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

==== 5.1.2.3 Holocene Changes ====

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Atmospheric GHG concentrations were much less variable during the pre-industrial Holocene (from 11.7 ka to 1750 CE). Atmospheric CH <sub>4</sub> concentrations decreased at the beginning of the Holocene, consistent with a general weakening of boreal sources ( [[#Yang--2017|Yang et al., 2017]] ; [[#Beck--2018|Beck et al., 2018]] ) and further decline during the mid-Holocene owing to a reduction in Southern Hemisphere emissions concomitant with a southward shift of the ITCZ ( [[#Singarayer--2011|Singarayer et al., 2011]] ; [[#Beck--2018|Beck et al., 2018]] ). Atmospheric CH <sub>4</sub> concentrations increased about 5 ka, which prompted the hypothesis of an early anthropogenic influence related to land-use changes in South East Asia ( [[#Ruddiman--2016|Ruddiman et al., 2016]] ). However, stable isotope compositions on CH <sub>4</sub> extracted from Greenland and Antarctic ice ( [[#Beck--2018|Beck et al., 2018]] ) reveal that natural emissions located in the southern tropics were responsible for the rise in atmospheric CH <sub>4</sub> concentrations, in line with model simulations ( [[#Singarayer--2011|Singarayer et al., 2011]] ) thus disputing the early anthropogenic influence on the global CH <sub>4</sub> budget. Atmospheric N <sub>2</sub> O concentrations increased slightly (20 ppb) across the Holocene, associated with a gradual decline in its nitrogen stable isotope composition (H. [[#Fischer--2019|]] [[#Fischer--2019|Fischer et al., 2019]] ). The combined signal is consistent with a small increase in terrestrial emissions, offset by a reduction in marine emissions ( [[#Schilt--2010b|Schilt et al., 2010b]] ; [[#Fischer--2019|]] [[#Fischer--2019|Fischer et al., 2019]] ).

The early Holocene decrease in CO <sub>2</sub> concentration by about 5 ppm ( [[#Schmitt--2012|Schmitt et al., 2012]] ) has been attributed to post-glacial regrowth in terrestrial biomass and a gradual increase in peat reservoirs over the Holocene, resulting in the sequestration of several hundred PgC ( [[#Yu--2010|Yu et al., 2010]] ; [[#Nichols--2019|Nichols and Peteet, 2019]] ). Peat accumulation rates in boreal and temperate regions were higher under warmer summer conditions in the early to mid-Holocene ( [[#Loisel--2014|Loisel et al., 2014]] ; [[#Stocker--2017|Stocker et al., 2017]] ). The 20 ppm gradual increase of atmospheric CO <sub>2</sub> starting 7 ka has been attributed to a decrease in natural terrestrial biomass due to climate change, carbonate compensation and enhanced shallow water carbonate deposition ( [[#Menviel--2012|Menviel and Joos, 2012]] ; [[#Brovkin--2016|Brovkin et al., 2016]] ), consistent with stable carbon isotope measurements on CO <sub>2</sub> extracted from Antarctic ice ( [[#Elsig--2009|Elsig et al., 2009]] ; [[#Schmitt--2012|Schmitt et al., 2012]] ). These isotopic measurements do not support an early anthropogenic influence on atmospheric CO <sub>2</sub> due to land-use change and forest clearing ( [[#Ruddiman--2016|Ruddiman et al., 2016]] ). Recent paleoceanographic evidence suggests that remineralized carbon outgassing associated with increased Southern Ocean circulation and upwelling ( [[#Studer--2018|Studer et al., 2018]] ), possibly promoted by stronger Southern Hemisphere westerly winds ( [[#Saunders--2018|Saunders et al., 2018]] ), could have additionally contributed to the late Holocene increase in atmospheric CO <sub>2</sub> concentrations. However, the role of these mechanisms remained insignificant in transient Holocene ESM simulations ( [[#Brovkin--2019|Brovkin et al., 2019]] ). Overall, as in AR5 (WGI, Chapter 5), there is ''medium confidence'' in the key drivers of the CO <sub>2</sub> increase between the early Holocene and the beginning of the industrial era, yet there is ''low confidence'' in the relative contributions of these drivers due to insufficient quantitative constraints on particular processes.

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