Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.5.3.3 Estimates of ECS from Warm Periods of the Pre-Quaternary ====

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In the pre-Quaternary (prior to about 2.5 million years ago), the forcings and response are generally of the same sign and similar magnitude as future projections of climate change ( [[#Burke--2018|Burke et al., 2018]] ; [[#Tierney--2020a|Tierney et al., 2020a]] ). Similar uncertainties as for the LGM apply, but in this case a major uncertainty relates to the forcing, because prior to the ice-core record there are only indirect estimates of CO <sub>2</sub> concentration. However, advances in pre-ice-core CO <sub>2</sub> reconstruction (e.g., [[#Foster--2016|Foster and Rae, 2016]] ; [[#Super--2018|Super et al., 2018]] ; [[#Witkowski--2018|Witkowski et al., 2018]] ) mean that the estimates of pre-Quaternary CO <sub>2</sub> have less uncertainty than at the time of AR5, and these time periods can now contribute to an assessment of climate sensitivity (Table 7.11). The mid-Pliocene Warm Period (MPWP; Cross-Chapter Box 2.1 and Cross-Chapter Box 2.4) has been targeted for constraints on ECS ( [[#Martínez-Botí--2015|Martínez-Botí et al., 2015]] ; [[#Sherwood--2020|Sherwood et al., 2020]] ), due to the fact that CO <sub>2</sub> concentrations were relatively high at this time (350–425 ppm) and because the MPWP is sufficiently recent that topography and continental configuration are similar to modern-day. As such, a comparison of the MPWP with the pre-industrial climate provides probably the closest natural geological analogue for the modern day that is useful for assessing constraints on ECS, despite the effects of different geographies not being negligible (global surface temperature patterns; ocean circulation). Furthermore, the global surface temperature of the MPWP was such that non-linearities in feedbacks ( [[#7.4.3|Section 7.4.3]] ) were relatively modest. Within the MPWP, the KM5c interglacial has been identified as a particularly useful time period for assessing ECS ( [[#Haywood--2013|Haywood et al., 2013]] , 2016b) because Earth’s orbit during that time was very similar to that of the modern day.

Further back in time, in the Early Eocene (Cross-Chapter Box 2.1), uncertainties in forcing and temperature change become larger, but the signals are generally larger too ( [[#Anagnostou--2016|Anagnostou et al., 2016]] , 2020; [[#Shaffer--2016|Shaffer et al., 2016]] ; [[#Inglis--2020|Inglis et al., 2020]] ). Caution must be applied when estimating ECS from these time periods, due to differing continental position and topography/bathymetry ( [[#Farnsworth--2019|Farnsworth et al., 2019]] ), and due to temperature-dependence of feedbacks ( [[#7.4.3|Section 7.4.3]] ). On even longer time scales of the last 500 million years ( [[#Royer--2016|Royer, 2016]] ) the temperature and CO <sub>2</sub> measurements are generally asynchronous, presenting challenges in using this information for assessments of ECS.

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