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

===== 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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