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

==== 2.5.2.2 Projected Changes Globally at the Biome Level ====

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Climate change and the associated change in atmospheric CO 2 levels already exacerbate other human-caused impacts on the structure and composition of land and freshwater ecosystems, such as LULCC, nitrogen deposition and pollution. The relative importance of these drivers for ecosystems over the coming decades will likely differ between biomes, but climate change and atmospheric CO 2 will be pervasive unless there is a rapid lowering of fossil-fuel emissions and warming ( ''high confidence'' ) ( [[#Pereira--2010|Pereira et al., 2010]] ; [[#Warren--2011|Warren, 2011]] ; [[#Ostberg--2013|Ostberg et al., 2013]] ; [[#Davies-Barnard--2015|Davies-Barnard et al., 2015]] ; [[#Pecl--2017|Pecl et al., 2017]] ; [[#Ostberg--2018|Ostberg et al., 2018]] ). Global vegetation and ESMs agree on climate change-driven shifts of biome boundaries of potentially hundreds of kilometres over this century, combined with several substantial alterations that take place within biomes (e.g., changes in phenology, canopy structure and functional diversity, etc.). Large discrepancies exist between models and between scenarios regarding the region and the speed of change ( [[#Gonzalez--2010|Gonzalez et al., 2010]] ; [[#Pereira--2010|Pereira et al., 2010]] ; [[#Pecl--2017|Pecl et al., 2017]] ), but robust understanding is emerging in that the degree of impact increases in high-emission and high-warming scenarios ( ''high confidence'' ) (Figure 2.9).

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[[File:f307bbaf4831631b9ffa621bd753e09d IPCC_AR6_WGII_Figure_2_009.png]]

'''Figure 2.9 | Projected fraction of global terrestrial area that could experience a biome shift by 2100.''' Shifts due to climate change (filled symbols) or a combination of climate change and LUC (outline symbols), from publications in Supplementary Table SM2 '''.''' 3 (projected vulnerabilities and risks of ecosystems to biome shifts). Filled circles ( [[#Bergengren--2011|Bergengren et al., 2011]] ), filled squares ( [[#Alo--2008|Alo and Wang, 2008]] ), filled diamonds ( [[#Gonzalez--2010|Gonzalez et al., 2010]] ), filled triangle pointing up ( [[#Scholze--2006|Scholze et al., 2006]] ), filled triangle pointing down ( [[#Sitch--2008|Sitch et al., 2008]] ), filled triangle on its side ( [[#Li--2018|Li et al., 2018]] ), filled cross ( [[#Warszawski--2013|Warszawski et al., 2013]] ), outlined circle ( [[#Ostberg--2018|Ostberg et al., 2018]] )and outlined diamond ( [[#Eigenbrod--2015|Eigenbrod et al., 2015]] ).

Substantial changes in vegetation structure and ecosystem processes are already happening (see [[#2.4|Section 2.4]] ). Many of these observations have already been projected to take place as early as at least IPCC AR3 ( [[#Rosenzweig--2007|Rosenzweig et al., 2007]] ), and can they now be increasingly tested for their robustness with observational evidence. These multiple changes in response to warming (and changes in precipitation and increasing atmospheric CO 2 levels that go hand-in-hand with warming) are further expected for already relatively small additional temperature increases. In particular, in cold (boreal and tundra) regions, as well as in dry regions ( ''high confidence'' ), alterations of 2–47% of the areal extent of terrestrial ecosystems in scenarios of &lt;2°C warming above pre-industrial levels have been projected, increasing drastically with higher-warming scenarios ( [[#Warren--2011|Warren, 2011]] ; [[#Wårlind--2014|Wårlind et al., 2014]] ). More recent work, applying also probabilistic methods, confirm the risk of drastic changes in vegetation cover (e.g., forest to non-forest or vice versa) at the end of the 21st century even for approximately 2°C warming scenarios, especially in tundra, and also in tropical forest and savannah regions, with more subtle changes (within a given biome type) likely to occur in all regions ( [[#Ostberg--2013|Ostberg et al., 2013]] ; [[#Ostberg--2018|Ostberg et al., 2018]] ). Model studies have found 5–20% of terrestrial ecosystems affected by warming of around 2°C–3°C, increasing to above one-third at a warming of 4°C–5°C ( [[#Ostberg--2013|Ostberg et al., 2013]] ; [[#Warszawski--2013|Warszawski et al., 2013]] ).

In general, vegetation types are projected to be moving into their ‘neighbouring’ climates, depending on whether temperature or precipitation is expected to be the predominant factor and how vegetation interacts with the increasing CO 2 levels in the atmosphere ( [[#Wårlind--2014|Wårlind et al., 2014]] ; [[#Scheiter--2015|Scheiter et al., 2015]] ; [[#Schimel--2015|Schimel et al., 2015]] ; [[#Huntzinger--2017|Huntzinger et al., 2017]] ). For instance, boreal or temperate forest vegetation is simulated to migrate polewards, closed tropical (moist) forest is expected to transition towards dry tropical forest types, while climate-driven degradation might expand arid vegetation cover (Sections 2.5.2.2–2.5.2.9). However, ‘novel ecosystems’, that is, communities with no current or historical equivalent because of the novel combinations of abiotic conditions under climate change, are expected to be increasingly common in the future ( ''medium confidence'' ), although the regions where these novel ecosystems might emerge are still disputed ( [[#Reu--2014|Reu et al., 2014]] ; [[#Radeloff--2015|Radeloff et al., 2015]] ; [[#Ordonez--2016|Ordonez et al., 2016]] ). The possibility of these novel ecosystems and the communities that live within them are a challenge for current modelling of ecosystem shifts, and new approaches to conservation will be required that are designed to adapt to rapid changes in species composition and the ensuing challenges.

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