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

=== 1.3.2 Lines of Evidence: Paleoclimate ===

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With the gradual acceptance of evidence for geological ‘deep time’ in the 19th century came investigation of fossils, geological strata, and other evidence pointing to large shifts in the Earth’s climate, from ice ages to much warmer periods, across thousands to billions of years. This awareness set off a search for the causes of climatic changes. The long-term perspective provided by paleoclimate studies is essential to understanding the causes and consequences of natural variations in climate, as well as crucial context for recent anthropogenic climatic change. The reconstruction of climate variability and change over recent millennia began in the 1800s ( [[#Brückner--1890|Brückner, 1890]] ; [[#Stehr--2000|Stehr and von Storch, 2000]] ; [[#Coen--2018|Coen, 2018]] , 2020). In brief, paleoclimatology reveals the key role of CO <sub>2</sub> and other greenhouse gases in past climatic variability and change, the magnitude of recent climate change in comparison to past glacial–interglacial cycles, and the unusualness of recent climate change ( [[#1.2.1.2|Section 1.2.1.2]] and Cross-Chapter Box 2.1; [[#Tierney--2020a|Tierney et al., 2020a]] ). FAQ 1.3 provides a plain-language summary of its importance.

Paleoclimate studies reconstruct the evolution of Earth’s climate over hundreds to billions of years using pre-instrumental historical archives, indigenous knowledge, and natural archives left behind by geological, chemical and biological processes (Figure 1.7). Paleoclimatology covers a wide range of temporal scales, ranging from the human historical past (decades to millennia) to geological deep time (millions to billions of years). Paleoclimate reference periods are presented in Cross-Chapter Box 2.1.

Historical climatology aids near-term paleoclimate reconstructions using media such as diaries, almanacs and merchant accounts that describe climate-related events such as frosts, thaws, flowering dates, harvests, crop prices and droughts ( [[#Lamb--1965|Lamb, 1965]] , 1995; [[#Le%20Roy%20Ladurie--1967|Le Roy Ladurie, 1967]] ; [[#Brázdil--2005|Brázdil et al., 2005]] ). Meticulous records by Chinese scholars and government workers, for example, have permitted detailed reconstructions of China’s climate back to 1000 CE, and even beyond ( [[#Louie--2003|Louie and Liu, 2003]] ; [[#Ge--2008|Ge et al., 2008]] ). Climatic phenomena such as large-scale, regionally and temporally distributed warmer and cooler periods of the past 2000 years were reconstructed from European historical records ( [[#Lamb--1965|Lamb, 1965]] , 1995; [[#Le%20Roy%20Ladurie--1967|Le Roy Ladurie, 1967]] ; [[#Neukom--2019|Neukom et al., 2019]] ).

Indigenous and local knowledge has played an increasing role in historical climatology, especially in areas where instrumental observations are sparse. Peruvian fishermen named the periodic El Niño warm current in the Pacific, which was linked by later researchers to the Southern Oscillation ( [[#Cushman--2004|Cushman, 2004]] ). Inuit communities have contributed to climatic history and community-based monitoring across the Arctic ( [[#Riedlinger--2001|Riedlinger and Berkes, 2001]] ; [[#Gearheard--2010|Gearheard et al., 2010]] ). Indigenous Australian knowledge of climatic patterns has been offered as a complement to sparse observational records ( [[#Green--2010|Green et al., 2010]] ; [[#Head--2014|Head et al., 2014]] ), such as those of sea-level rise ( [[#Nunn--2016|Nunn and Reid, 2016]] ). Ongoing research seeks to conduct further dialogue, utilize indigenous and local knowledge as an independent line of evidence complementing scientific understanding, and analyse their utility for multiple purposes, especially adaptation ( [[#Laidler--2006|Laidler, 2006]] ; [[#Alexander--2011|Alexander et al., 2011]] ; [[#IPCC--2019c|IPCC, 2019c]] ). Indigenous and local knowledge is used most extensively by IPCC WGII.

Certain geological and biological materials preserve evidence of past climate changes. These ‘natural archives’ include corals, trees, glacier ice, speleothems (stalactites and stalagmites), loess deposits (dust sediments), fossil pollen, peat, lake sediment and marine sediment ( [[#Stuiver--1965|Stuiver, 1965]] ; [[#Eddy--1976|Eddy, 1976]] ; [[#Haug--2001|Haug et al., 2001]] ; [[#Wang--2001|Wang et al., 2001]] ; [[#Jones--2009|Jones et al., 2009]] ; [[#Bradley--2015|Bradley, 2015]] ). By the early 20th century, laboratory research had begun to use tree rings to reconstruct precipitation and the possible influence of sunspots on climatic change ( [[#Douglass--1914|Douglass, 1914]] , 1919, 1922). Radiocarbon dating, developed in the 1940s ( [[#Arnold--1949|Arnold and Libby, 1949]] ), allows accurate determination of the age of carbon-containing materials from the past 50,000 years; this dating technique ushered in an era of rapid progress in paleoclimate studies.

