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

=== 2.2.3 Well-mixed Greenhouse Gases (WMGHGs) ===

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Well-mixed greenhouse gases generally have lifetimes of more than several years. The AR5 assigned ''medium confidence'' to the values of atmospheric CO <sub>2</sub> concentrations (mixing ratios) during the warm geological periods of the early Eocene and Pliocene. It concluded with ''very high confidence'' that, by 2011, the mixing ratios of CO <sub>2</sub> , CH <sub>4</sub> , and N <sub>2</sub> O in the atmosphere exceeded the range derived from ice cores for the previous 800 kyr, and that the observed rates of increase of the greenhouse gases were unprecedented on centennial timescales over at least the past 22 kyr. It reported that over 2005–2011 atmospheric burdens of CO <sub>2</sub> , CH <sub>4</sub> , and N <sub>2</sub> O increased, with 2011 levels of 390.5 parts per million (ppm), 1803.2 parts per billion (ppb) and 324.2 ppb, respectively. Increases of CO <sub>2</sub> and N <sub>2</sub> O over 2005–2011 were comparable to those over 1996–2005, while CH <sub>4</sub> resumed increasing in 2007, after remaining nearly constant over 1999–2006. A comprehensive process-based assessment of changes in CO <sub>2</sub> , CH <sub>4</sub> , and N <sub>2</sub> O is undertaken in Chapter 5.

<div id="2.2.3.1" class="h3-container"></div>

<span id="co-2-during-450-ma-to-800-ka"></span>
==== 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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<span id="glacialinterglacial-wmghg-fluctuations-from-800-ka"></span>
==== 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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[[File:2f2d17b5b6508d0f035037b4a5fb2f75 IPCC_AR6_WGI_Figure_2_4.png]]

'''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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<span id="carbon-dioxide-co-2"></span>
===== 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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<span id="methane-ch-4"></span>
===== 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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<span id="nitrous-oxide-n-2-o"></span>
===== 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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<span id="modern-measurements-of-wmghgs"></span>

==== 2.2.3.3 Modern Measurements of WMGHGs ====

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In this section and for calculation of ERF, surface global averages are determined from measurements representative of the well-mixed lower troposphere. Global averages that include sites subject to significant anthropogenic activities or influenced by strong regional biospheric emissions are typically larger than those from remote sites, and require weighting accordingly (Table 2.2). This section focusses on global mean mixing ratios estimated from networks with global spatial coverage, and updated from the CMIP6 historical dataset ( [[#Meinshausen--2017|Meinshausen et al., 2017]] ) for periods prior to the existence of global networks.

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Table 2.2 '''|''' '''Atmospheric global annual mean mixing ratios (dry-air mole fraction) for well-mixed greenhouse gases.''' The table provides observed values for 2011 and 2019, and relative changes since 2011, for selected well-mixed, radiatively important gases (ERF &gt;0.001 W m <sup>–2</sup> ), estimated from various measurement networks or compilations. Units are parts per million (ppm) for CO <sub>2</sub> , parts per billion (ppb) for CH <sub>4</sub> and N <sub>2</sub> O, parts per trillion (ppt) for all other gases. Time series since 1750, data for additional gases, references, and network information can be found in [[IPCC:Wg1:Chapter:Annex-iii|Annex III]] and the corresponding electronic supplement. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

{| class="wikitable"
|-
| '''Species'''

| '''Lifetime,'''

'''AR6, ERF'''

| '''2011'''

| '''2019'''

| '''Change'''

| '''Network'''

| rowspan="35"|
| '''Species'''

| '''Lifetime,'''

'''AR6, ERF'''

| '''2011'''

| '''2019'''

| '''Change'''

| '''Network'''

|-
| rowspan="5"| '''CO''' <sub>2</sub>

| \#

| 390.5

| 409.9 (0.17)

| 5.0%

| NOAA* <sup>a</sup>

| rowspan="3"| '''HCFC-22'''

| 11.9

| 212.6

| 246.8 (0.5)

| 16.1%

| NOAA*

|-
| 409.9 (0.4)

| 389.7

| 409.5 (0.37)

| 5.1%

| SIO

| 246.8 (0.6)

| 213.7

| 246.7 (0.4)

| 15.5%

| AGAGE*

|-
| 2.156

| 390.2

| 409.6 (0.31)

| 5.0%

| CSIRO

| 0.053

| 209.0

| 244.1 (3.0)

| 22.0%

| UCI

|-
|
| 390.9

| 410.5 (0.30)

| 5.0%

| WMO

| rowspan="3"| '''HCFC-141b'''

