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

===== 2.3.1.1.1 Temperatures of the deep past (65 Ma to 8 ka) =====

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This assessment of the paleo reference periods (Cross-Chapter Box 2.1) draws from studies based mostly or entirely on indirect observational evidence from geological archives (i.e., proxy records) rather than reconstructions that rely more heavily on modelled parameters and those based on deep-ocean temperatures (e.g., [[#Köhler--2015|Köhler et al., 2015]] ; [[#Friedrich--2016|Friedrich et al., 2016]] ). In contrast to AR5, temperature estimates from climate models are not included in the assessed values for paleo reference periods in this chapter. The AR5 concluded that the reconstructed GMST during the PETM was 4°C–7°C warmer than pre-PETM mean climate ( ''low confidence'' ), and that the EECO and the MPWP were 9°C–14°C and 1.9°C–3.6°C warmer than pre-industrial, respectively ( ''medium confidence'' ). The GMST during the LIG was assessed at 1°C–2°C warmer than pre-industrial ''(medium confidence'' ), whereas SROCC narrowed the range to 0.5°C–1.0°C warmer, but did not state a confidence level. The AR5 further concluded that it was ''very'' ''likely'' that the LGM was 3°C–8°C colder than pre-industrial, and ''likely'' that the maximum rate of global warming during the subsequent deglacial period was 1°C–1.5°C kyr <sup>–1</sup> .

For the PETM, new reconstructions agree with those assessed by AR5. A major new compilation of proxy temperature data ( [[#Hollis--2019|Hollis et al., 2019]] ) analysed using multiple statistical approaches ( [[#Inglis--2020|Inglis et al., 2020]] ) indicates that GMST was 10°C–25°C (90% range) warmer than 1850–1900, or about 5°C warmer relative to the pre-PETM state. A related synthesis study also estimates that PETM warmed by 5°C (no uncertainty assigned; [[#Zhu--2019|Zhu et al., 2019]] ). A recent benthic isotope compilation ( [[#Westerhold--2020|Westerhold et al., 2020]] ) transformed to GMST based on the formulation by J. Hansen et al. (2013; Cross-Chapter Box 2.1, Figure 1), and adjusted to 1850–1900 by adding 0.36°C, shows an increase of GMST by about 10°C during the PETM. This reflects the expected higher variability at single sites that were used to splice together the composite time series, compared to the globally averaged composite time series of [[#Zachos--2008|Zachos et al. (2008)]] . The latter was originally used by J. [[#Hansen--2013|]] [[#Hansen--2013|Hansen et al. (2013)]] to reconstruct GMST, and is the preferred representation of the global average bottom water conditions, despite its less well-refined chronology.

For the EECO, new GMST reconstructions fall at the high end of the range assessed by AR5. These include estimates of 7°C–18°C (90% range; [[#Inglis--2020|Inglis et al., 2020]] ) and 12°C–18°C (95% range; [[#Zhu--2019|Zhu et al., 2019]] ) warmer than 1850–1900, and 10°C–16°C warmer than 1995–2014 ‘recent past’ conditions (2 standard error range; [[#Caballero--2013|Caballero and Huber, 2013]] ). Together, they indicate that GMST was 10°C–18°C warmer during the EECO compared with 1850–1900 ( ''medium confidence'' ).

The AR5 did not assess the GMST for the MCO. Reconstructions based on data from multiple study sites include estimates of about 4°C (uncertainty range not specified; [[#You--2009|You et al., 2009]] ) and 5°C–10°C (2 standard error range; [[#Goldner--2014|Goldner et al., 2014]] ) warmer than 1850–1900. Together, these studies indicate that GMST was 4°C–10°C warmer during the MCO ( ''medium confidence'' ).

For the MPWP, new proxy-based estimates of global sea surface temperatures (SST) are about 2.0°C–3.5°C warmer than 1850–1900, depending on which proxy types are included in the analysis ( [[#Foley--2019|Foley and Dowsett, 2019]] ; [[#McClymont--2020|McClymont et al., 2020]] ). On the basis of model-derived relationships between land versus sea surface temperatures under different climate states (Figure 3.2b), the increase in GMST is estimated to have been roughly 15% greater than the increase in global SST. Therefore, GMST during the MPWP is estimated to have been 2.5°C–4.0°C warmer than 1850–1900 ( ''medium confidenc'' e).

For the LIG (Cross-Chapter Box 2.1, Figure 1, and Figure 2.11), a major new compilation of marine proxy data ( [[#Turney--2020|Turney et al., 2020]] ) from 203 sites indicates that the average SST from 129–125 ka was 1.0°C ± 0.2°C (2 SD) warmer than 1850–1900 (reported relative to 1981–2010 and adjusted here by 0.8°C). These temperatures represent the time of peak warmth, which may not have been synchronous among these sites. This compares with two other SST estimates for 125 ka of 0.5°C ± 0.3°C (± 2 SD) warmer at 125 ka relative to 1870–1889 ( [[#Hoffman--2017|Hoffman et al., 2017]] ), and about 1.4°C (no uncertainty stated) warmer at 125 ka relative to 1850–1900 ( [[#Friedrich--2020|Friedrich and Timmermann, 2020]] ; reported relative to 10–5 ka and adjusted here by 0.4°C; [[#Kaufman--2020a|Kaufman et al., 2020a]] ). The average of these post-AR5 global SST anomalies is 1°C. Commensurately (Figure 3.2b), GMST is estimated to have been roughly 1.1°C above 1850–1900 values, although this value could be too high if peak warmth was not globally synchronous ( [[#Capron--2017|Capron et al., 2017]] ). A further estimate of peak GMST anomalies of 1.0°C–3.5°C (90% range; adjusted here to 1850–1900 by adding 0.2°C) based on 59 marine sediment cores ( [[#Snyder--2016|Snyder, 2016]] ) is considerably warmer than remaining estimates and is therefore given less weight in the final assessment. The warmest millennium of the LIG GMST reconstruction in J. [[#Hansen--2013|]] [[#Hansen--2013|Hansen et al. (2013)]] is 1.5°C above 1850–1900. In summary, GMST during the warmest millennia of the LIG (within the interval of around 129–125 ka) is estimated to have reached 0.5°C–1.5°C higher values than the 1850–1990 reference period ( ''medium confidence'' ).

