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

==== 2.2.3.1 CO <sub>2</sub> During 450 Ma to 800 ka ====

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Isotopes from continental and marine sediments using improved analytical techniques and sampling resolution have reinforced the understanding of long-term changes in atmospheric CO <sub>2</sub> during the past 450 Myr (Table 2.1 and Figure 2.3). In particular, for the last 60 Myr, sampling resolution and accuracy of the boron isotope proxy in ocean sediments has improved ( [[#Penman--2014|Penman et al., 2014]] ; [[#Anagnostou--2016|Anagnostou et al., 2016]] , 2020; [[#Chalk--2017|Chalk et al., 2017]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ; [[#Babila--2018|Babila et al., 2018]] ; [[#Dyez--2018|Dyez et al., 2018]] ; [[#Raitzsch--2018|Raitzsch et al., 2018]] ; [[#Sosdian--2018|Sosdian et al., 2018]] ; [[#Henehan--2019|Henehan et al., 2019]] , 2020; [[#de%20la%20Vega--2020|de la Vega et al., 2020]] ; [[#Harper--2020|Harper et al., 2020]] ), the understanding of the alkenone CO <sub>2</sub> proxy has increased (e.g., [[#Badger--2019|Badger et al., 2019]] ; [[#Stoll--2019|Stoll et al., 2019]] ; Y. [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ; [[#Zhang--2020|Zhang et al., 2020]] ; [[#Rae--2021|Rae et al., 2021]] ) and new phytoplankton proxies have been developed and applied (e.g., [[#Witkowski--2018|Witkowski et al., 2018]] ). Understanding of the boron isotope CO <sub>2</sub> proxy has improved since AR5 with studies showing very good agreement between boron-CO <sub>2</sub> estimates and co-existing ice core CO <sub>2</sub> ( [[#Hönisch--2005|Hönisch and Hemming, 2005]] ; [[#Foster--2008|Foster, 2008]] ; [[#Henehan--2013|Henehan et al., 2013]] ; [[#Chalk--2017|Chalk et al., 2017]] ; [[#Raitzsch--2018|Raitzsch et al., 2018]] ; see Figure 2.3c). Such independent validation has proven difficult to achieve with the other available CO <sub>2</sub> proxies (e.g., [[#Badger--2019|Badger et al., 2019]] ; [[#Da--2019|Da et al., 2019]] ; [[#Stoll--2019|Stoll et al., 2019]] ; Y. [[#Zhang--2019|]] [[#Zhang--2019|Zhang et al., 2019]] ). Remaining uncertainties in these ocean sediment based proxies ( [[#Hollis--2019|Hollis et al., 2019]] ) partly limit the applicability of the alkenone δ <sup>13</sup> C and boron δ <sup>11</sup> B proxies beyond the Cenozoic, although new records are emerging, for example, [[#Jurikova--2020|Jurikova et al. (2020)]] . CO <sub>2</sub> estimates from the terrestrial CO <sub>2</sub> proxies, such as stomatal density in fossil plants and δ <sup>13</sup> C of palaeosol carbonates, are available for much of the last 420 Myr. Given the low sampling density, relatively large CO <sub>2</sub> uncertainty, and high age uncertainty (relative to marine sediments) of the terrestrial proxies, preference here is given to the marine based proxies (and boron in particular) where possible.

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'''Table 2.1 |''' '''Concentration (mixing ratios) and, where applicable, century time-scale rate of change of atmospheric CO''' <sub>2</sub> '''based on multiple datasets for target paleoclimate reference (Cross-Chapter Box 2.1, and Figure 2.34) and selected other periods.''' Modern data are from [[#2.2.3.3|Section 2.2.3.3]] and Annex III. ‘AR6’ denotes best estimates assessed in this report and propagated to Figure 2.34. Units for the rate of change are given only for centennial periods characterized by rapid changes. ''confidence'' levels are ''very high'' for instrumentally derived concentrations, ''high'' for values derived from air in glacier ice (back to LIG), ''medium'' for values supported by multiple proxy types (MPWP, EECO), and ''low'' for values from a single sedimentary proxy type (PETM). ‘ ''→'' ’ indicates transition from the beginning to the end of the time interval. Uncertainties for Modern are based on 2019 estimates. Last Millennium rate of range shows lowest and highest values attained during this period; LDT shows highest rate of change. N/A indicates that values are not available. See chapter data table for bibliographic citation and auxiliary information for each dataset (Table 2.SM.1).