On longer time scales, tiny air bubbles trapped in polar ice sheets provide direct evidence of past atmospheric composition, including CO <sub>2</sub> levels ( [[#Petit--1999|Petit et al., 1999]] ), and the <sup>18</sup> O isotope in frozen precipitation serves as a proxy marker for temperature ( [[#Dansgaard--1954|Dansgaard, 1954]] ). Sulphate deposits in glacier ice and as ash layers within sediment record major volcanic eruptions, providing another mechanism for dating. The first paleoclimate reconstructions used an almost 100-kyr ice core taken at Camp Century, Greenland ( [[#Dansgaard--1969|Dansgaard et al., 1969]] ; [[#Langway%20Jr--2008|Langway Jr, 2008]] ). Subsequent cores from Antarctica extended this climatic record to 800 kyr ( [[#EPICA%20Community%20Members--2004|EPICA Community Members, 2004]] ; [[#Jouzel--2013|Jouzel, 2013]] ). Comparisons of air contained in these ice samples against measurements from the recent past enabled AR5 WGI to assess that atmospheric concentrations of CO <sub>2</sub> , methane (CH <sub>4</sub> ), and nitrous oxide (N <sub>2</sub> O) had all increased to levels unprecedented in at least the last 800,000 years (Figure 1.5; [[#IPCC--2013b|IPCC, 2013b]] ).

Global reconstructions of sea surface temperature were developed from material contained in deep-sea sediment cores (CLIMAP Project Members et al., 1976), providing the first quantitative constraints for model simulations of ice-age climates (e.g., [[#Rind--1985|Rind and Peteet, 1985]] ). Paleoclimate data and modelling showed that the Atlantic Ocean circulation has not been stable over glacial–interglacial time periods, and that many changes in ocean circulation are associated with abrupt transitions in climate in the North Atlantic region ( [[#Ruddiman--1981|Ruddiman and McIntyre, 1981]] ; [[#Broecker--1985|Broecker et al., 1985]] ; [[#Boyle--1987|Boyle and Keigwin, 1987]] ; [[#Manabe--1988|Manabe and Stouffer, 1988]] ).

By the early 20th century, cyclical changes in insolation due to the interacting periodicities of orbital eccentricity, axial tilt and axial precession had been hypothesized as a chief pacemaker of ice age–interglacial cycles on multi-millennial time scales ( [[#Milankovitch--1920|Milankovitch, 1920]] ). Paleoclimate information derived from marine sediment provides quantitative estimates of past temperature, ice volume and sea level over millions of years (Figure 1.5; [[#Emiliani--1955|Emiliani, 1955]] ; [[#Shackleton--1973|Shackleton and Opdyke, 1973]] ; [[#Siddall--2003|Siddall et al., 2003]] ; [[#Lisiecki--2005|Lisiecki and Raymo, 2005]] ; [[#Past%20Interglacials%20Working%20Group%20of%20PAGES--2016|Past Interglacials Working Group of PAGES, 2016]] ). These estimates have bolsteredthe orbital cycles hypothesis ( [[#Hays--1976|Hays et al., 1976]] ; [[#Berger--1977|Berger, 1977]] , 1978). However, paleoclimatology of multi-million to billion-year periods reveals that CH <sub>4</sub> , CO <sub>2</sub> , continental drift, silicate rock weathering and other factors played a greater role than orbital cycles in climate changes during ice-free ‘hothouse’ periods of Earth’s distant past ( [[#Frakes--1992|Frakes et al., 1992]] ; [[#Bowen--2015|Bowen et al., 2015]] ; [[#Zeebe--2016|Zeebe et al., 2016]] ).

The AR5 WGI ( [[#IPCC--2013b|IPCC, 2013b]] ) used paleoclimatic evidence to put recent warming and sea level rise in a multi-century perspective and assessed that 1983–2012 was ''likely'' to have been the warmest 30-year period of the last 1400 years in the Northern Hemisphere ( ''medium confidence'' ). The AR5 also assessed that the rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia ( ''hi'' ''gh confidence'' ).

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