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

===== 7.4.4.1.2 Polar amplification from proxies and models during past climates associated with CO 2 change =====

<div id="h4-14-siblings" class="h4-siblings"></div>

Paleoclimate proxy data provide observational evidence of large-scale patterns of surface warming in response to past forcings, and allow an evaluation of the modelled response to these forcings (Sections 3.3.1.1 and 3.8.2.1). In particular, paleoclimate data provide evidence for long-term changes in polar amplification during time periods in which the primary forcing was a change in atmospheric CO <sub>2</sub> , although data sparsity means that for some time periods this evidence may be limited to a single hemisphere or ocean basin, or the evidence may come primarily from the mid-latitudes as opposed to the polar regions. In this context, there has been a modelling and data focus on the Last Glacial Maximum (LGM) in the context of PMIP4 ( [[#Cleator--2020|Cleator et al., 2020]] ; [[#Tierney--2020b|Tierney et al., 2020b]] ; [[#Kageyama--2021|Kageyama et al., 2021]] ), the mid-Pliocene Warm Period (MPWP) in the context of PlioMIP2 (Cross-Chapter Box 2.4; [[#Salzmann--2013|Salzmann et al., 2013]] ; [[#Haywood--2020|Haywood et al., 2020]] ; [[#McClymont--2020|McClymont et al., 2020]] ), the Early Eocene Climatic Optimum (EECO) in the context of DeepMIP ( [[#Hollis--2019|Hollis et al., 2019]] ; [[#Lunt--2021|Lunt et al., 2021]] ), and there is growing interest in the Miocene ( [[#Goldner--2014b|Goldner et al., 2014b]] ; [[#Steinthorsdottir--2021|Steinthorsdottir et al., 2021]] ; for definitions of time periods see Cross-Chapter Box 2.1). For all these time periods, in addition to the CO <sub>2</sub> forcing there are long-term feedbacks associated with ice sheets ( [[#7.4.2.6|Section 7.4.2.6]] ), and in particular for the Early Eocene there is a forcing associated with paleogeographic change ( [[#Farnsworth--2019|Farnsworth et al., 2019]] ). However, because these non-CO <sub>2</sub> effects can all be included as boundary conditions in model simulations, these time periods allow an assessment of the patterns of modelled response to known forcing (although uncertainty in the forcing increases further back in time). Because these changes to boundary conditions can be complex to implement in models, and because long simulations (typically longer than 500 years) are required to approach equilibrium, these simulations have been carried out mostly by pre-CMIP6 models, with relatively few (or none for the Early Eocene) fully coupled CMIP6 models in the ensembles.

At the time of AR5, polar amplification was evident in proxy reconstructions of paleoclimate sea surface temperature (SST) and surface air temperature (SAT) from the LGM, MPWP and the Early Eocene, but uncertainties associated with proxy calibrations ( [[#Waelbroeck--2009|Waelbroeck et al., 2009]] ; [[#Dowsett--2012|Dowsett et al., 2012]] ; [[#Lunt--2012a|Lunt et al., 2012a]] ) and the role of orbital forcing (for the MPWP; [[#Lisiecki--2005|Lisiecki and Raymo, 2005]] ) meant that the degree of polar amplification during these time periods was not accurately known. Furthermore, although some models (CCSM3; [[#Winguth--2010|Winguth et al., 2010]] ; [[#Huber--2011|Huber and Caballero, 2011]] ) at that time were able to reproduce the strong polar amplification implied by temperature proxies of the Early Eocene, this was achieved at higher CO <sub>2</sub> concentrations (&gt;2000 ppm) than those indicated by CO <sub>2</sub> proxies (&lt;1500 ppm; [[#Beerling--2011|Beerling and Royer, 2011]] ).

Since AR5 there has been progress in improving the accuracy of proxy temperature reconstructions of the LGM ( [[#Cleator--2020|Cleator et al., 2020]] ; [[#Tierney--2020b|Tierney et al., 2020b]] ), the MPWP ( [[#McClymont--2020|McClymont et al., 2020]] ), and the Early Eocene ( [[#Hollis--2019|Hollis et al., 2019]] ) time periods. In addition, reconstructions of the MPWP have been focused on a short time slice with an orbit similar to modern-day (isotopic stage KM5C; [[#Haywood--2013|Haywood et al., 2013]] , 2016b). Furthermore, there are more robust constraints on CO <sub>2</sub> concentrations from the MPWP ( [[#Martínez-Botí--2015|Martínez-Botí et al., 2015]] ; [[#de%20la%20Vega--2020|de la Vega et al., 2020]] ) and the Early Eocene ( [[#Anagnostou--2016|Anagnostou et al., 2016]] , 2020). As such, polar amplification during the LGM, MPWP, and Early Eocene time periods can now be better quantified than at the time of AR5, and the ability of climate models to reproduce this pattern can be better assessed; model-data comparisons for SAT and SST for these three time periods are shown in Figure 7.13.

