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

===== 2.3.1.3.1 Paleo perspective of the global hydrological cycle =====

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The AR5 assessed large-scale indicators of terrestrial paleo hydroclimate, including as part of its assessment of paleo floods and droughts, but did not assess proxy evidence for paleo hydroclimate indicators over continental and larger scales. The paleoclimate evidence assessed in AR5 was broadly consistent with global hydroclimate scaling with temperature: warmer periods were wetter (e.g., the Pliocene; increased precipitation) with colder periods being drier (e.g., the LGM; decreased precipitation).

Substantial limitations exist in reconstructing the global hydrological cycle prior to the Quaternary, particularly during the Eocene, due to the lack of high-resolution proxy records and their sparsity. Spatial heterogeneity complicates identification of wetting and drying signals during the PETM and the EECO, with paleo data and model simulations suggesting an intensified global hydrological cycle ( [[#Carmichael--2016|Carmichael et al., 2016]] , 2017; [[#Hyland--2017|Hyland et al., 2017]] ; [[#West--2020|West et al., 2020]] ), in particular an increased specific humidity ( [[#Winnick--2015|Winnick et al., 2015]] ; [[#van%20Dijk--2020|van Dijk et al., 2020]] ). Conditions wetter than present were inferred for the MPWP (Cross Chapter Box 2.4), with intensified Asian monsoons ( [[#An--2015|An et al., 2015]] ) but with nevertheless drier conditions over tropical and subtropical SH locations ( [[#Pontes--2020|Pontes et al., 2020]] ). A new global reconstruction of hydroclimate proxies for the LIG points to stronger boreal precipitation compared to 1850–1900 over high latitudes and especially over monsoon areas, with a more heterogeneous signal for the SH ( [[#Scussolini--2019|Scussolini et al., 2019]] ). This heterogeneity is also present in the tropics, characterized by large zonal differences in precipitation change due to the variations in the intensity of Walker circulation ( [[#2.3.1.4.1|Section 2.3.1.4.1]] ). Available records indicate reduced global vegetation cover and abundant atmospheric dust deposition during the LGM (increased aridity), particularly over the tropics and high latitudes ( [[#Lamy--2014|Lamy et al., 2014]] ; [[#Újvári--2017|Újvári et al., 2017]] ). This agrees with models and moisture-sensitive proxies, suggesting an overall decrease in global precipitation during the LGM relative to recent decades, albeit with regional-scale heterogeneity ( [[#Cao--2019|Cao et al., 2019]] ). Despite lower global precipitation amounts, research since AR5 has identified a wetting of mid-latitudes during the LGM ( [[#Putnam--2017|Putnam and Broecker, 2017]] ; [[#Lowry--2018|Lowry and Morrill, 2018]] ; [[#Morrill--2018|Morrill et al., 2018]] ), thereby complicating the characterization of the LGM as a relatively ‘dry’ period. Low evaporation rates and increased top-soil moisture during the LGM may have contributed to elevated levels of large closed-basin lakes located in the 30°–45° latitudinal belts ( [[#Putnam--2017|Putnam and Broecker, 2017]] ; [[#Scheff--2017|Scheff et al., 2017]] ), such as the south-west United States (e.g., [[#Ibarra--2018|Ibarra et al., 2018]] ), southern Australia ( [[#Petherick--2013|Petherick et al., 2013]] ; [[#Fitzsimmons--2015|Fitzsimmons et al., 2015]] ; [[#Sniderman--2019|Sniderman et al., 2019]] ) and Patagonia (e.g., [[#Quade--2017|Quade and Kaplan, 2017]] ).

New analyses suggest that during the Holocene, the NH mid-latitudes became increasingly wet, in phase with the strength of the latitudinal temperature and insolation gradients ( [[#Shuman--2016|Shuman and Marsicek, 2016]] ; [[#Routson--2019|Routson et al., 2019]] ). Nevertheless, there was also considerable spatial heterogeneity and variability on centennial to millennial timescales ( [[#Newby--2014|Newby et al., 2014]] ; [[#Shuman--2016|Shuman and Marsicek, 2016]] ; H. [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|]] [[#Zhang--2018|Zhang et al., 2018]] ; [[#Liefert--2020|Liefert and Shuman, 2020]] ). The NH tropics and many regions of the SH deep tropics experienced wetting up until the early to mid-Holocene but drying thereafter ( [[#Shanahan--2015|Shanahan et al., 2015]] ; [[#Nash--2016|Nash et al., 2016]] ; [[#Muñoz--2017|Muñoz et al., 2017]] ; [[#Quade--2018|Quade et al., 2018]] ). ''Evidence'' for the SH is ''limited'' , with a wetting trend during the Holocene in low latitudes of South America ( [[#Kanner--2013|Kanner et al., 2013]] ; [[#Mollier-Vogel--2013|Mollier-Vogel et al., 2013]] ) and parts of the African tropics ( [[#Schefuß--2011|Schefuß et al., 2011]] ; [[#Chevalier--2015|Chevalier and Chase, 2015]] ) but a drying tendency over southern Australia and New Zealand ( [[#van%20den%20Bos--2018|van den Bos et al., 2018]] ; [[#Barr--2019|Barr et al., 2019]] ) and South America ( [[#Quade--2017|Quade and Kaplan, 2017]] ; [[#Moreno--2018|Moreno et al., 2018]] ).

For the CE, new proxy records have led to the creation of continental drought atlases ( [[#Cook--2015|Cook et al., 2015]] ; [[#Palmer--2015|Palmer et al., 2015]] ; [[#Stahle--2016|Stahle et al., 2016]] ; [[#Morales--2020|Morales et al., 2020]] ) and millennial reanalyses ( [[#Steiger--2018|Steiger et al., 2018]] ; [[#Tardif--2019|Tardif et al., 2019]] ). These reconstructions highlighted the occurrence of multi-decadal regional mega-droughts in the NH before 1600 CE, particularly during 800–1200 CE, with a predominance of wet periods after 1700 CE ( [[#Cook--2015|Cook et al., 2015]] ; [[#Rodysill--2018|Rodysill et al., 2018]] ; [[#Shuman--2018|Shuman et al., 2018]] ). In the SH, much of South America and the African tropics experienced a reduction of precipitation during 900–1200 CE and a wetting peak during 1500–1800 CE ( [[#Tierney--2015|Tierney et al., 2015]] ; [[#Nash--2016|Nash et al., 2016]] ; [[#Fletcher--2018|Fletcher et al., 2018]] ; [[#Lüning--2018|Lüning et al., 2018]] ; [[#Campos--2019|Campos et al., 2019]] ), with an opposite pattern in southern subtropical Africa ( [[#Woodborne--2015|Woodborne et al., 2015]] ; [[#Lüning--2018|Lüning et al., 2018]] ). Large multi-decadal variability was documented over Australia and New Zealand during the 800–1300 CE period, followed by a well-defined wet period during 1500–1800 CE ( [[#Barr--2014|Barr et al., 2014]] ; [[#Evans--2019|Evans et al., 2019]] ).

To summarize, since AR5 there has been considerable progress in detecting the variations of the global hydrological cycle prior to the instrumental period. There are indications from multiple sources of a wetting trend during the Holocene, particularly for the NH and parts of the SH tropics ( ''medium confidence'' ). Hydroclimate during the CE is dominated by regional variability, generally precluding definitive statements on changes at continental and larger scales, with a general reduction of mega-drought occurrences over the last about 500 years ( ''medium confidence'' ). Availability of proxy data for assessing Holocene hydroclimate variability is biased towards the NH, with ''medium evidence'' but ''low agreement'' for the assessment of SH changes.

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