Editing IPCC:Wg1:Chapter:Chapter-1-comments (section)

===== ''1.5.1.1.5 Paleoclimate'' =====

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Major paleoreconstruction efforts completed since AR5 include a variety of large-scale, multi-proxy temperature datasets and associated reconstructions spanning the last 2000 years ( [[#PAGES%202k%20Consortium--2017|PAGES 2k Consortium, 2017]] , 2019; [[#Neukom--2019|Neukom et al., 2019]] ), the Holocene ( [[#Kaufman--2020|Kaufman et al., 2020]] ), the Last Glacial Maximum ( [[#Cleator--2020|Cleator et al., 2020]] ; [[#Tierney--2020b|Tierney et al., 2020b]] ), the mid-Pliocene Warm Period ( [[#McClymont--2020|McClymont et al., 2020]] ), and the Early Eocene Climatic Optimum ( [[#Hollis--2019|Hollis et al., 2019]] ). Newly compiled borehole data ( [[#Cuesta-Valero--2019|Cuesta-Valero et al., 2019]] ), as well as advances in statistical applications to tree ring data, result in more robust reconstructions of key indices such as Northern Hemisphere temperature over the last millennium (e.g., [[#Wilson--2016|Wilson et al., 2016]] ; [[#Anchukaitis--2017|Anchukaitis et al., 2017]] ). Such reconstructions provide a new context for recent warming trends (Chapter 2) and serve to constrain the response of the climate system to natural and anthropogenic forcing (Chapters 3 and 7).

Ongoing efforts have expanded the number of large-scale, tree ring-based drought reconstructions that span the last centuries to millennium at annual resolution (Chapter 8; [[#Cook--2015|Cook et al., 2015]] ; [[#Stahle--2016|Stahle et al., 2016]] ; [[#Aguilera-Betti--2017|Aguilera-Betti et al., 2017]] ; [[#Morales--2020|Morales et al., 2020]] ). Likewise, stalagmite records of oxygen isotopes have increased in number, resolution and geographic distribution since AR5, providing insights into regional-to-global-scale hydrological change over the last centuries to millions of years (Chapter 8; [[#Cheng--2016|Cheng et al., 2016]] ; [[#Denniston--2016|Denniston et al., 2016]] ; [[#Comas-Bru--2019|Comas-Bru and Harrison, 2019]] ). A new global compilation of water isotope-based paleoclimate records spanning the last 2000 years (PAGES Iso2K) lays the groundwork for quantitative multi-proxy reconstructions of regional- to global-scale hydrological and temperature trends and extremes ( [[#Konecky--2020|Konecky et al., 2020]] ).

Recent advances in the reconstruction of climate extremes – aside from temperature and drought – include expanded datasets of past El Niño–Southern Oscillation extremes ( [[IPCC:Wg1:Chapter:Chapter-2#2.4.2|Section 2.4.2]] ; e.g., [[#Barrett--2018|Barrett et al., 2018]] ; [[#Freund--2019|Freund et al., 2019]] ; [[#Grothe--2020|Grothe et al., 2020]] ) and other modes of variability ( [[#Hernández--2020|Hernández et al., 2020]] ), hurricane activity (e.g., [[#Burn--2015|Burn and Palmer, 2015]] ; [[#Donnelly--2015|Donnelly et al., 2015]] ), jet stream variability ( [[#Trouet--2018|Trouet et al., 2018]] ) and wildfires (e.g., [[#Taylor--2016|Taylor et al., 2016]] ).

New datasets as well as recent data compilations and syntheses of sea level over the last millennia ( [[#Kopp--2016|Kopp et al., 2016]] ; [[#Kemp--2018|Kemp et al., 2018]] ), the last 20 kyr ( [[#Khan--2019|Khan et al., 2019]] ), the last interglacial period ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] : [[#Dutton--2015|Dutton et al., 2015]] ), and the Pliocene (Cross-Chapter Box 2.4; [[#Dumitru--2019|Dumitru et al., 2019]] ; [[#Grant--2019|Grant et al., 2019]] ) help constrain sea level variability and its relationship to global and regional temperature variability, and to estimates of contributions to sea level change from different sources on centennial to millennial time scales (Section 9.6.2).

Reconstructions of paleo ocean pH ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.5|Section 2.3.3.5]] ) have increased in number and accuracy, providing new constraints on ocean pH across the last centuries (e.g., [[#Wu--2018|Wu et al., 2018]] ), the last glacial cycles (e.g., [[#Moy--2019|Moy et al., 2019]] ), and the last several million years (e.g., [[#Anagnostou--2020|Anagnostou et al., 2020]] ). Such reconstructions inform processes and act as benchmarks for Earth system models of the global carbon cycle over the recent geologic past (Section 5.3.1), including previous high-CO <sub>2</sub> warm intervals such as the Pliocene (Cross-Chapter Box 2.4). Particularly relevant to such investigations are reconstructions of atmospheric CO <sub>2</sub> ( [[#Honisch--2012|Honisch et al., 2012]] ; [[#Foster--2017|Foster et al., 2017]] ) that span the past millions to tens of millions of years.

Constraints on the timing and rates of past climate changes have improved since AR5. Analytical methods have increased the precision and reduced sample-size requirements for key radiometric dating techniques, including radiocarbon ( [[#Gottschalk--2018|Gottschalk et al., 2018]] ; [[#Lougheed--2018|Lougheed et al., 2018]] ) and uranium–thorium dating ( [[#Cheng--2013|Cheng et al., 2013]] ). More accurate ages of many paleoclimate records are also facilitated by recent improvements in the radiocarbon calibration datasets (IntCal20, [[#Reimer--2020|Reimer et al., 2020]] ). A recent compilation of global cosmogenic nuclide-based exposure dates ( [[#Balco--2020b|Balco, 2020b]] ) allows for a more rigorous assessment of the evolution of glacial landforms since the Last Glacial Maximum ( [[#Balco--2020a|Balco, 2020a]] ).

Advances in paleoclimate data assimilation (Section 10.2.3.2) leverage the expanded set of paleoclimate observations to create physically consistent gridded fields of climate variables for data-rich intervals of interest (e.g., over the last millennium, ( [[#Hakim--2016|Hakim et al., 2016]] ) or last glacial period ( [[#Cleator--2020|Cleator et al., 2020]] ; [[#Tierney--2020b|Tierney et al., 2020b]] )). Such efforts mirror advances in our understanding of the relationship between proxy records and climate variables of interest, as formalized in so-called proxy system models (e.g., [[#Tolwinski-Ward--2011|Tolwinski-Ward et al., 2011]] ; [[#Dee--2015|Dee et al., 2015]] ; [[#Dolman--2018|Dolman and Laepple, 2018]] ).

Overall, the number, temporal resolution and chronological accuracy of paleoclimate reconstructions have increased since AR5, leading to improved understanding of climate system processes (or Earth system processes) ( ''hi'' ''gh confidence'' ).

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