| 9.4

| 21.3

| 24.4 (0.1)

| 14.4%

| NOAA*

|-
|
| 390.9

|
| CMIP6

| 24.4 (0.3)

| 21.4

| 24.3 (0.1)

| 13.7%

| AGAGE*

|-
| rowspan="6"| '''CH''' <sub>4</sub>

| 9.1–11.8

| 1803.1

| 1866.6 (1.0)

| 3.5%

| NOAA*

| 0.004

| 20.8

| 26.0 (0.3)

| 25.0%

| UCI

|-
| 1866.3 (3.3)

| 1803.6

| 1866.1 (2.0)

| 3.5%

| AGAGE*

| rowspan="3"| '''HCFC-142b'''

| 18

| 20.9

| 22.0 (0.1)

| 5.3%

| NOAA*

|-
| 0.544

| 1791.8

| 1860.8 (3.5)

| 3.9%

| UCI

| 22.3 (0.4)

| 21.5

| 22.5 (0.1)

| 5.0%

| AGAGE*

|-
|
| 1802.3

| 1862.5 (2.4)

| 3.3%

| CSIRO

| 0.004

| 21.0

| 22.8 (0.2)

| 8.6%

| UCI

|-
|
| 1813

| 1877 (3)

| 3.5%

| WMO

| rowspan="3"| '''HFC-134a'''

| 14

| 62.7

| 107.8 (0.4)

| 72%

| NOAA*

|-
|
| 1813.1

|
| CMIP6

| 107.6 (1.0)

| 62.8

| 107.4 (0.2)

| 71%

| AGAGE*

|-
| rowspan="5"| '''N''' <sub>2</sub> '''O'''

| 116–109

| 324.2

| 331.9 (0.2)

| 2.4%

| NOAA*

| 0.018

| 63.4

| 107.6 (1.7)

| 70%

| UCI

|-
| 332.1 (0.4)

| 324.7

| 332.3 (0.1)

| 2.4%

| AGAGE*

| rowspan="3"| '''HFC-125'''

| 30

| 10.1

| 29.1 (0.3)

| 187%

| NOAA*

|-
| 0.208

| 324.0

| 331.6 (0.3)

| 2.3%

| CSIRO

| 29.4 (0.6)

| 10.4

| 29.7 (0.1)

| 186%

| AGAGE*

|-
|
| 324.3

| 332.0 (0.2)

| 2.4%

| WMO

| 0.007

|
|-
|
| 324.2

|
| CMIP6

| rowspan="3"| '''HFC-23'''

| 228

| 24.1

| 32.4 (0.1)

| 35%

| AGAGE*

|-
| rowspan="3"| '''CFC-12'''

| 102

| 526.9

| 501.5 (0.3)

| –4.8%

| NOAA*

| 32.4 (0.1)

|
|-
| 503.1 (3.2)

| 529.6

| 504.6 (0.2)

| –4.7%

| AGAGE*

| 0.006

|
|-
| 0.180

| 525.3

| 508.4 (2.5)

| –3.2%

| UCI

| rowspan="3"| '''HFC-143a'''

| 51

| 11.9

| 23.8 (0.1)

| 100%

| NOAA*

|-
| rowspan="3"| '''CFC-11'''

| 52

| 237.2

| 226.5 (0.2)

| –4.5%

| NOAA*

| 24.0 (0.4)

| 12.1

| 24.2 (0.1)

| 100%

| AGAGE*

|-
| 226.2 (1.1)

| 237.4

| 225.9 (0.1)

| –4.8%

| AGAGE*

| 0.004

|
|-
| 0.066

| 237.9

| 224.9 (1.3)

| –5.5%

| UCI

| rowspan="3"| '''HFC-32'''

| 5.4

| 4.27

| 19.2 (0.3)

| 350%

| NOAA*

|-
| rowspan="3"| '''CFC-113'''

| 93

| 74.5

| 69.7 (0.1)

| –6.4%

| NOAA*

| 20.0 (1.4)

| 5.15

| 20.8 (0.2)

| 304%

| AGAGE*

|-
| 69.8 (0.3)

| 74.6

| 69.9 (0.1)

| –6.3%

| AGAGE*

| 0.002

|
|-
| 0.021

| 74.9

| 70.0 (0.5)

| –6.5%

| UCI

| rowspan="3"| '''CF''' <sub>4</sub>

| 50,000

| 79.0

| 85.5 (0.1)

| 8.2%

| AGAGE*

|-
| rowspan="3"| '''CFC-114'''

