Palaeogeography, Palaeoclimatology, Palaeoecology 532 (2019) 109251

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Review article The Medieval Anomaly in T ⁎ Sebastian Lüninga, , Mariusz Gałkab, Fritz Vahrenholtc a Institute for Hydrography, Geoecology and Climate Sciences, Hauptstraße 47, 6315 Ägeri, Switzerland b Department of Geobotany and Plant Ecology, Faculty of Biology and Environmental Protection, University of Lodz, 12/16 Banacha Str., Lodz, Poland c Department of Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany

ABSTRACT

The Medieval Climate Anomaly (MCA) is a well-recognized climate perturbation in many parts of the world, with a core period of 1000–1200 CE. Here we are mapping the MCA across the region based on the analysis of published palaeotemperature data from 60 sites. In addition to the conventionally used ice core data, we are integrating temperature proxy records from marine and terrestrial sediment cores as well as radiocarbon ages of glacier moraines and elephant seal colonies. A generally warm MCA compared to the subsequent (LIA) was found for the Islands south of the Antarctic Convergence, the , Victoria Land and central West Antarctica. A somewhat less clear MCA warm signal was detected for the majority of East Antarctica. MCA cooling occurred in the Ross Ice Shelf region, and probably in the Weddell Sea and on Filchner-Ronne Ice Shelf. Spatial distribution of MCA cooling and warming follows modern dipole patterns, as reflected by areas of opposing temperature trends. Main drivers of the multi-centennial scale climate variability appear to be the Southern Annular Mode (SAM) and El Niño-Southern Oscillation (ENSO) which are linked to solar activity changes by nonlinear dynamics.

1. Introduction 2014; O'Donnell et al., 2011; Ramesh and Soni, 2018). Cooling and an increase in snowfall in East Antarctica seems to have led to a gain in ice Until recently, the Antarctic Peninsula and West Antarctica were sheet mass and thickening of ice rises over the past 15 years (Goel et al., among the most rapidly warming regions on Earth. Between the 1950s 2017; Martin-Español et al., 2017; Philippe et al., 2016; Zwally et al., and 1990s temperatures on the Antarctic Peninsula increased by more 2015). East Antarctic marine-terminating glaciers show no systematic than 0.3 °C/decade (Stenni et al., 2017; Turner et al., 2016; Vaughan change over the past 50 years (Lovell et al., 2017). Lastly, also the et al., 2003), with even higher warming rates reported for Byrd Station surface layers of the Southern Ocean south of 45°S has predominantly in West Antarctica (Bromwich et al., 2013; Bunde et al., 2014; Nicolas cooled over that past three decades (Armour et al., 2016; Fan et al., and Bromwich, 2014). Since the late 1990s, however, this warming has 2014; Kusahara et al., 2017; Latif et al., 2017), whilst the subpolar essentially stalled. Rapid cooling of nearly 0.5 °C per decade occurred abyssal waters are warming (Sallée, 2018). on the Antarctic Peninsula (Favier et al., 2017; Fernandoy et al., 2018; On a century scale, no significant statistical warming trend can be Turner et al., 2016). This already impacted the cryosphere in parts of found for most Antarctic stations (Ludescher et al., 2016). Antarctic the Antarctic Peninsula, including slow-down of glacier recession, temperatures appear to be still well within the range of natural varia- surface mass gains of the peripheral glaciers and a thinning of the active bility. Natural climate factors such as multidecadal ocean cycles still layer of permafrost in the northern Antarctic Peninsula islands (Engel dominate over anthropogenic climate drivers, such as CO2 (Chylek et al., 2018; Oliva et al., 2017; Seehaus et al., 2018). At the same time, et al., 2010; Favier et al., 2017; Jones et al., 2016; Ludescher et al., temperatures in West Antarctica over the past two decades appear to 2016; Smith and Polvani, 2017; Steig et al., 2013; Turner et al., 2016). have plateaued or slightly cooled (Bromwich et al., 2013; Jones et al., On interannual to decadal scale, a general anti-phase has been ob- 2016; Steig et al., 2009). However, temperature records in West Ant- served between the temperatures of East Antarctica and Antarctic arctica are few, often discontinuous and show opposing trends from Peninsula/West Antarctica (Schneider et al., 2006). The Transantarctic location to location (Shuman and Stearns, 2001) which complicates Mountains act as a dividing line for the temperature trends of the two modern trend analysis in this part of Antarctica. In contrast, East Ant- regions, reflecting the topographic constraint on warm air advection arctica has not experienced any significant temperature change since from the Amundsen Sea basin (Nicolas and Bromwich, 2014). Un- the 1950s (e.g. Yang et al., 2018) and some areas appear to have even fortunately, climate models still fail to reproduce the regional Antarctic cooled during the most recent decades (Clem et al., 2018; Favier et al., temperature and snowfall development and generally overestimate the 2017; Jones et al., 2016; Marshall et al., 2014; Nicolas and Bromwich, magnitude of the forced response (Favier et al., 2017; Jones et al., 2016;

