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

===== 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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