| 189

| 16.36

| 16.28 (0.03)

| –0.5%

| AGAGE*

| 85.5 (0.2)

|
|-
| 16.0 (0.05)

|
| 0.005

|
|-
| 0.005

|
| rowspan="3"| '''C''' <sub>2</sub> '''F''' <sub>6</sub>

| 10,000

| 4.17

| 4.85 (0.01)

| 16.3%

| AGAGE*

|-
| rowspan="3"| '''CFC-115'''

| 540

| 8.39

| 8.67 (0.02)

| 3.3%

| AGAGE*

| 4.85 (0.1)

|
|-
| 8.67 (0.02)

|
| 0.001

|
|-
| 0.002

|
| rowspan="3"| '''SF''' <sub>6</sub> <sub></sub>

| About 1000

| 7.32

| 9.96 (0.02)

| 36.1%

| NOAA*

|-
| rowspan="3"| '''CCl''' <sub>4</sub>

| 32

| 86.9

| 78.4 (0.1)

| –9.8%

| NOAA*

| 9.95 (0.01)

| 7.28

| 9.94 (0.02)

| 36.5%

| AGAGE*

|-
| 77.9 (0.7)

| 85.3

| 77.3 (0.1)

| –9.4%

| AGAGE*

| 0.006

|
|-
| 0.013

| 87.8

| 77.7 (0.7)

| –11.5%

| UCI

|
|}

AGAGE: Advanced Global Atmospheric Gases Experiment; SIO: Scripps Institution of Oceanography; NOAA: National Oceanic and Atmospheric Administration, Global Monitoring Laboratory; UCI: University of California, Irvine; CSIRO: Commonwealth Scientific and Industrial Research Organization, Aspendale, Australia; WMO: World Meteorological Organization, Global Atmosphere Watch, CMIP6 (Climate Model Intercomparison Project Phase 6). Mixing ratios denoted by AR6 are representative of the remote, unpolluted troposphere, derived from one or more measurement networks (denoted by *). Minor differences between 2011 values reported here and in the previous Assessment Report (AR5) are due to updates in calibration and data processing. ERF in 2019 is taken from Table 7.5, and the difference with the AR5 assessment reflects updates in the estimates of AR6 global mixing ratios and updated radiative calculations. Uncertainties, in parenthesis, are estimated at 90% confidence interval. Networks use different methods to estimate uncertainties. Some uncertainties have been rounded up to be consistent with the number of decimal places shown. Lifetime is reported in years: # indicates multiple lifetimes for CO <sub>2</sub> . For CH <sub>4</sub> and N <sub>2</sub> O the two values represent total atmospheric lifetime and perturbation lifetime.

<div id="2.2.3.3.1" class="h4-container"></div>

<span id="carbon-dioxide-co-2-1"></span>
===== 2.2.3.3.1 Carbon dioxide (CO <sub>2</sub> ) =====

<div id="h4-4-siblings" class="h4-siblings"></div>

There has been a positive trend in globally averaged surface CO <sub>2</sub> mixing ratios since 1958 (Figure 2.5a), that reflects the imbalance of sources and sinks (Section 5.2). The growth rate has increased overall since the 1960s (Figure 2.5a inset), while annual growth rates have varied substantially, for example, reaching a peak during the strong El Niño events of 1997–1998 and 2015–2016 ( [[#Bastos--2013|Bastos et al., 2013]] ; [[#Betts--2016|Betts et al., 2016]] ). The average annual CO <sub>2</sub> increase from 2000 through 2011 was 2.0 ppm yr <sup>–1</sup> (standard deviation 0.3 ppm yr <sup>–1</sup> ), similar to what was reported in AR5. From 2011 through 2019 it was 2.4 ppm yr <sup>–</sup> <sup>1</sup> (standard deviation 0.5 ppm yr <sup>–1</sup> ), which is higher than that of any comparable time period since global measurements began. Global networks consistently show that the globally averaged annual mean CO <sub>2</sub> has increased by 5.0% since 2011, reaching 409.9 ± 0.4 ppm in 2019 (NOAA measurements). Further assessment of changing seasonality is undertaken in [[#2.3.4.1|Section 2.3.4.1]] .