{| class="wikitable"
|-
| '''Reference Period'''

| '''CO''' <sub>2</sub> '''Concentration (ppm) and Dataset Details'''

| '''Rate of Change (ppm per century)'''

|-
| Modern

(1995–2014)

| 359.6 to 360.4 ''→'' 396.7 to 397.5 (AR6)

| 192.3 to 198.3 <sup>a</sup> (AR6)

|-
| Last 100 years (1919–2019)

| 302.8 to 306.0 ''→'' 409.5 to 410.3 (AR6)

| 103.9 to 107.1 (AR6)

|-
| Approximate pre-industrial baseline (1850–1900; see Cross-Chapter Box 1.2)

| 283.4 to 287.6 ''→'' 294.8 to 298.0 (AR6);

284.3 <sup>b</sup> ''→'' 295.7 <sup>b</sup> (CMIP6)

| 16.5 to 27.1 <sup>a</sup> (AR6)

22.8 <sup>b,a</sup> (CMIP6)

|-
| Last millennium (1000–1750)

| 278.0 to 285.0 (AR6; average of WAIS Divide, Law Dome and EDML core data)

| –6.9 ~ 4.7 <sup>b</sup> (Law Dome); –1.9 ~ 3.2 <sup>b</sup> (EDML); –5.2 ~ 4.2 <sup>b</sup> (WAIS Divide)

|-
| MH

| 260.1 to 268.1 (Dome C; CMIP6)

| N/A

|-
| LDT

| 193.2 <sup>b</sup> ''→'' 271.2 <sup>b</sup> (AR6); 195.2 <sup>b</sup> ''→'' 265.3 <sup>b</sup> (Dome C); 191.2 <sup>b</sup> → 277.0 <sup>b</sup> (WAIS Divide)

| 9.6 <sup>b</sup> (WAIS Divide);

7.1 <sup>b</sup> (Dome C)

|-
| LGM

| 188.4 to 194.2 (AR6); 190.5 to 200.1 (WAIS Divide); 186.8 to 202.0 (Byrd); 184.9 to 193.1 (Dome C); 180.5 to 192.7 (Siple Dome); 190 <sup>b</sup> (PMIP6); 174.2 to 205.8 ( δ <sup>11</sup> B proxy)

| N/A

|-
| LIG

| 265.9 to 281.5 (AR6); 259.4 to 283.8 (Vostok); 266.2 to 285.4 (Dome C); 275 <sup>b</sup> (PMIP4) 282.2 to 305.8 ( δ <sup>11</sup> B proxy)

| N/A

|-
| MPWP (KM5c)

| 360 to 420 (AR6)

| N/A

|-
| EECO

| 1150 to 2500 (AR6)

| N/A

|-
| PETM

| 800 to 1000 ''→'' 1400 to 3150 (AR6)

| 4 to 42 (AR6)

|}

<sup>a</sup> Centennial rate of change estimated by extrapolation of data from a shorter time period. The values (x to y) represent ''very likely'' ranges (90% CIs).

<sup>b</sup> Data uncertainty is not estimated.

