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

==== 2.3.4.3 Terrestrial Biosphere ====

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===== 2.3.4.3.1 Growing season and phenology changes =====

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The AR5 WGII briefly discussed large-scale changes in the length of the growing season but made no confidence statement about observed trends. However, AR5 did conclude with ''high confidence'' that warming contributed to an overall spring advancement in the NH.

Recent in situ analyses document increases in the length of the thermal growing season (i.e., the period of the year when temperatures are warm enough to support growth) over much of the extratropical land surface since at least the mid-20th century. Over the NH as a whole, an increase of about 2.0 days per decade is evident for 1951–2018 ( [[#Dunn--2020|Dunn et al., 2020]] ), with slightly larger increases north of 45°N ( [[#Barichivich--2013|Barichivich et al., 2013]] ). Over North America, a rise of about 1.3 days per decade is apparent in the United States for 1900–2014 ( [[#Kukal--2018|Kukal and Irmak, 2018]] ), with larger increases after 1980 ( [[#McCabe--2015|McCabe et al., 2015]] ); likewise, all ecozones in Canada experienced increases from 1950–2010 ( [[#Pedlar--2015|Pedlar et al., 2015]] ). Growing season length in China increased by at least 1.0 days per decade since 1960 ( [[#Xia--2018|Xia et al., 2018]] ) and by several days per decade in South Korea since 1970 ( [[#Jung--2015|Jung et al., 2015]] ). In general, changes in phenological indicators are consistent with the increase in growing season length documented by instrumental data ( [[#Parmesan--2015|Parmesan and Hanley, 2015]] ). Several long-term, site-specific records illustrate the unusualness of recent phenological changes relative to interannual variability; for example, peak bloom dates for cherry blossoms in Kyoto, Japan have occurred progressively earlier in the growing season in recent decades, as have grape harvest dates in Beaune, France (Figure 2.32).

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[[File:d764bd8f474028bd82eba6568ef91e06 IPCC_AR6_WGI_Figure_2_32.png]]

'''Figure 2.32 |''' '''Phenological indicators of changes in growing season. (a)''' Cherry blossom peak bloom in Kyoto, Japan; '''(b)''' grape harvest in Beaune, France; '''(c)''' spring phenology index in eastern China; '''(d)''' full flower of Piedmont species in Philadelphia, USA; '''(e)''' grape harvest in Central Victoria, Australia; '''(f)''' start of growing season in Tibetan Plateau, China. Red lines depict the 25-year moving average (top row) or the nine-year moving average (middle and bottom rows) with the minimum roughness boundary constraint of Mann (2004). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Changes in the length of the photosynthetically active growing season (derived from the Normalized Difference Vegetation Index (NDVI)) are also evident over many land areas since the early 1980s. Increases of about 2.0 days per decade are apparent north of 45°N since the early 1980s (centred over mid-latitude Eurasia and north-eastern North America), with indications of a reversal to a decline in season length starting in the early 2000s ( [[#Barichivich--2013|Barichivich et al., 2013]] ; [[#Zhao--2015|Zhao et al., 2015]] ; [[#Garonna--2016|Garonna et al., 2016]] ; Q. [[#Liu--2016|]] [[#Liu--2016|Liu et al., 2016]] ; [[#Park--2016|Park et al., 2016]] ). Satellite-based records suggest that most NH regions have experienced both an earlier start and a later end to the growing season, a finding supported by ground-based data ( [[#Piao--2020|Piao et al., 2020]] ). A number of studies also capture increases in growing season length over the Canadian Arctic (W. [[#Chen--2016|]] [[#Chen--2016|Chen et al., 2016]] ), Fennoscandia ( [[#Høgda--2013|Høgda et al., 2013]] ), most of Europe ( [[#Garonna--2014|Garonna et al., 2014]] ), and parts of sub-Saharan Africa ( [[#Vrieling--2013|Vrieling et al., 2013]] ).

The general consistency between in situ and satellite estimates over the NH is noteworthy given that many factors independently contribute to uncertainty in observed changes. For example, there is no universally accepted definition of growing season length across in situ analyses; some define the growing season as the period based on a temperature threshold (e.g., 5°C) whereas others use the frost-free period. Spatial and temporal coverage can also affect conclusions based upon in situ studies ( [[#Donat--2013b|Donat et al., 2013b]] ). For satellite analyses, uncertainties can be related to the satellite datasets themselves (e.g., satellite drift, sensor differences, calibration uncertainties, atmospheric effects); and to the methods for determining phenological metrics (e.g., start, end, and length of season; S. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ).

In summary, based on multiple independent analyses of in situ, satellite, and phenological data, there is ''high confidence'' that the length of the growing season has increased over much of the extratropical NH since at least the mid-20th century.

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===== 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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===== 2.3.4.3.3 Global greening and browning =====

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The AR5 WGII briefly discussed changes in global vegetation greenness derived from satellite proxies for photosynthetic activity. Observed trends varied in their strength and consistency, and AR5 thus made no confidence statement on observed changes. The SRCCL subsequently concluded that greening had increased globally over the past 2–3 decades ( ''high confidence'' ).