<div id="_idContainer018" class="Basic-Text-Frame"></div>

[[File:3e2fe664c93efaa7c3c0198d50490cdd IPCC_AR6_WGI_Figure_2_5.png]]

'''Figure 2.5 |''' '''Globally averaged dry-air mole fractions of greenhouse gases. (a)''' CO <sub>2</sub> from SIO, CSIRO, and NOAA/GML '''(b)''' CH <sub>4</sub> from NOAA, AGAGE, CSIRO, and UCI; and '''(c)''' N <sub>2</sub> O from NOAA, AGAGE, and CSIRO (Table 2.2). Growth rates, calculated as the time derivative of the global means after removing seasonal cycle are shown as inset figures. Note that the CO <sub>2</sub> series is 1958–2019 whereas CH <sub>4</sub> , and N <sub>2</sub> O are 1979–2019. Units are parts per million (ppm) or parts per billion (ppb). Further details on data are in Annex III, and on data sources and processing are available in the chapter data table (Table 2.SM.1).

<div id="2.2.3.3.2" class="h4-container"></div>

<span id="methane-ch-4-1"></span>
===== 2.2.3.3.2 Methane (CH <sub>4</sub> ) =====

<div id="h4-5-siblings" class="h4-siblings"></div>

The globally averaged surface mixing ratio of CH <sub>4</sub> in 2019 was 1866.3 ± 3.3 ppb, which is 3.5% higher than 2011, while observed increases from various networks range from 3.3–3.9% (Table 2.2 and Figure 2.5b). There are marked growth rate changes over the period of direct observations, with a decreasing rate from the late-1970s through the late-1990s, very little change in concentrations from 1999–2006, and resumed increases since 2006. Atmospheric CH <sub>4</sub> fluctuations result from complex variations of sources and sinks. A detailed discussion of recent methane trends and our understanding of their causes is presented in Cross-Chapter Box 5.2.

<div id="2.2.3.3.3" class="h4-container"></div>

<span id="nitrous-oxide-n-2-o-1"></span>
===== 2.2.3.3.3 Nitrous oxide (N <sub>2</sub> O) =====

<div id="h4-6-siblings" class="h4-siblings"></div>

The AR5 reported 324.2 ± 0.1 ppb for global surface annual mean N <sub>2</sub> O in 2011; since then, it has increased by 2.4% to 332.1 ± 0.4 ppb in 2019. Independent measurement networks agree well for both the global mean mixing ratio and relative change since 2011 (Table 2.2). Over 1995–2011, N <sub>2</sub> O increased at an average rate of 0.79 ± 0.05 ppb yr <sup>–1</sup> . The growth rate has been higher in recent years, amounting to 0.96 ± 0.05 ppb yr <sup>–</sup> <sup>1</sup> from 2012 to 2019 (Figure 2.5c and Section 5.2.3.5).

<div id="2.2.3.4" class="h3-container"></div>

<span id="summary-of-changes-in-wmghgs"></span>

==== 2.2.3.4 Summary of Changes in WMGHGs ====

<div id="h3-4-siblings" class="h3-siblings"></div>

In summary, CO <sub>2</sub> has fluctuated by at least 2000 ppm over the last 450 Myr ( ''medium confidence'' ). The last time CO <sub>2</sub> concentrations were similar to the present-day was over 2 Ma ( ''high confidence'' ). Further, it is certain that WMGHG mixing ratios prior to industrialization were lower than present-day levels and the growth rates of the WMGHGs from 1850 are unprecedented on centennial timescales in at least the last 800 kyr. During the glacial-interglacial climate cycles over the last 800 kyr, the concentration variations of the WMGHG were 50–100 ppm for CO <sub>2</sub> , 210–430 ppb for CH <sub>4</sub> and 60–90 ppb for N <sub>2</sub> O. Between 1750–2019 mixing ratios increased by 131.6 ± 2.9 ppm (47%), 1137 ± 10 ppb (156%), and 62 ± 6 ppb (23%), for CO <sub>2</sub> , CH <sub>4</sub> , and N <sub>2</sub> O, respectively ( ''very high confidence'' ). Since 2011 (AR5) mixing ratios of CO <sub>2</sub> , CH <sub>4</sub> , and N <sub>2</sub> O have further increased by 19 ppm, 63 ppb, and 7.7 ppb, reaching in 2019 levels of 409.9 (± 0.4) ppm, 1866.3 (± 3.3) ppb, and 332.1 (± 0.4) ppb, respectively. By 2019, the combined ERF (relative to 1750) of CO <sub>2</sub> , CH <sub>4</sub> and N <sub>2</sub> O was 2.9 ± 0.5 W m <sup>–2</sup> (Table 2.2; Section 7.3.2).

<div id="2.2.4" class="h2-container"></div>

<span id="halogenated-greenhouse-gases-cfcs-hcfcs-hfcs-pfcs-sf6-and-others"></span>