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

==== 5.3.1.1 Paleocene–Eocene Thermal Maximum ====

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The Paleocene–Eocene Thermal Maximum (PETM) was an episode of global warming exceeding pre-industrial temperatures by 4°C–8°C ( [[#McInerney--2011|McInerney and Wing, 2011]] ; [[#Dunkley%20Jones--2013|Dunkley]] [[#Jones--2013|]] [[#Jones--2013|Jones et al., 2013]] ) that occurred 55.9–55.7 Ma. The PETM involved a large pulse of geologic CO <sub>2</sub> released into the ocean–atmosphere system in 3–20 kyr ( [[#Zeebe--2016|Zeebe et al., 2016]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ; [[#Kirtland%20Turner--2017|Kirtland]] [[#Turner--2017|Turner et al., 2017]] ; [[#Kirtland%20Turner--2018|Kirtland Turner, 2018]] ; [[#Gingerich--2019|Gingerich, 2019]] ; [[#5.2.1.1|Section 5.2.1.1]] ). In response, observationally constrained model simulations report an increase in atmospheric CO <sub>2</sub> concentrations ranging from about 900 ppm to &gt;2000 ppm (Chapter 2; [[#Gutjahr--2017|Gutjahr et al., 2017]] ; [[#Cui--2018|Cui and Schubert, 2018]] ; [[#Anagnostou--2020|Anagnostou et al., 2020]] ). The PETM thus provides a test for our understanding of the ocean’s response to the increase in carbon (and heat) emissions over geologically short time scales.

A limited number of independent proxy records indicate that the PETM was associated with a surface ocean pH decline ranging from 0.15 to 0.30 units ( [[#Penman--2014|Penman et al., 2014]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ; [[#Babila--2018|Babila et al., 2018]] ) ''.'' It was also accompanied by a rapid (&lt;10 ka) shallowing of the carbonate saturation horizon, resulting in the widespread dissolution of sedimentary carbonate, followed by a gradual (100 kyr) recovery ( [[#Zachos--2005|Zachos et al., 2005]] ; [[#Bralower--2018|Bralower et al., 2018]] ). The remarkable similarity among sedimentary records spanning a wide range of ecosystems suggests with ''medium confidence'' that the perturbation in the ocean carbonate saturation was global ( [[#Babila--2018|Babila et al., 2018]] ) and directly resulted from elevated atmospheric CO <sub>2</sub> levels. The degree of acidification is similar to the 0.4 pH unit decrease projected for the end of the 21st century under RCP8.5 ( [[#Gattuso--2015|Gattuso et al., 2015]] ) and is estimated to have occurred at a rate about one order of magnitude slower than the current rate of ocean acidification ( [[#Zeebe--2016|Zeebe et al., 2016]] ). There is ''low confidence'' in the inferred rates of ocean acidification inherent to the range of uncertainties affecting rates estimates based on marine sediments ( [[#5.1.2.1|Section 5.1.2.1]] ).

Recent model outputs and globally distributed geochemical data reveal with ''medium confidence'' widespread ocean deoxygenation during the PETM ( [[#Dickson--2012|Dickson et al., 2012]] , 2014; [[#Winguth--2012|Winguth et al., 2012]] ; [[#Chang--2018|Chang et al., 2018]] ; [[#Remmelzwaal--2019|Remmelzwaal et al., 2019]] ), with parts of the ocean potentially becoming drastically oxygen-depleted (anoxic; [[#Yao--2018|Yao et al., 2018]] ; [[#Clarkson--2021|Clarkson et al., 2021]] ). Deoxygenation affected the surface ocean globally (including the Arctic Ocean; [[#Sluijs--2006|Sluijs et al., 2006]] ), due to vertical and lateral expansion of OMZs ( [[#Zhou--2014|Zhou et al., 2014]] ) that resulted from warming and related changes in ocean stratification ''.'' Expansion of OMZs may have stimulated N <sub>2</sub> O production through water-column (de)nitrification ( [[#Junium--2018|Junium et al., 2018]] ). The degree to which N <sub>2</sub> O production impacted PETM warming, however, has not yet been established.

The feedbacks associated with recovery from the PETM are uncertain, yet could include drawdown associated with silicate weathering ( [[#Zachos--2005|Zachos et al., 2005]] ) and regrowth of terrestrial and marine organic carbon stocks ( [[#Bowen--2010|Bowen and Zachos, 2010]] ; [[#Gutjahr--2017|Gutjahr et al., 2017]] ).

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