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

===== 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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