Levels were close to 1750 values during at least one prolonged interval during the Carboniferous and Permian (350–252 Ma). During the Triassic (251.9–201.3 Ma), atmospheric CO <sub>2</sub> mixing ratios reached a maximum of between 2000–5000 ppm (200–220 Ma). During the PETM (56 Ma) CO <sub>2</sub> rapidly rose from about 900 ppm to about 2000 ppm (Table 2.1; [[#Schubert--2013|Schubert and Jahren, 2013]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ; [[#Anagnostou--2020|Anagnostou et al., 2020]] ) in 3–20 kyr ( [[#Zeebe--2016|Zeebe et al., 2016]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ; [[#Turner--2018|Turner, 2018]] ). Estimated multi-millennial rates of CO <sub>2</sub> accumulation during this event range from 0.3–1.5 PgC yr <sup>–1</sup> ( [[#Gingerich--2019|Gingerich, 2019]] ), at least 4–5 times lower than current centennial rates (Section 5.3.1.1). Based on boron and carbon isotope data, supported by other proxies ( [[#Hollis--2019|Hollis et al., 2019]] ), atmospheric CO <sub>2</sub> during the EECO (50 Ma) was between 1150 and 2500 ppm ( ''medium confidence'' ), and then gradually declined over the last 50 Myr at a long-term rate of about 16 ppm Myr <sup>–1</sup> (Figure 2.3). The last time the CO <sub>2</sub> mixing ratio was as high as 1000 ppm (the level reached by some high emissions scenarios by 2100; Annex III) was prior to the Eocene-Oligocene transition (33.5 Ma; Figure 2.3) that was associated with the first major advance of the AIS ( [[#Pearson--2009|Pearson et al., 2009]] ; [[#Pagani--2011|Pagani et al., 2011]] ; [[#Anagnostou--2016|Anagnostou et al., 2016]] ; [[#Witkowski--2018|Witkowski et al., 2018]] ; [[#Hollis--2019|Hollis et al., 2019]] ). The compilation of [[#Foster--2017|Foster et al. (2017)]] constrained CO <sub>2</sub> concentration to between 290 and 450 ppm during the MPWP, based primarily on the boron-isotope data reported by [[#Martínez-Botí--2015b|Martínez-Botí et al. (2015b)]] , consistent with the AR5 range of 300–450 ppm. A more recent high-resolution boron isotope-based study revealed that CO <sub>2</sub> cycled during the MPWP from about 330 to about 390 ppm on orbital timescales, with a mean of about 370 ppm ( [[#de%20la%20Vega--2020|de la Vega et al., 2020]] ). Although data from other proxy types (e.g., stomatal density or δ <sup>13</sup> C of alkenones) have too low resolution to resolve the orbital-related variability of CO <sub>2</sub> during this interval (e.g., [[#Kürschner--1996|Kürschner et al., 1996]] ; [[#Stoll--2019|Stoll et al., 2019]] ) there is general agreement among the different proxy types with the boron-derived mean (e.g., [[#Stoll--2019|Stoll et al., 2019]] ). High-resolution sampling (about 1 sample per 3 kyr) with the boron-isotope proxy indicates mean CO <sub>2</sub> mixing ratios for the Marine Isotope Stage KM5c interglacial were 360–420 ppm ( ''medium confidence'' ) ( [[#de%20la%20Vega--2020|de la Vega et al., 2020]] ).

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[[File:d037d36a9535e99702f37968c89d6d5c IPCC_AR6_WGI_Figure_2_3.png]]

'''Figure 2.3 | The evolution of atmospheric CO''' <sub>2</sub> '''through the last 450 million years (450 Myr).''' The periods covered are 0–450 Ma '''(a)''' , 0–58 Ma '''(b)''' , and 0–3500 ka '''(c)''' , reconstructed from continental rock, marine sediment and ice core records. Note different time scales and axes ranges in panels (a), (b) and (c). Dark and light green bands in (a) are uncertainty envelopes at 68% and 95% uncertainty, respectively. 100 ppm in each panel is shown by the marker in the lower right-hand corner to aid comparison between panels. In panel (b) and (c) the major paleoclimate reference periods (CCB2.1) have been labelled, and in addition: MPT (Mid Pleistocene Transition), MCO (Miocene Climatic Optimum). Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Following the MPWP, the atmospheric CO <sub>2</sub> mixing ratio generally decreased at a rate of about 30 ppm Myr <sup>–1</sup> . It is ''very likely'' that CO <sub>2</sub> levels as high as the present were not experienced in the last 2 Myr ( [[#Hönisch--2009|Hönisch et al., 2009]] ; [[#Bartoli--2011|Bartoli et al., 2011]] ; [[#Martínez-Botí--2015a|Martínez-Botí et al., 2015a]] ; [[#Chalk--2017|Chalk et al., 2017]] ; [[#Dyez--2018|Dyez et al., 2018]] ; [[#Da--2019|Da et al., 2019]] ; [[#Stoll--2019|Stoll et al., 2019]] ). Related to the shift of glacial-interglacial cycle frequency from 40 to 100 kyr at 0.8–1.2 Ma, there was a decrease of glacial-period CO <sub>2</sub> ( [[#Chalk--2017|Chalk et al., 2017]] ; [[#Dyez--2018|Dyez et al., 2018]] ). These boron isotope-based CO <sub>2</sub> results agree with available records based on ancient ice exposed near the surface of the AIS ( [[#Yan--2019|Yan et al., 2019]] ), however, direct comparison is limited due to a lack of ancient ice cores with sufficiently continuous stratigraphy ( [[#Higgins--2015|Higgins et al., 2015]] ; [[#Brook--2018|Brook and Buizert, 2018]] ).

To conclude, there is ''high confidence'' that average EECO and MPWP (KM5c) CO <sub>2</sub> concentrations were higher than those preceding industrialization at 1150–2500 ppm and 360–420 ppm, respectively. Although there is some uncertainty due to the non-continuous nature of marine sediment records, the last time atmospheric CO <sub>2</sub> mixing ratio was as high as present was ''very likely'' more than 2 Ma.

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