⁎ Corresponding author. E-mail addresses: [email protected] (S. Lüning), [email protected] (M. Gałka), [email protected] (F. Vahrenholt). https://doi.org/10.1016/j.palaeo.2019.109251 Received 2 April 2019; Received in revised form 24 June 2019; Accepted 25 June 2019 Available online 29 June 2019 0031-0182/ © 2019 Elsevier B.V. All rights reserved. S. Lüning, et al. Palaeogeography, Palaeoclimatology, Palaeoecology 532 (2019) 109251

Tang et al., 2018). A better understanding of natural variability on (IOD) and the Atlantic Multidecadal Oscillation (AMO) (e.g. Li et al., multi-decadal, centennial and millennial-scales will help to further re- 2014; Zhang et al., 2015). The various ocean cycles are partly inter- fine these models and calibrate them with the pre-industrial climate linked (Wyatt and Curry, 2014). Other factors that may have affected development by way of hindcasts (Fogt et al., 2017; Goosse, 2017; pre-industrial are solar activity changes (e.g. Barbara Mayewski et al., 2017). et al., 2016; Bentley et al., 2009; Christ et al., 2015; Costa et al., 2007) This paper focuses on the Medieval Climate Anomaly (MCA), a and volcanic eruptions (e.g. Yang and Xiao, 2018). A detailed overview distinctive rapid climate fluctuation that has been documented from of important potential drivers of natural climate variability in Antarc- many parts of the world. The best-studied region is the mid and high tica can be found in Supplement Texts 1a–h. latitude North Atlantic realm where numerous studies documented a pronounced warm phase during the MCA. This historically led to the 4. Material and methods introduction of the term ‘Early Medieval Warm Epoch’ (Lamb, 1965), which subsequently changed in the literature to ‘Medieval Warm 4.1. Literature review Period’ (MWP), and finally to ‘Medieval Climate Anomaly’ (MCA) (e.g. Grove and Switsur, 1994). It is generally agreed today that the core The mapping project is based on a comprehensive literature period of the MCA comprises ca. 1000–1200 CE. Nevertheless, different screening process during which all available published Antarctic pa- time schemes and durations have historically been used in the litera- laeotemperature case studies were evaluated towards their temporal ture, with the widest scheme comprising 800–1300 CE (e.g. Crowley coverage, types of climate information and data resolution. Suitable and Lowery, 2000; Esper and Frank, 2009; Mann et al., 2009). papers including the MCA core period 1000–1200 CE were earmarked Here we concentrate on the MCA core period 1000–1200 CE and for a thorough analysis. Generally, studies should contain at least one attempt to map temperature patterns across the Antarctic region based palaeotemperature data point within the MCA period, either quantita- on proxy data from 60 published sites (Fig. 1). We investigate which tive or just qualitative. A total of 60 Antarctic localities with one or parts of Antarctica experienced warming during the MCA and which more MCA palaeotemperature proxy curves were identified (Figs. 1–3; areas cooled. Likely drivers of the observed pre-industrial south polar Tables 1 and S1). This Antarctic review forms part of an attempt to climate change of the past 1500 years are discussed. A recent Antarctic palaeoclimatically map the MCA on a global scale (Lüning et al., 2019a; temperature reconstruction by Stenni et al. (2017) based solely on ice Lüning et al., 2018; Lüning et al., 2019b; Lüning et al., 2017). cores suggests a warm MCA that is followed by significant cooling of the Little Ice Age (LIA) after 1250 CE that lasts until 1950 CE. Overall, the 4.2. Palaeoclimate archives and data types authors note a long-term cooling trend for the last 2000 years. We are complementing the ice core data with other palaeotemperature proxies MCA climate reconstructions of the publications high-graded by the from marine and terrestrial sediment cores as well as radiocarbon ages screening process comprised of several natural archives, namely (1) of glacier moraines, elephant seal colonies and penguin rookeries. sediment cores from marine, lakes, peatlands, (2) ice cores from the Whilst this approach adds more data points, it also diversifies the Antarctic Ice Sheet and smaller peripheral ice caps, (3) dating of glacier source of palaeoclimatic information. A better understanding of the pre- moraines and elephant seal colonies. Data types include (a) pa- industrial Antarctic climate history, and in particular of natural warm laeontology (diatoms, palynology), (b) inorganic geochemistry (δ18O, phases, provides crucial input for climate model calibration and re- δ2H, deuterium excess in ice cores; δ18O in foraminifera and lake car- finement. This compilation also highlights major data gaps, which fu- bonates; biogenic silica, elemental sediment composition; ikaite crys- ture studies hopefully will be able to infill. tallization), (c) organic geochemistry (total organic carbon; loss-on-ig-

