Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.2.1.2 Atmosphere ====

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Atmospheric CO <sub>2</sub> concentration measurements in remote locations began in 1957 at the South Pole Observatory (SPO) and in 1958 at Mauna Loa Observatory (MLO), Hawaii, USA ( [[#Keeling--1960|Keeling, 1960]] ) (Figure 5.6a). Since then, measurements have been extended to multiple locations around the world ( [[#Bacastow--1980|Bacastow et al., 1980]] ; [[#Conway--1994|Conway et al., 1994]] ; [[#Nakazawa--1997|Nakazawa et al., 1997]] ). In addition, high-density global observations of total column CO <sub>2</sub> measurements by dedicated GHG-observing satellites began in 2009 ( [[#Yoshida--2013|Yoshida et al., 2013]] ; [[#O’Dell--2018|O’Dell et al., 2018]] ). Annual mean CO <sub>2</sub> growth rates are observed to be 1.56 ± 0.18 ppm yr <sup>–1</sup> (average and range from 1 standard deviation of annual values) over the 61 years of atmospheric measurements (1959–2019), with the rate of CO <sub>2</sub> accumulation almost tripling from an average of 0.82 ± 0.29 ppm yr <sup>–1</sup> during the decade of 1960–1969 to 2.39 ± 0.37 ppm yr <sup>–1</sup> during the decade of 2010–2019 (Chapter 2). The latter agrees well with that derived for total column (XCO <sub>2</sub> ) measurements by the Greenhouse Gases Observing Satellite (GOSAT; Figure 5.6b). The interannual oscillations in monthly mean CO <sub>2</sub> growth rates (Figure 5.6b) show a close relationship with the El Niño–Southern Oscillation (ENSO) cycle (Figure 5.6b) due to the ENSO-driven changes in terrestrial and ocean CO <sub>2</sub> sources and sinks on the Earth’s surface ( [[#5.2.1.4|Section 5.2.1.4]] ).

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[[File:4bed2863808f5cd02d942ac319456bac IPCC_AR6_WGI_Figure_5_6.png]]

'''Figure 5.6 |''' '''Time series of CO''' <sub>2</sub> '''concentrations and related measurements in ambient air''' . '''(a)''' Concentration time series and MLO-SPO difference, '''(b)''' growth rates, '''(c)''' <sup>14</sup> C and <sup>13</sup> C isotopes, and '''(d)''' O <sub>2</sub> /N <sub>2</sub> ratio. The data for Mauna Loa Observatory (MLO) and South Pole Observatory (SPO) are taken from the Scripps Institution of Oceanography (SIO)/University of California, San Diego ( [[#Keeling--2001|Keeling et al., 2001]] ). The global mean CO <sub>2</sub> are taken from National Oceanic and Atmospheric Administration (NOAA) cooperative network (as in Chapter 2), and Greenhouse Gases Observing Satellite (GOSAT) monthly mean XCO <sub>2</sub> (mixing ratio) time series are taken from National Institute for Environmental Studies ( [[#Yoshida--2013|Yoshida et al., 2013]] ). CO <sub>2</sub> growth rates are calculated as the time derivative of deseasonalized time series ( [[#Nakazawa--1997|Nakazawa et al., 1997]] ). The D(O <sub>2</sub> /N 2 ) are expressed in per meg units (= (FF/M) × 10 <sup>6</sup> , where FF = moles of O <sub>2</sub> consumed by fossil-fuel burning, M = 3.706 × 10 <sup>19</sup> , total number of O <sub>2</sub> molecules in the atmosphere ( [[#Keeling--2014|Keeling and Manning, 2014]] ). The <sup>14</sup> CO <sub>2</sub> time series at Barring Head, Wellington, New Zealand (BHD) is taken from GNS Science and NIWA ( [[#Turnbull--2017|Turnbull et al., 2017]] ). The multivariate ENSO index (MEI) is shown as the shaded background in panel (b); (warmer shade indicates El Niño). Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

