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

==== 2.2.3.2 Glacial–Interglacial WMGHG Fluctuations from 800 Ka ====

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Since AR5, the number of ice cores for the last 800 kyr has increased and their temporal resolution has improved (Figure 2.4), especially for the last 60 kyr and when combined with analyses of firn air, leading to improved quantification of greenhouse gas concentrations prior to the mid-20th century.

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'''Figure 2.4 | Atmospheric well-mixed greenhouse gas (WMGHG) concentrations''' '''from ice cores. (a)''' Records during the last 800 kyr with the Last Glacial Maximum (LGM) to Holocene transition as inset. '''(b)''' Multiple high-resolution records over the CE. The horizontal black bars in panel (a) inset indicate LGM and Last Deglacial Termination (LDT) respectively. The red and blue lines in (b) are 100-year running averages for CO <sub>2</sub> and N <sub>2</sub> O concentrations, respectively. The numbers with vertical arrows in (b) are instrumentally measured concentrations in 2019. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

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===== 2.2.3.2.1 Carbon dioxide (CO <sub>2</sub> ) =====

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Records of CO <sub>2</sub> from the AIS formed during the last glacial period and the LDT show century-scale fluctuations of up to 9.6 ppm ( [[#Ahn--2012|Ahn et al., 2012]] ; [[#Ahn--2014|Ahn and Brook, 2014]] ; [[#Marcott--2014|Marcott et al., 2014]] ; [[#Bauska--2015|Bauska et al., 2015]] ; [[#Rubino--2019|Rubino et al., 2019]] ). Although these rates are an order of magnitude lower than those directly observed over 1919–2019 CE ( [[#2.2.3.3.1|Section 2.2.3.3.1]] ), they provide information on non-linear responses of climate-biogeochemical feedbacks (Section 5.1.2). Multiple records for 0–1850 CE show CO <sub>2</sub> mixing ratios of 274–285 ppm. Offsets among ice core records are about 1%, but the long-term trends agree well and show coherent multi-centennial variations of about 10 ppm ( [[#Ahn--2012|Ahn et al., 2012]] ; [[#Bauska--2015|Bauska et al., 2015]] ; [[#Rubino--2019|Rubino et al., 2019]] ). Multiple records show CO <sub>2</sub> concentrations of 278.3 ± 2.9 ppm in 1750 and 285.5 ± 2.1 ppm in 1850 ( [[#Siegenthaler--2005|Siegenthaler et al., 2005]] ; [[#MacFarling%20Meure--2006|MacFarling Meure et al., 2006]] ; [[#Ahn--2012|Ahn et al., 2012]] ; [[#Bauska--2015|Bauska et al., 2015]] ). CO <sub>2</sub> concentration increased by 5.0 ± 0.8 ppm during 970–1130 CE, followed by a decrease of 4.6 ± 1.7 ppm during 1580–1700 CE. The greatest rate of change over the CE prior to 1750 is observed at about 1600 CE, and ranges from –6.9 to +4.7 ppm per century in multiple high-resolution ice core records ( [[#Siegenthaler--2005|Siegenthaler et al., 2005]] ; [[#MacFarling%20Meure--2006|MacFarling Meure et al., 2006]] ; [[#Ahn--2012|Ahn et al., 2012]] ; [[#Bauska--2015|Bauska et al., 2015]] ; [[#Rubino--2019|Rubino et al., 2019]] ). Although ice core records present low-pass filtered time series due to gas diffusion and gradual bubble close-off in the snow layer over the ice sheet ( [[#Fourteau--2020|Fourteau et al., 2020]] ), the rate of increase since 1850 CE (about 125 ppm increase over about 170 years) is far greater than implied for any 170-year period by ice core records that cover the last 800 ka ( ''very high confidence'' ).

