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

==== 5.1.2.1 Cenozoic Proxy CO <sub>2</sub> Record ====

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Quantifying past changes in the rate of CO <sub>2</sub> accumulation in the atmosphere based on reconstructions using marine sediment proxies is complex as age model uncertainties, assumptions and shortcomings underlying proxy applications and sedimentary processes conspire to alter and confound rate estimates ( [[#Ajayi--2020|Ajayi et al., 2020]] ). Differential sediment mixing and bioturbation contribute to smooth and attenuate proxy records ( [[#Hupp--2020|Hupp and Kelly, 2020]] ), thereby tending to underestimate maximum rates of change ( [[#Kemp--2015|Kemp et al., 2015]] ). Considering the extent to which uncertainties can affect sediment-based rate estimates, and notwithstanding recent effort in minimizing their inherent contribution, there is generally ''low to medium confidence'' in quantifying rates of change on a time scale less than a decade back thousands of years, and less than a millennium back millions of years in the past based on marine sediments.

In the past, atmospheric CO <sub>2</sub> concentrations reached much higher levels than present day (Cross-Chapter Box 2.1 and Figure 5.3). In particular, the Paleocene–Eocene thermal maximum (PETM), 55.9–55.7 Ma (Figure 5.3), provides some level of comparison with the current and projected anthropogenic increase in CO <sub>2</sub> emissions (Chapter 2). Atmospheric CO <sub>2</sub> concentrations increased from about 900 to around 2000 ppm in 3–20 kyr as a result of geological carbon release to the ocean–atmosphere system ( [[#Zeebe--2016|Zeebe et al., 2016]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ; [[#Cui--2018|Cui and Schubert, 2018]] ; [[#Kirtland%20Turner--2018|Kirtland Turner, 2018]] ). There is ''low'' to ''medium confidence'' in evaluations of the total amount of carbon released during the PETM, as proxy data constrained estimates vary from around 3000 to more than 7000 PgC, with methane hydrates, volcanic emissions, terrestrial and/or marine organic carbon, or some combination thereof, as the probable sources of carbon ( [[#Zeebe--2009|Zeebe et al., 2009]] ; [[#Cui--2011|Cui et al., 2011]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ; [[#Elling--2019|Elling et al., 2019]] ; [[#Jones--2019|]] [[#Jones--2019|Jones et al., 2019]] ; [[#Haynes--2020|Haynes and Hönisch, 2020]] ). Methane emissions related to hydrate/permafrost thawing and fossil carbon oxidation may have acted as positive feedbacks ( [[#Lunt--2011|Lunt et al., 2011]] ; [[#Armstrong%20McKay--2018|Armstrong McKay and Lenton, 2018]] ; [[#Lyons--2019|Lyons et al., 2019]] ), as the inferred increase in atmospheric CO <sub>2</sub> can only account for approximately half of the reported warming ( [[#Zeebe--2009|Zeebe et al., 2009]] ). The estimated, time-integrated carbon input is broadly similar to the RCP8.5 extension scenario, although CO <sub>2</sub> emissions rates (0.3–1.5 Pg yr <sup>–1</sup> ) and by inference the rate of CO <sub>2</sub> accumulation in the atmosphere (4–42 ppm per century) during the PETM were at least 4–5 lower than during the modern era (from 1995 to 2014; Table 2.1; [[#Zeebe--2016|Zeebe et al., 2016]] ; [[#Gingerich--2019|Gingerich, 2019]] ).

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[[File:216de2b6c3c945cc85550154b813b7ff IPCC_AR6_WGI_Figure_5_3.png]]

'''Figure 5.3 |''' '''Atmospheric CO''' <sub>2</sub> '''concentrations and growth rates for the past 60 million years (Myr) and projections to 2100.''' '''(a)''' CO <sub>2</sub> concentrations data for the period 60 Myr to the time prior to 800 kyr (left column) are shown as the LOESS Fit and 68% range (data from Chapter 2) ( [[#Foster--2017|Foster et al., 2017]] ). Concentrations from 1750 and projections through 2100 are taken from Shared Socio-economic Pathways of IPCC AR6 ( [[#Meinshausen--2017|Meinshausen et al., 2017]] ). '''(b)''' Growth rates are shown as the time derivative of the concentration time series. Inserts in (b) show growth rates at the scale of the sampling resolution. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

The last 50 Myr (50 million years) have been characterized by a gradual decline in atmospheric CO <sub>2</sub> levels at a rate of about 16 ppm Myr <sup>–1</sup> (Figure 5.3; [[#Foster--2017|Foster et al., 2017]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ). The exact cause of this long-term change in CO <sub>2</sub> remains uncertain, but may be related to an imbalance between long-term sources of CO <sub>2</sub> (volcanic outgassing) and long-term sinks (organic carbon burial and silicate weathering).

The most recent time interval when atmospheric CO <sub>2</sub> concentration was as high as 1000 ppm (i.e., similar to the end of 21st century projection for the high-end emissions scenario RCP8.5) was around 33.5 Ma, prior to the Eocene-Oligocene transition ( [[#Zhang--2013|]] [[#Zhang--2013|Zhang et al., 2013]] ; [[#Anagnostou--2016|Anagnostou et al., 2016]] ). Atmospheric CO <sub>2</sub> levels then reached a critical threshold (1000–750 ppm; [[#DeConto--2008|DeConto et al., 2008]] ) to allow for the development of permanent regional ice sheets on Antarctica, associated with changes in Southern Ocean hydrography, which would have increased deep ocean CO <sub>2</sub> storage ( [[#Leutert--2020|Leutert et al., 2020]] ).

The most recent interval characterized by atmospheric CO <sub>2</sub> levels similar to modern (i.e., 360–420 ppm) was the mid-Pliocene Warm Period (MPWP, 3.3–3.0 Ma; Martínez-Botí et al., 2015; [[#de%20la%20Vega--2020|de la Vega et al., 2020]] ) (Chapter 2). The relatively high atmospheric CO <sub>2</sub> concentration during the MPWP are related to vigorous ocean circulation and a rather inefficient marine biological carbon pump ( [[#Burls--2017|Burls et al., 2017]] ), which would have reduced deep ocean carbon storage. After the MPWP, atmospheric CO <sub>2</sub> concentrations declined gradually at a rate of 30 ppm Myr <sup>–1</sup> (Figure 5.3; [[#de%20la%20Vega--2020|de la Vega et al., 2020]] ), as an increase in ocean stratification led to enhanced ocean carbon storage, allowing for major, sustained advances in Northern Hemisphere ice sheets, 2.7 Ma ( [[#Sigman--2004|Sigman et al., 2004]] ; [[#DeConto--2008|DeConto et al., 2008]] ).

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