Vegetation index data derived from AVHRR and MODIS depicts increases in aspects of vegetation greenness (i.e., green leaf area and/or mass) over the past four decades ( [[#Piao--2020|Piao et al., 2020]] ). NDVI increased globally from the early 1980s through the early 2010s (Y. [[#Liu--2015|]] [[#Liu--2015|Liu et al., 2015]] a). N. [[#Pan--2018|]] [[#Pan--2018|Pan et al. (2018)]] found NDVI increases over about 70% of the Earth’s vegetated surface through 2013, and [[#Osborne--2018|Osborne et al. (2018)]] noted strong upward changes in NDVI in the circumpolar Arctic through 2016. Globally integrated Leaf Area Index (LAI) also rose from the early 1980s through at least the early 2010s ( [[#Zhu--2016|Zhu et al., 2016]] ; [[#Forzieri--2017|Forzieri et al., 2017]] ; [[#Jiang--2017|Jiang et al., 2017]] ; [[#Xiao--2017|Xiao et al., 2017]] ) and probably through near-present; for example, [[#Chen--2019|]] [[#Chen--2019|C. Chen et al. (2019)]] documented an LAI increase over one-third of the global vegetated area from 2000–2017. Although less frequently analysed for temporal trends, Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) likewise increased over many global land areas (particularly China, India, and eastern Europe) in the past two decades (Figure 2.33; [[#Forkel--2014|Forkel et al., 2014]] ; [[#Gobron--2018|Gobron, 2018]] ; [[#Keenan--2018|Keenan and Riley, 2018]] ). There are also documented changes in specific vegetation types, such as a 7% rise in global tree cover for 1982–2016 ( [[#Song--2018|Song et al., 2018]] ) and an expansion of shrub extent in the Arctic tundra over 1982–2017 ( [[#Myers-Smith--2020|Myers-]] [[#Smith--2020|]] [[#Smith--2020|Smith et al., 2020]] ). The increased greening is largely consistent with CO <sub>2</sub> fertilization at the global scale, with other changes being noteworthy at the regional level ( [[#Piao--2020|Piao et al., 2020]] ); examples include agricultural intensification in China and India (X. [[#Chen--2019|]] [[#Chen--2019|Chen et al., 2019]] ; [[#Gao--2019|Gao et al., 2019]] ) and temperature increases in the northern high latitudes ( [[#Kong--2017|Kong et al., 2017]] ; [[#Keenan--2018|Keenan and Riley, 2018]] ) and in other areas such as the Loess Plateau in central China (Y. [[#Wang--2018|]] [[#Wang--2018|Wang et al., 2018]] ). Notably, some areas (such as parts of Amazonia, central Asia, and the Congo basin) have experienced browning (i.e., decreases in green leaf area and/or mass) ( [[#Hoogakker--2015|Hoogakker et al., 2015]] ; [[#Gottschalk--2016|Gottschalk et al., 2016]] ; [[#Anderson--2019|Anderson et al., 2019]] ). Because rates of browning have exceeded rates of greening in some regions since the late 1990s, the increase in global greening has been somewhat slower in the last two decades (T.-Y. [[#Pan--2018|]] [[#Pan--2018|Pan et al., 2018]] ).

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[[File:6ff276a39d9fc3ae50a6b58d2122d61b IPCC_AR6_WGI_Figure_2_33.png]]

'''Figure 2.''' '''33 |''' '''Satellite-based trends in fraction of absorbed photosynthetically active radiation (per decade) for 1998–2019.''' Trends are calculated using OLS regression with significance assessed following AR(1) adjustment after [[#Santer--2008|Santer et al. (2008)]] ; ‘×’ marks denote non-significant trend). Unvegetated areas such as barren deserts (grey) and ice sheets (white) have no trend in FAPAR. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Global-scale linear trends differ substantially across products for the same periods and trend metrics used ( [[#Jiang--2017|Jiang et al., 2017]] ). Several factors contribute to this large span in estimated changes. Remotely sensed vegetation products vary in their spatial and temporal completeness as well as resolution and are sensitive to contamination from atmospheric composition, clouds, snow cover, and anisotropy, as well as orbital changes and sensor degradations ( [[#de%20Jong--2012|de Jong et al., 2012]] ; [[#Zhu--2016|Zhu et al., 2016]] ; [[#Jiang--2017|Jiang et al., 2017]] ; [[#Xiao--2017|Xiao et al., 2017]] ; N. [[#Pan--2018|]] [[#Pan--2018|Pan et al., 2018]] ). Ground-based measurements suitable for calibration and validation are scarce before 2000 ( [[#Xiao--2017|Xiao et al., 2017]] ), and the recalibration of satellite records (e.g., as in from MODIS Collection 5 to 6) can affect trends ( [[#Piao--2020|Piao et al., 2020]] ). It is possible that the increase in greenness over 2000–2015 is larger than the increase in gross primary production (based on flux tower measurements and MODIS Collection 6 data) (L. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ). Land use changes and altered disturbance regimes (e.g., floods, fires, diseases) may mask large-scale signals ( [[#Franklin--2016|Franklin et al., 2016]] ). In addition, there is a plethora of models for the identification of phenological metrics from satellite data as well as a variety of statistical techniques for analysing historical changes (S. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ).

In summary, there is ''high confidence'' that vegetation greenness (i.e., green leaf area and/or mass) has increased globally since the early 1980s. However, there is ''low confidence'' in the magnitude of this increase owing to the large range in available estimates.

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