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

===== 2.3.1.1.2 Temperatures of the post-glacial period (past 7000 years) =====

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The AR5 did not include an assessment of large-scale temperature estimates for the MH, although it assigned ''high confidence'' to the long-term cooling trend over mid- to high-latitudes of the Northern Hemisphere (NH) during the 5 kyr that preceded recent warming. For average annual NH temperatures, the period 1983–2012 was assessed as ''very likely'' the warmest 30-year period of the past 800 years ( ''high confidence'' ) and ''likely'' the warmest 30-year period of the past 1.4 kyr ( ''medium confidence'' ); the warm multi-decadal periods prior to the 20th century were unsynchronized across regions, in contrast to the warming since the mid-20th century ( ''high confidence'' ), although only sparse information was available from the SH.

This section concerns the Holocene period prior to industrialization when GMST was overall highest. Whereas SR1.5 focussed upon the ‘Holocene thermal maximum’ when regional temperatures were up to 1°C higher than 1850–1900, though peak warming occurred regionally at different times between around 10 and 5 ka greatly complicating interpretation. A multi-method reconstruction ( [[#Kaufman--2020a|Kaufman et al., 2020a]] ) based on a quality-controlled, multi-proxy synthesis of paleo-temperature records from 470 terrestrial and 209 marine sites globally ( [[#Kaufman--2020b|Kaufman et al., 2020b]] ) indicates that the median GMST of the warmest two-century-long interval was 0.7 [0.3 to 1.8] °C warmer than 1800–1900 (which averaged 0.03°C colder than 1850–1900; [[#PAGES%202k%20Consortium--2019|PAGES 2k Consortium, 2019]] ), and was centred around 6.5 ka. This is similar to [[#Marcott--2013|Marcott et al. (2013)]] , which is based on a smaller dataset (73 sites) and different procedures to estimate a maximum warmth of 0.8°C ± 0.3°C (2 SD) at around 7.0 ka, adjusted here by adding 0.3°C to account for differences in reference periods. These may be underestimates because averaging inherently smoothed proxy records with uncertain chronologies reduces the variability in the temperature reconstruction (e.g., Dolman and Laepple, (2018) for sedimentary archives). However, the general coincidence between peak warmth and astronomically driven boreal summer insolation might reflect a bias toward summer conditions ( [[#Liu--2014|Liu et al., 2014]] ; [[#Hou--2019|Hou et al., 2019]] ; [[#Bova--2021|Bova et al., 2021]] ), suggesting that the estimate is too high. This possibility is supported by AR5-generation proxy data focusing on 6 ka ( [[#Harrison--2014|Harrison et al., 2014]] ), the long-standing MH modelling target (Cross-Chapter Box 2.1), that indicate surface temperatures for land and ocean were indistinguishable from ‘pre-industrial’ climate (Figure 1c in [[#Harrison--2015|Harrison et al., 2015]] ; Figure 7 in [[#Harrison--2016|Harrison et al., 2016]] ). In contrast, the GMST estimate from the multi-method global reconstruction ( [[#Kaufman--2020a|Kaufman et al., 2020a]] ) for the millennium centred on 6 ka is only about 0.1°C colder than the warmest millennium.

Taking all lines of evidence into account, the GMST averaged over the warmest centuries of the current interglacial period (sometime between around 6 and 7 ka) is estimated to have been 0.2°C–1.0°C higher than 1850–1900 ( ''medium confidence'' ). It is therefore ''more likely than not'' that no multi-centennial interval during the post-glacial period was warmer globally than the most recent decade (which was 1.1°C warmer than 1850–1900; [[#2.3.1.1.3|Section 2.3.1.1.3]] ); the LIG (129–116 ka) is the next most recent candidate for a period of higher global temperature. Zonally averaged mean annual temperature reconstructions ( [[#Routson--2019|Routson et al., 2019]] ) indicate that MH warmth was most pronounced north of 30°N latitude, and that GMST subsequently decreased in general, albeit with multi-century variability, with greater cooling in the NH than in the SH ( [[#Kaufman--2020a|Kaufman et al., 2020a]] ).

The temperature history of the last millennium and the methods used to reconstruct it have been studied extensively, both prior to and following AR5, as summarized recently by [[#Smerdon--2016|Smerdon and Pollack (2016)]] and [[#Christiansen--2017|Christiansen and Ljungqvist (2017)]] . New regional (e.g., [[#Shi--2015|Shi et al., 2015]] ; [[#Stenni--2017|Stenni et al., 2017]] ; [[#Werner--2018|Werner et al., 2018]] ), global ocean ( [[#McGregor--2015|McGregor et al., 2015]] ), quasi-hemispheric ( [[#Neukom--2014|Neukom et al., 2014]] ; [[#Schneider--2015|Schneider et al., 2015]] ; [[#Anchukaitis--2017|Anchukaitis et al., 2017]] ), and global ( [[#Tardif--2019|Tardif et al., 2019]] ) temperature reconstructions, and new regional proxy data syntheses ( [[#Lüning--2019a|Lüning et al., 2019a]] , b) have been published, extending back 1–2 kyr. In addition, a major new global compilation of multiproxy, annually resolved paleo-temperature records for the CE ( [[#PAGES%202k%20Consortium--2017|PAGES 2k Consortium, 2017]] ) has been analysed using a variety of statistical methods for reconstructing temperature ( [[#PAGES%202k%20Consortium--2019|PAGES 2k Consortium, 2019]] ). The median of the multi-method GMST reconstruction from this synthesis (Figure 2.11a) generally agrees with the AR5 assessment, while affording more robust estimates of the following major features of GMST during the CE: (i) an overall millennial-scale cooling trend of –0.18 [–0.28 to 0.00] °C kyr <sup>–1</sup> prior to 1850; (ii) a multi-centennial period of relatively low temperature beginning around the 15th century, with GMST averaging –0.03 [–0.30 to 0.06] °C between 1450 and 1850 relative to 1850–1900; (iii) the warmest multi-decadal period occurring most recently; and (iv) the rate of warming during the second half of the 20th century (from instrumental data) exceeding the 99th percentile of all 51-year trends over the past 2 kyr. Moreover, the new proxy data compilation shows that the warming of the 20th century was more spatially uniform than any other century-scale temperature change of the CE ( ''medium confidence'' ) ( [[#Neukom--2019|Neukom et al., 2019]] ). A new independent temperature reconstruction extending back to 1580 is based on an expanded database of subsurface borehole temperature profiles, along with refined methods for inverse modelling ( [[#Cuesta-Valero--2021|Cuesta-Valero et al., 2021]] ). The borehole data, converted to GMST based on the modelled relation between changes in land versus sea surface temperature outlined previously, indicate that average GMST for 1600–1650 was 0.12°C colder than 1850–1900, which is similar to the PAGES 2k reconstruction (0.09°C colder), although both estimates are associated with relatively large uncertainties (0.8°C (95% range) and 0.5°C (90% range), respectively).

To conclude, following approximately 6 ka, GMST generally decreased, culminating in the coldest multi-century interval of the post-glacial period (since 8 ka), which occurred between around 1450 and 1850 ( ''high confidence'' ). This multi-millennial cooling trend was reversed in the mid-19th century. Since around 1950, GMST has increased at an observed rate unprecedented for any 50-year period in at least the last 2000 years ( ''high confidence'' ).

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