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[[File:db18d2c58474b72f5157a77d07c68327 IPCC_AR6_WGI_Figure_2_11.png]]

'''Figure''' '''2.11 |''' '''Earth’s surface temperature history with key findings annotated within each panel. (a)''' GMST over the Holocene divided into three time scales: (i) 12 kyr–1 kyr in 100-year time steps; (ii) 1000–1900 CE, 10-year smooth; and (iii) 1900–2020 CE (from panel (c)). Median of the multi-method reconstruction (bold lines), with 5th and 95th percentiles of the ensemble members (thin lines). Vertical bars are the assessed ''medium confidence'' ranges of GMST for the Last Interglacial and mid-Holocene ( [[#2.3.1.1|Section 2.3.1.1]] ). The last decade value and ''very likely'' range arises from [[#2.3.1.1.3|Section 2.3.1.1.3]] . '''(b)''' Spatially resolved trends (°C per decade) for HadCRUTv5 over (upper map) 1900–1980, and (lower map) 1981–2020. Significance is assessed following AR(1) adjustment after [[#Santer--2008|Santer et al. (2008)]] , ‘×’ marks denote non-significant trends. '''(c)''' Temperature from instrumental data for 1850–2020, including (upper panel) multi-product mean annual time series assessed in [[#2.3.1.1.3|Section 2.3.1.1.3]] for temperature over the oceans (blue line) and temperature over the land (red line) and indicating the warming to the most recent 10 years; and annually (middle panel) and decadally (bottom panel) resolved averages for the GMST datasets assessed in [[#2.3.1.1.3|Section 2.3.1.1.3]] . The grey shading in each panel shows the uncertainty associated with the HadCRUT5 estimate ( [[#Morice--2021|Morice et al., 2021]] ). All temperatures relative to the 1850–1900 reference period. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

New GMST reconstructions for the LGM fall near the middle of AR5’s ''very likely'' range, which was based on a combination of proxy reconstructions and model simulations. Two of these new reconstructions use marine proxies to reconstruct global SST that were scaled to GMST based on different assumptions. One indicates that GMST was 6.2 [4.5 to 8.1] °C cooler than the late Holocene average ( [[#Snyder--2016|Snyder, 2016]] ), and the other, 5.7°C ± 0.8°C (2 SD) cooler than the average of the first part of the Holocene (10–5 ka) ( [[#Friedrich--2020|Friedrich and Timmermann, 2020]] ). A third new estimate ( [[#Tierney--2020|Tierney et al., 2020]] ) uses a much larger compilation of marine proxies along with a data-assimilation procedure, rather than scaling, to reconstruct a GMST of 6.1°C ± 0.4°C (2 SD) cooler than the late Holocene. Assuming that the 1850–1900 reference period was 0.2°C and 0.4°C cooler than the late and first part of the Holocene, respectively ( [[#Kaufman--2020a|Kaufman et al., 2020a]] ), the midpoints of these three new GMST reconstructions average –5.8°C relative to 1850–1900. The coldest multi-century period of the LGM in the J. [[#Hansen--2013|]] [[#Hansen--2013|Hansen et al. (2013)]] reconstruction is 4.3°C colder than 1850–1900. This compares to land- and SST-only estimates of about –6.1°C ± 2°C and –2.2°C ± 1°C, respectively (2 SD), which are based on AR5-generation studies that imply a warmer GMST than more recent reconstructions (Figure 1c in [[#Harrison--2015|Harrison et al., 2015]] ; Figure 7 in [[#Harrison--2016|Harrison et al., 2016]] ). A major new pollen-based data-assimilation reconstruction averages 6.9°C cooler over northern extratropical land ( [[#Cleator--2020|Cleator et al., 2020]] ). LGM temperature variability on centennial scales was about four times higher globally than during the Holocene, and even greater at high latitudes ( [[#Rehfeld--2018|Rehfeld et al., 2018]] ). In summary, GMST is estimated to have been 5°C–7°C lower during the LGM (around 23–19 ka) compared with 1850–1900 ( ''medium confidence'' ).

For the LDT (Cross-Chapter Box 2.1, Figure 1), no new large-scale studies have been published since AR5 ( [[#Shakun--2012|Shakun et al., 2012]] ) to further assess the rate of GMST change during this period of rapid global warming (estimated at 1°C–1.5°C per kyr). The reconstruction of [[#Shakun--2012|Shakun et al. (2012)]] was based primarily on SST records and therefore underrepresents the change in GMST during the LDT. Temperature over Greenland increased by about ten times that rate during the centuries of most rapid warming ( [[#Jansen--2020|Jansen et al., 2020]] ).

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