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

===== 2.3.4.3.2 Terrestrial ecosystems =====

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The AR5 WGII concluded that many terrestrial species have shifted their geographic ranges in recent decades ( ''high confidence'' ). Similarly, SRCCL assessed that many land species have experienced range size and location changes as well as altered abundances over recent decades ( ''high confidence'' ). SROCC noted that species composition and abundance have markedly changed in high mountain ecosystems in recent decades ( ''very high confidence'' ).

Paleoclimate reconstructions document large-scale biome shifts from the deep past through the Holocene (e.g., [[#Hoogakker--2016|Hoogakker et al., 2016]] ). The northernmost location of the treeline is a representative indicator in this regard (Figure 2.34). During the MPWP, boreal forest extended to the Arctic coast, with the northernmost treeline being about 4° to 10° latitude further north than at present; temperate forests and grasslands were also shifted poleward (with reduced tundra extent), while savannahs and woodlands were more expansive in Africa and Australia at the expense of deserts (Cross-Chapter Box 2.4, Figure 1b; [[#Salzmann--2008|Salzmann et al., 2008]] , 2013; [[#Sniderman--2016|Sniderman et al., 2016]] ; [[#Andrae--2018|Andrae et al., 2018]] ). During the LGM, tundra and steppe expanded whereas forests were globally reduced in extent ( [[#Prentice--2000|Prentice et al., 2000]] ; [[#Binney--2017|Binney et al., 2017]] ), the northern treeline being about 17° to 23° latitude south of its present-day location in most areas. During the LDT, pervasive ecosystem transformations occurred in response to warming and other climatic changes ( [[#Nolan--2018|Nolan et al., 2018]] ; [[#Fordham--2020|Fordham et al., 2020]] ). By the MH, North Africa had experienced a widespread conversion from grasslands to desert ( [[#Hoelzmann--1998|Hoelzmann et al., 1998]] ; [[#Prentice--2000|Prentice et al., 2000]] ; [[#Sha--2019|Sha et al., 2019]] ), and the northernmost treeline had shifted poleward again to about 1° to 3° latitude north of its current location ( [[#MacDonald--2000|MacDonald et al., 2000]] ; [[#Binney--2009|Binney et al., 2009]] ; [[#Williams--2011|Williams et al., 2011]] ). Over the past half century, there has been an increase in the spatial synchrony of annual tree growth across all continents that is unprecedented during the past millennium ( [[#Manzanedo--2020|Manzanedo et al., 2020]] ). Elevated rates of vegetation change in the Holocene are consistent with climate variability ( [[#Shuman--2019|Shuman et al., 2019]] ), intensified human land use ( [[#Fyfe--2015|Fyfe et al., 2015]] ; [[#Marquer--2017|Marquer et al., 2017]] ), and resulting increased ecosystem novelty ( [[#Finsinger--2017|Finsinger et al., 2017]] ; K.D. [[#Burke--2019|]] [[#Burke--2019|Burke et al., 2019]] ).

