detected upslope shifts of boreal and sub-alpine conifer forest into alpine grassland at 143 sites on four continents (41 m from 1901–2018) ( [[#Lu--2021|Lu et al., 2021]] ) and at individual sites in Canada (54 m from 1900–2010) ( [[#Davis--2020|Davis et al., 2020]] ); China (300 m from 1910–2000) ( [[#Liang--2016|Liang et al., 2016]] ) (33 m from 1985–2014) ( [[#Du--2018|Du et al., 2018]] ); Nepal (50 m from 1860–2000) ( [[#Sigdel--2018|Sigdel et al., 2018]] ); Russia (150 m from 1954–2006) ( [[#Gatti--2019|Gatti et al., 2019]] ); and the USA (19 m from 1950–2016) ( [[#Smithers--2018|Smithers et al., 2018]] ) (38 m from 1953–2015) ( [[#Terskaia--2020|Terskaia et al., 2020]] ). Other upslope cases include shifts of temperate conifer forest in Canada ( [[#Jackson--2016|Jackson et al., 2016]] ) and the USA ( [[#Lubetkin--2017|Lubetkin et al., 2017]] ), temperate deciduous forest in Switzerland ( [[#Rigling--2013|Rigling et al., 2013]] ) and temperate shrubland in the USA ( [[#Donato--2016|Donato et al., 2016]] ).

In summary, anthropogenic climate change caused latitudinal and elevational biome shifts in at least 19 sites in boreal, temperate and tropical ecosystems between 1700 and 2007, where temperature increased to 0.4°C–1.6°C above the pre-industrial period ( ''robust evidence'' , ''high agreement'' ). Additional cases of 5–20 km northward and 20–300 m upslope biome shifts between 1860 and 2016, under a mean global temperature increase of approximately 0.9°C above the pre-industrial period, are consistent with climate change ( ''medium evidence'' , ''high agreement'' ).

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===== 2.4.3.2.2 Observed biome shifts from combined land use change and climate change =====

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Research has detected biome shifts in areas where agriculture, fire use or suppression, livestock grazing, harvesting of timber and wood for fuel and other local land use substantially altered vegetation, in addition to changes in climatic factors and CO 2 fertilisation. These studies were not designed or conducted in a manner to make climate change attribution possible, although many vegetation changes are consistent with climate change. For example, a global review of observed changes in tree lines found that, globally, two-thirds of tree lines have shifted upslope in elevation over the past 50 years or more, (( [[#Hansson--2021|Hansson et al., 2021]] ).

Upslope and poleward forest shifts have occurred where timber harvesting or livestock grazing has been abandoned, allowing the regeneration of trees at sites in Canada ( [[#Brice--2019|Brice et al., 2019]] ; [[#Wang--2020b|Wang et al., 2020b]] ), France ( [[#Feuillet--2020|Feuillet et al., 2020]] ), Italy ( [[#Vitali--2017|Vitali et al., 2017]] ), Spain ( [[#Ameztegui--2016|Ameztegui et al., 2016]] ) and the USA ( [[#Wang--2020b|Wang et al., 2020b]] ) as well as in mountainous areas across Europe ( [[#Cudlin--2017|Cudlin et al., 2017]] ). Intentional use of fire drove an upslope forest shift in Peru ( [[#Bush--2015|Bush et al., 2015]] ) while mainly human-ignited fires drove the conversion of shrubland to grassland in a drought-affected area of the USA ( [[#Syphard--2019b|Syphard et al., 2019b]] ). In eastern Canada, timber harvesting and wildfire drove the conversion of mixed conifer–broadleaf forests to broadleaf-dominated forests ( [[#Brice--2020|Brice et al., 2020]] ; [[#Wang--2020b|Wang et al., 2020b]] ).

Shrub encroachment onto savanna has occurred at numerous sites, particularly across the Southern Hemisphere, mainly between 1992 and 2010 ( [[#Criado--2020|Criado et al., 2020]] ). Globally, overgrazing initiates shrub encroachment by reducing grasses more than woody plants, while fire exclusion maintains the shrub cover ( [[#D’Odorico--2012|D’Odorico et al., 2012]] ; [[#Caracciolo--2016|Caracciolo et al., 2016]] ; [[#Bestelmeyer--2018|Bestelmeyer et al., 2018]] ). The magnitude of woody cover change in savannas is not correlated with mean annual temperature change ( [[#Criado--2020|Criado et al., 2020]] ); however, higher atmospheric CO 2 increases shrub growth in savannas ( [[#Nackley--2018|Nackley et al., 2018]] ; [[#Manea--2019|Manea and Leishman, 2019]] ). A global remote-sensing analysis of biome changes from all causes, including agricultural and grazing expansion and deforestation, estimated that 14% of pixels changed between 1981 and 2012, although this approach can overestimate global changes, since it uses a new biome classification system which doubles the conventional biome classifications ( [[#Higgins--2016|Higgins et al., 2016]] ). In addition to climate change, LULCC causes vegetation changes at the biome level ( ''robust evidence'' , ''high agreement'' ).

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