Quantifying uncertainties in global monthly mean sea surface temperature and sea ice at the Last Glacial Maximum&#160;
Authors: Ivanovic Ruza , Gregoire Lauren , Astfalk Lachlan , Williamson Danny , Gandy Niall , Burke Andrea , Schmidt Dani 
Published: March 2022

&lt;div&gt;
&lt;p&gt;&lt;span&gt;Studying the Last Glacial Maximum&amp;#160;(LGM), 21000 years ago, provides insights into climate sensitivity to greenhouse gases and critical interactions within the earth system (e.g.&amp;#160;atmosphere, ocean, cryosphere) operating in a climate different from today. Much effort has been put into reconstructing the Sea Surface Temperatures (SST) at the LGM using a range of&amp;#160;palaeoclimate&amp;#160;records, statistical techniques and models. Large disagreements exist amongst reconstructions and&amp;#160;between&amp;#160;models&amp;#160;and data. Disentangling the causes of these differences is challenging. How much of these differences are due to the choice of data used, their interpretation, the statistical method or climate models used? The polar regions are particularly&amp;#160;difficult&amp;#160;to&amp;#160;reconstruct, yet&amp;#160;are key for assessing polar amplification and key processes driving&amp;#160;cryospheric&amp;#160;changes.&amp;#160;Combining the information gained from&amp;#160;sea ice&amp;#160;and&amp;#160;SST proxies&amp;#160;has the potential&amp;#160;to&amp;#160;improve&amp;#160;reconstructions&amp;#160;in those regions.&amp;#160;&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
&lt;/div&gt;&lt;div&gt;
&lt;p&gt;&lt;span&gt;Here,&amp;#160;we provide a new probabilistic joint reconstruction of&amp;#160;global&amp;#160;SST and&amp;#160;sea ice concentration&amp;#160;(SIC)&amp;#160;that incorporates&amp;#160;information from the ensemble of PMIP3 and PMIP4&amp;#160;models&amp;#160;(Kageyama et al., 2021)&amp;#160;and&amp;#160;existing compilations of SST and sea ice. Our reconstruction was specifically designed to provide ensembles of plausible&amp;#160;monthly mean fields&amp;#160;that can be used to drive&amp;#160;atmosphere models&amp;#160;to&amp;#160;investigate uncertainty in LGM climate and their potential effects/interactions on&amp;#160;e.g.&amp;#160;vegetation, ice and atmospheric&amp;#160;circulation.&amp;#160;&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
&lt;/div&gt;&lt;div&gt;
&lt;p&gt;&lt;span&gt;We present our statistical approach&amp;#160;(Astfalk&amp;#160;et al., 2021)&amp;#160;in simplified terms for non-specialists,&amp;#160;and&amp;#160;discuss&amp;#160;how&amp;#160;different&amp;#160;interpretations&amp;#160;of the&amp;#160;palaeo-records&amp;#160;can be&amp;#160;included&amp;#160;in&amp;#160;our statistical framework.&amp;#160;Our results are compared to other recent reconstructions such as Tierney et al.&amp;#160;(2020)&amp;#160;and Paul et al.&amp;#160;(2021).&amp;#160;To interpret these differences, we test the effect of the&amp;#160;choices of input proxy data and models&amp;#160;on&amp;#160;the reconstructed monthly mean SSTs and SIC.&amp;#160;&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
&lt;/div&gt;&lt;div&gt;
&lt;p&gt;&lt;strong&gt;&lt;span&gt;References:&lt;/span&gt;&lt;/strong&gt;&amp;#160;&lt;/p&gt;
&lt;ul&gt;&lt;li&gt;Astfalck, L., Williamson, D., Gandy, N., Gregoire, L. &amp; Ivanovic, R.&amp;#160;Coexchangeable&amp;#160;process modelling for uncertainty quantification in joint climate reconstruction.&amp;#160;&lt;em&gt;arXiv:2111.12283 [stat]&lt;/em&gt;&amp;#160;(2021).&lt;/li&gt;
&lt;li&gt;Kageyama, M.&amp;#160;&lt;em&gt;et al.&lt;/em&gt;&amp;#160;The PMIP4 Last Glacial Maximum experiments: preliminary results and comparison with the PMIP3 simulations.&amp;#160;&lt;em&gt;Climate of the Past&lt;/em&gt;&amp;#160;17, 1065&amp;#8211;1089 (2021).&lt;/li&gt;
&lt;li&gt;Paul, A.,&amp;#160;Mulitza, S., Stein, R. &amp; Werner, M. A global climatology of the ocean surface during the Last Glacial Maximum mapped on a regular grid (GLOMAP).&amp;#160;&lt;em&gt;Climate of the Past&lt;/em&gt;&amp;#160;17, 805&amp;#8211;824 (2021).&lt;/li&gt;
&lt;li&gt;Tierney, J. E.&amp;#160;&lt;em&gt;et al.&lt;/em&gt;&amp;#160;Glacial cooling and climate sensitivity revisited.&amp;#160;&lt;em&gt;Nature&lt;/em&gt;&amp;#160;584, 569&amp;#8211;573 (2020).&lt;/li&gt;
&lt;/ul&gt;&lt;/div&gt;