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===== 2.2.3.2.2 Methane (CH <sub>4</sub> ) =====

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CH <sub>4</sub> concentrations over the past 110 kyr are higher in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH), but closely correlated on centennial and millennial timescales ( [[#Buizert--2015|Buizert et al., 2015]] ). On glacial to interglacial cycles, approximately 450 ppb oscillations in CH <sub>4</sub> concentrations have occurred ( [[#Loulergue--2008|Loulergue et al., 2008]] ). On millennial timescales, most rapid climate changes observed in Greenland and other regions are coincident with rapid CH <sub>4</sub> changes ( [[#Buizert--2015|Buizert et al., 2015]] ; [[#Rhodes--2015|Rhodes et al., 2015]] , 2017). The variability of CH <sub>4</sub> on centennial timescales during the early Holocene does not significantly differ from that of the late Holocene prior to about 1850 ( [[#Rhodes--2013|Rhodes et al., 2013]] ; [[#Yang--2017|Yang et al., 2017]] ). The LGM concentration was 390.5 ± 6.0 ppb ( [[#Kageyama--2017|Kageyama et al., 2017]] ). The global mean concentrations during 0–1850 CE varied between 625 and 807 ppb. High-resolution ice core records from Antarctica and Greenland exhibit the same trends with an inter-polar difference of 36–47 ppb ( [[#Sapart--2012|Sapart et al., 2012]] ; L. [[#Mitchell--2013|]] [[#Mitchell--2013|Mitchell et al., 2013]] ). There is a long-term positive trend of about 0.5 ppb per decade during the CE until 1750 CE. The most rapid CH <sub>4</sub> changes prior to industrialization were as large as 30–50 ppb on multi-decadal timescales. Global mean CH <sub>4</sub> concentrations estimated from Antarctic and Greenland ice cores are 729.2 ± 9.4 ppb in 1750 and 807.6 ± 13.8 ppb in 1850 (L. [[#Mitchell--2013|]] [[#Mitchell--2013|Mitchell et al., 2013]] ).

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===== 2.2.3.2.3 Nitrous oxide (N <sub>2</sub> O) =====

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New records show that N <sub>2</sub> O concentration changes are associated with glacial-interglacial transitions ( [[#Schilt--2014|Schilt et al., 2014]] ). The most rapid change during the last glacial termination is a 30 ppb increase in a 200-year period, which is an order of magnitude smaller than the modern rate ( [[#2.2.3.3|Section 2.2.3.3]] ). During the LGM, N <sub>2</sub> O was 208.5 ± 7.7 ppb ( [[#Kageyama--2017|Kageyama et al., 2017]] ). Over the Holocene the lowest value was 257 ± 6.6 ppb during 6–8 ka, but millennial variation is not clearly detectable due to analytical uncertainty and insufficient ice core quality ( [[#Flückiger--2002|Flückiger et al., 2002]] ; [[#Schilt--2010|Schilt et al., 2010]] ). Recently acquired high-resolution records from Greenland and Antarctica for the last 2 kyr consistently show multi-centennial variations of about 5–10 ppb (Figure 2.4), although the magnitudes vary over time ( [[#Ryu--2020|Ryu et al., 2020]] ). Three high temporal resolution records exhibit a short-term minimum at about 600 CE of 261 ± 4 ppb ( [[#MacFarling%20Meure--2006|MacFarling Meure et al., 2006]] ; [[#Ryu--2020|Ryu et al., 2020]] ). It is ''very likely'' that industrial N <sub>2</sub> O increase started before 1900 CE ( [[#Machida--1995|Machida et al., 1995]] ; [[#Sowers--2001|Sowers, 2001]] ; [[#MacFarling%20Meure--2006|MacFarling Meure et al., 2006]] ; [[#Ryu--2020|Ryu et al., 2020]] ). Multiple ice cores show N <sub>2</sub> O concentrations of 270.1 ± 6.0 ppb in 1750 and 272.1 ± 5.7 ppb in 1850 ( [[#Machida--1995|Machida et al., 1995]] ; [[#Flückiger--1999|Flückiger et al., 1999]] ; [[#Sowers--2001|Sowers, 2001]] ; [[#Rubino--2019|Rubino et al., 2019]] ; [[#Ryu--2020|Ryu et al., 2020]] ).

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