Multiple lines of evidence unequivocally establish the dominant role of human activities in the growth of atmospheric CO <sub>2</sub> . First, the systematic increase in the difference between the MLO and SPO records (Figure 5.6a) is caused primarily by the increase in emissions from fossil fuel combustion in industrialized regions that are situated predominantly in the Northern Hemisphere ( [[#Ciais--2019|Ciais et al., 2019]] ). Second, measurements of the stable carbon isotope in the atmosphere (d <sup>13</sup> C–CO <sub>2</sub> ) are more negative over time because CO <sub>2</sub> from fossil fuels extracted from geological storage is depleted in <sup>13</sup> C (Figure 5.6c; [[#Rubino--2013|Rubino et al., 2013]] ; [[#Keeling--2017|Keeling et al., 2017]] ). Third, measurements of the d(O <sub>2</sub> /N <sub>2</sub> ) ratio show a declining trend because for every molecule of carbon burned, 1.17 to 1.98 molecules of oxygen (O <sub>2</sub> ) is consumed (Figure 5.6d; [[#Ishidoya--2012|Ishidoya et al., 2012]] ; [[#Keeling--2014|Keeling and Manning, 2014]] ). These three lines of evidence confirm unambiguously that the atmospheric increase of CO <sub>2</sub> is due to an oxidative process (i.e., combustion). Fourth, measurements of radiocarbon ( <sup>14</sup> C–CO <sub>2</sub> ) at sites around the world ( [[#Levin--2010|Levin et al., 2010]] ; [[#Graven--2017|Graven et al., 2017]] ; [[#Turnbull--2017|Turnbull et al., 2017]] ) show a continued long-term decrease in the <sup>14</sup> C/ <sup>12</sup> C ratio. Fossil fuels are devoid of <sup>14</sup> C and therefore fossil fuel-derived CO <sub>2</sub> additions decrease the atmospheric <sup>14</sup> C/ <sup>12</sup> C ratio ( [[#Suess--1955|Suess, 1955]] ).

Over the past six decades, the fraction of anthropogenic CO <sub>2</sub> emissions that has accumulated in the atmosphere (referred to as airborne fraction) has remained near constant at approximately 44% (Figure 5.7) ( [[#Ballantyne--2012|Ballantyne et al., 2012]] ; [[#Ciais--2019|Ciais et al., 2019]] ; [[#Gruber--2019b|Gruber et al., 2019b]] ; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). This suggests that the land and ocean CO <sub>2</sub> sinks have continued to grow at a rate consistent with the growth rate of anthropogenic CO <sub>2</sub> emissions, albeit with large interannual and sub-decadal variability dominated by the land sinks (Figure 5.7).

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[[File:3ce7222807994e5277656bbc62e28bdc IPCC_AR6_WGI_Figure_5_7.png]]

'''Figure 5.7 |''' '''Airborne fraction and anthropogenic (fossil fuel and land-use change) CO''' <sub>2</sub> '''emissions.''' Data as in [[#5.2.1.1|Section 5.2.1.1]] . The multivariate El Niño–Southern Oscillation (ENSO) index (shaded) and the major volcanic eruptions are marked along the x-axis. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

Since AR5, an alternative observable diagnostic to the airborne fraction has been proposed to understand the trends in land and ocean sinks in response to its driving atmospheric CO <sub>2</sub> concentrations ( [[#Raupach--2014|Raupach et al., 2014]] ; [[#Bennedsen--2019|Bennedsen et al., 2019]] ). It is the sink rate that is defined as the combined ocean and land sink flux per unit of atmospheric excess of CO <sub>2</sub> above pre-industrial levels ( [[#Raupach--2014|Raupach et al., 2014]] ). The sink rate has declined over the past six decades, which indicates that the combined ocean and land sinks are not growing as fast as the growth in atmospheric CO <sub>2</sub> ( [[#Raupach--2014|Raupach et al., 2014]] ; [[#Bennedsen--2019|Bennedsen et al., 2019]] ). Possible explanations for the sink rate decline are that the land and/or ocean CO <sub>2</sub> sinks are no longer responding linearly with CO <sub>2</sub> concentrations or that anthropogenic emissions are slower than exponential (Figure 5.7 and Sections 5.2.1.3 and 5.2.1.4; [[#Gloor--2010|Gloor et al., 2010]] ; [[#Raupach--2014|Raupach et al., 2014]] ; [[#Bennedsen--2019|Bennedsen et al., 2019]] ). In addition, both diagnostics are influenced by major climate modes (e.g., ENSO) and volcanic eruptions that contribute to high interannual variability ( [[#Gloor--2010|Gloor et al., 2010]] ; [[#Frölicher--2013|Frölicher et al., 2013]] ; [[#Raupach--2014|Raupach et al., 2014]] ), suggesting high sensitivity to future climate change. Uncertain land-use change fluxes ( [[#5.2.1.2|Section 5.2.1.2]] ) influence the robustness of the trends. Based on the airborne fraction (AF), it is concluded with ''medium confidence'' that both ocean and land CO <sub>2</sub> sinks have grown consistent with the rising of anthropogenic emissions. Further research is needed to understand the drivers of changes in the CO <sub>2</sub> sink rate.

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