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

===== 2.3.1.4.2 Global monsoon (GM) changes =====

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The AR5 reported a weakening of the global monsoon (GM) circulation as well as a decrease of global land monsoon rainfall over the second half of the 20th century. Nevertheless, there was ''low confidence'' in the observed circulation trends due to uncertainties in reanalysis products and in the definition of the monsoon area. From a paleo perspective, AR5 only assessed regional monsoon changes.

New research based on high-resolution proxies reinforces previous findings on the influence of orbital cycles on GM variability on millennial time scales. The intensity of the monsoon systems is generally out of phase between hemispheres, being associated with the precession cycle (about 21–23 kyr) ( [[#An--2015|An et al., 2015]] ; P.X. [[#Wang--2017|Wang et al., 2017]] ; [[#Seth--2019|Seth et al., 2019]] ), with intensified NH monsoon systems during precession minima ( [[#Toucanne--2015|Toucanne et al., 2015]] ; [[#Wagner--2019|Wagner et al., 2019]] ). The eccentricity forcing (about 100 kyr cycle) shows stronger GM during interglacial periods (P.X. [[#Wang--2014|]] [[#Wang--2014|Wang et al., 2014]] , 2017; [[#An--2015|An et al., 2015]] ; [[#Mohtadi--2016|Mohtadi et al., 2016]] ). Changes in obliquity (about 41 kyr cycle) modify the strength of monsoon systems, with increased summer monsoon rainfall when obliquity is maximal (Y. [[#Liu--2015|]] [[#Liu--2015|Liu et al., 2015]] b; [[#Mohtadi--2016|Mohtadi et al., 2016]] ). Millennial scale variability in GM during the LDT was also linked to the occurrences of Heinrich stadials, resulting in weakened NH monsoons and intensified SH monsoons ( [[#An--2015|An et al., 2015]] ; P.X. [[#Wang--2017|Wang et al., 2017]] ; [[#Margari--2020|Margari et al., 2020]] ).

An intensification of the NH monsoons in the early to mid-Holocene with increased precipitation and regional expansions of rainfall areas identified through a variety of proxy records is shown by [[#Biasutti--2018|Biasutti et al. (2018)]] and P.X. [[#Wang--2017|Wang et al. (2017)]] . The response for the SH monsoons during this period indicates a weakening in both summer and winter precipitation (P.X. [[#Wang--2014|]] [[#Wang--2014|Wang et al., 2014]] , 2017; [[#Sachs--2018|Sachs et al., 2018]] ). A decline in GM precipitation and a retraction of the northern fringes of monsoon areas was inferred from the mid-Holocene onwards, with some regions experiencing wetter conditions during the mid to late Holocene compared with present and a strengthening of the SH monsoons (P.X. [[#Wang--2014|]] [[#Wang--2014|Wang et al., 2014]] , 2017; [[#Sachs--2018|Sachs et al., 2018]] ). For the CE, GM reconstructions exhibit inter-hemispheric contrast during the period 950–1250 CE, with intensified NH monsoons and weakened SH monsoons, and the opposite pattern during 1400–1850 CE (P.X. [[#Wang--2014|]] [[#Wang--2014|Wang et al., 2014]] ; [[#An--2015|An et al., 2015]] ).

Direct observations highlight that the GM land precipitation, particularly over the NH, experienced a slight increase from 1900 through the early 1950s, followed by an overall decrease from the 1950s to the 1980s, and then an increase to present ( [[#Kitoh--2013|Kitoh et al., 2013]] ; [[#Wang--2018|]] [[#Wang--2018|B. Wang et al., 2018]] , 2021; X. [[#Huang--2019b|]] [[#Huang--2019|Huang et al., 2019]] b ). This highlights the existence of multi-decadal variations in the NH monsoon circulation patterns and precipitation intensity ( [[#Wang--2013|Wang et al., 2013]] ; P.X. [[#Wang--2014|]] [[#Wang--2014|Wang et al., 2014]] , 2017; [[#Monerie--2019|Monerie et al., 2019]] ). An overall increase in monsoon precipitation during extended boreal summer (JJAS) over the NH since 1979 is revealed by GPCP ( [[#Deng--2018|Deng et al., 2018]] ; [[#Han--2019|Han et al., 2019]] ) and CMAP for 1980–2010 ( [[#Jiang--2016|Jiang et al., 2016]] ). SH summer monsoon behaviour is dominated by strong interannual variability and large regional differences ( [[#Kitoh--2013|Kitoh et al., 2013]] ; [[#Lin--2014|Lin et al., 2014]] ; [[#Jiang--2016|Jiang et al., 2016]] ; [[#Kamae--2017|Kamae et al., 2017]] ; [[#Deng--2018|Deng et al., 2018]] ; [[#Han--2019|Han et al., 2019]] ), with no significant trends reported by GPCP and CMAP ( [[#Deng--2018|Deng et al., 2018]] ). Uncertainty predominantly arises from the observed increase in tropical precipitation seasonality ( [[#Feng--2013|Feng et al., 2013]] ) and the estimation of GM precipitation over the ocean areas, leading to a large apparent spread across datasets ( [[#Kitoh--2013|Kitoh et al., 2013]] ; [[#Kamae--2017|Kamae et al., 2017]] ).

In summary, observed trends during the last century indicate that the GM precipitation decline reported in AR5 has reversed since the 1980s, with a ''likely'' increase mainly due to a significant positive trend in the NH summer monsoon precipitation ( ''medium confidence'' ). However, GM precipitation has exhibited large multi-decadal variability over the last century, creating ''low confidence'' in the existence of centennial-length trends in the instrumental record. Proxy reconstructions show a ''likely'' NH monsoons weakening since the mid-Holocene, with opposite behaviour for the SH monsoons.

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