Since AR5, there has been progress in the simulation of polar amplification by paleoclimate models of the Early Eocene. Initial work indicated that changes to model parameters associated with aerosols and/or clouds could increase simulated polar amplification and improve agreement between models and paleoclimate data ( [[#Kiehl--2013|Kiehl and Shields, 2013]] ; [[#Sagoo--2013|Sagoo et al., 2013]] ), but such parameter changes were not physically based. In support of these initial findings, a more recent (CMIP5) climate model, that includes a process-based representation of cloud microphysics, exhibits polar amplification in better agreement with proxies when compared to the models assessed in AR5 ( [[#Zhu--2019a|Zhu et al., 2019a]] ). Since then, some other CMIP3 and CMIP5 models in the DeepMIP multi-model ensemble ( [[#Lunt--2021|Lunt et al., 2021]] ) have obtained polar amplification for the EECO that is consistent with proxy indications of both polar amplification and CO <sub>2</sub> . Although there is a lack of tropical proxy SAT estimates, both proxies and DeepMIP models show greater terrestrial warming in the high latitudes than the mid-latitudes in both hemispheres (Figure 7.13a,d). SST proxies also exhibit polar amplification in both hemispheres, but the magnitude of this polar amplification is too low in the models, in particular in the south-west Pacific (Figure 7.13g,j).

For the MPWP, model simulations are now in better agreement with proxies than at the time of AR5 ( [[#Haywood--2020|Haywood et al., 2020]] ; [[#McClymont--2020|McClymont et al., 2020]] ). In particular, in the tropics new proxy reconstructions of SSTs are warmer and in better agreement with the models, due in part to the narrower time window in the proxy reconstructions. There is also better agreement at higher latitudes (primarily in the North Atlantic), due in part to the absence of some very warm proxy SSTs due to the narrower time window ( [[#McClymont--2020|McClymont et al., 2020]] ), and in part to a modified representation of Arctic gateways in the most recent Pliocene model simulations ( [[#Otto-Bliesner--2017|Otto-Bliesner et al., 2017]] ), which have resulted in warmer modelled SSTs in the North Atlantic ( [[#Haywood--2020|Haywood et al., 2020]] ). Furthermore, as for the Eocene, improvements in the representation of aerosol–cloud interactions have also led to improved model-data consistency at high latitudes ( [[#Feng--2019|Feng et al., 2019]] ). Although all PlioMIP2 models exhibit polar amplification of SAT, due to the relatively narrow time window there are insufficient terrestrial proxies to assess this (Figure 7.13b,e). However, polar SST amplification in the PlioMIP2 ensemble mean is in reasonably good agreement with that from SST proxies in the Northern Hemisphere (Figure 7.13h,k).

The Last Glacial Maximum (LGM) also gives an opportunity to evaluate model simulation of polar amplification under CO <sub>2</sub> forcing, albeit under colder conditions than today ( [[#Kageyama--2021|Kageyama et al., 2021]] ). Terrestrial SAT and marine SST proxies exhibit clear polar amplification in the Northern Hemisphere, and the PMIP4 models capture this well (Figure 7.13c,f,i,l), particularly for SAT. There is less proxy data in the mid- to high latitudes of the Southern Hemisphere, but here the models exhibit polar amplification of both SST and SAT. LGM regional model-data agreement is also assessed in ( [[IPCC:Wg1:Chapter:Chapter-3|Chapter 3]] [[IPCC:Wg1:Chapter:Chapter-3#3.8.2|Section 3.8.2]] ).

Overall, the proxy reconstructions give ''high confidence'' that there was polar amplification in the LGM, MPWP and EECO, and this is further supported by model simulations of these time periods (Figure 7.13; [[#Zhu--2019a|Zhu et al., 2019a]] ; [[#Haywood--2020|Haywood et al., 2020]] ; [[#Kageyama--2021|Kageyama et al., 2021]] ; [[#Lunt--2021|Lunt et al., 2021]] ). For both the MPWP and EECO, models are more consistent with the temperature and CO <sub>2</sub> proxies than at the time of AR5 ( ''high confidence'' ). For the LGM Northern Hemisphere, which is the region with the most data and the time period with the least uncertainty in model boundary conditions, polar amplification in the PMIP4 ensemble mean is in good agreement with the proxies, especially for SAT ( ''medium confidence'' ). Overall, the confidence in the ability of models to accurately simulate polar amplification is higher than at the time of AR5, but a more complete model evaluation could be carried out if there were more CMIP6 paleoclimate simulations included in the assessment.

<div id="7.4.4.1.3" class="h4-container"></div>

<span id="overall-assessment-of-polar-amplification"></span>