Long-term ecological records capture extensive range shifts during the 20th and early 21st centuries ( [[#Lenoir--2015|Lenoir and Svenning, 2015]] ; [[#Pecl--2017|Pecl et al., 2017]] ). Research has been most extensive for North America and western Eurasia, with fewer studies for central Africa, eastern Asia, South America, Greenland, and Antarctica ( [[#Lenoir--2015|Lenoir and Svenning, 2015]] ). Most documented changes are toward cooler conditions – that is, poleward and upslope ( [[#Lenoir--2008|Lenoir et al., 2008]] ; [[#Harsch--2009|Harsch et al., 2009]] ; [[#Elmendorf--2015|Elmendorf et al., 2015]] ; [[#Parmesan--2015|Parmesan and Hanley, 2015]] ; [[#Evans--2017|Evans and Brown, 2017]] ). Notably, a large, quasi-global analysis ( [[#Chen--2011|Chen et al., 2011]] ) estimated that many insect, bird, and plant species had shifted by 17 (±3) km per decade toward higher latitudes and 11 (±2) m per decade toward higher elevations since the mid-20th century, with changes in both the leading and trailing edges of species ranges ( [[#Rumpf--2018|Rumpf et al., 2018]] ). Over the past century, long-term ecological surveys also show that species turnover (i.e., the total number of gains and losses of species within an area) has significantly increased across a broad array of ecosystems ( [[#Dornelas--2014|Dornelas et al., 2014]] , 2019), including undisturbed montane areas worldwide ( [[#Gibson-Reinemer--2015|Gibson-Reinemer et al., 2015]] ). Despite global losses to biodiversity, however, most local assemblages have experienced a change in biodiversity rather than a systematic loss ( [[#Pimm--2014|Pimm et al., 2014]] ). With increased species turnover, the novelty of contemporary communities relative to historical baselines has risen ( [[#Hobbs--2009|Hobbs et al., 2009]] ; [[#Radeloff--2015|Radeloff et al., 2015]] ) due to greater spatial homogenization, mixtures of exotic and native species, altered disturbance regimes, and legacies of current or historic land use ( [[#Olden--2006|Olden and Rooney, 2006]] ; [[#Schulte--2007|Schulte et al., 2007]] ; [[#Thompson--2013|Thompson et al., 2013]] ; [[#Goring--2016|Goring et al., 2016]] ). In general, terrestrial species have had lower rates of turnover than marine species ( [[#Dornelas--2018|Dornelas et al., 2018]] ; [[#Blowes--2019|Blowes et al., 2019]] ).

There are exceptions to the general pattern of poleward/upslope migration. For some species, various biotic and abiotic factors (such as precipitation and land use) supersede the physiological effects of temperature ( [[#Vanderwal--2013|Vanderwal et al., 2013]] ; [[#Gibson-Reinemer--2015|Gibson-Reinemer and Rahel, 2015]] ; [[#Ordonez--2016|Ordonez et al., 2016]] ; [[#Scheffers--2016|Scheffers et al., 2016]] ; [[#Lenoir--2020|Lenoir et al., 2020]] ). For other species, poleward migration is slower than expectations from the observed temperature increases. Trees are one such example because of their long lifespan and gradual maturity ( [[#Renwick--2015|Renwick and Rocca, 2015]] ); in fact, poleward advance is only evident at about half of the sites in a large global dataset of treeline dynamics for 1900-present ( [[#Harsch--2009|Harsch et al., 2009]] ). Furthermore, the northernmost extent of treeline at present (roughly 73°N) is actually somewhat south of its location in the MH ( [[#MacDonald--2008|MacDonald et al., 2008]] ) despite an expanding growing season in the extratropical NH since the mid-20th century ( [[#2.3.4.3.1|Section 2.3.4.3.1]] ).

Consistent with species range shifts, SRCCL noted that there have been changes in the geographical distribution of climate zones. Poleward shifts in temperate and continental climates are evident across the globe over 1950–2010, with decreases in the area (and increases in the average elevation) of polar climates ( [[#Chan--2015|Chan and Wu, 2015]] ). Zonal changes towards higher latitudes in winter plant hardiness regions are apparent since the 1970s over the central and eastern USA, with elevational changes also being important in the western USA ( [[#Daly--2012|Daly et al., 2012]] ). A clear northward shift in winter plant hardiness zones is detectable across western Canada since 1930, with somewhat lesser changes in the south-eastern part of the country ( [[#McKenney--2014|McKenney et al., 2014]] ). A northward migration of agro-climate zones is also evident over Europe since the mid-1970s ( [[#Ceglar--2019|Ceglar et al., 2019]] ). In addition, a shift toward more arid climate zones is apparent in some areas, such as the Asian monsoon region ( [[#Son--2015|Son and Bae, 2015]] ) as well as parts of South America and Africa ( [[#Spinoni--2015|Spinoni et al., 2015]] ).

In summary, there is ''very high confidence'' that many terrestrial species have shifted their geographic ranges poleward and/or upslope over the past century, with increased rates of species turnover. There is ''high confidence'' that the geographical distribution of climate zones has shifted in many parts of the world.

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