nition; Tex86 sea surface temperature), (d) geophysics (magnetic 2. Modern climate setting of Antarctica susceptibility, radio‑carbon 14C dating of moss and elephant seal skin), (e) sedimentology (sediment petrography, grain size), and (f) ice mi- Antarctica is positioned asymmetrically around the South Pole and croscopy (air bubble density). lies largely south of the Antarctic Circle (Fig. 1). About 98% of Ant- arctica is covered by the Antarctic ice sheet which has an average 4.3. Data processing and visualisation thickness of more than 1800 m. The average height of Antarctica is 1958 m, which represents the highest average elevation of all the 4.3.1. Qualitative continents. Antarctica is also the coldest, driest, and windiest continent Key information of the identified publications were captured on a of all. It is surrounded by the Southern Ocean which is connected georeferenced online map which is available at http://t1p.de/mwp. northwards to the Pacific, Atlantic and Indian Oceans. The Antarctic The MCA palaeotemperature was visually assessed and compared to the coastal zones are subjected to east wind drift. North of the Antarctic phases preceding and following the anomaly. The MCA trend was Divergence (~60°S), a west wind drift prevails that drives the Antarctic colour-coded with red dots marking a trend towards warmer conditions Circumpolar Current. Further north, between latitudes 50°S and 60°S, whilst blue dots represent cooling. In some cases the centuries pre- lies the Antarctic Convergence which is also known as Antarctic Polar ceding the MCA were as warm as the MCA itself. In this case the MCA Front (Fig. 1). Here, cold waters from the Antarctic region sink beneath was compared to the subsequent centuries of the LIA. The colour-coding the warm waters from the middle latitudes, forming the Antarctic in- provides much-needed initial orientation in the complex global MCA termediate water. climate puzzle.

3. Natural climate drivers of Antarctic temperature variability 4.3.2. Quantitative In order to robustly document, visualise and analyse MCA palaeo- Temperatures, precipitation and weather extremes across the globe climatic variability, all relevant climate curve data were collected in are characterized by systematic variability that is to a large extent digital form for flexible plotting and curve correlation. The tabulated driven by multidecadal and shorter-term ocean cycles. In the case of data were retrieved (a) from palaeoclimate online data repositories Antarctica, the most important drivers are the Southern Annular Mode (mainly Pangaea, NOAA's National Climatic Data Center NCDC and (SAM), El Niño Southern Oscillation (ENSO), the Southern Oscillation data supplements of papers), (b) from authors by email request and (c) Index (SOI) and the Interdecadal Pacific Oscillation (IPO) (e.g. Bukatov by digitizing and vectorising using WebPlotDigitizer (https:// et al., 2016; Ekaykin et al., 2014; Turner et al., 2014). Secondary automeris.io/WebPlotDigitizer/). Data sources for each site are listed Antarctic multidecadal climate factors are the Indian Ocean Dipole in Table S1. All curve data have been loaded into Lloyd's Register's

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Fig. 1. Location map of studied sites. Key to site numbers in Table 1. Zoom location maps of the Antarctic Peninsula and Victorialand are shown in Figs. 2 and 3. Brown lines connect the sites shown in the correlation panels in Supplement Figs. S2–S12. Basemap: Quantarctica. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) software IC™ which serves as a common database and correlation tool generate comprehensive and meaningful statistics for the Antarctic in the MCA mapping project. IC™ was originally developed for geolo- MCA dataset. Particularly challenging are differences in data resolution gical well correlations in the areas of water, minerals and petroleum and age shifts due to model uncertainties. Nevertheless, some basic exploration (e.g. Luthi, 2001). The software reliably handles large statistical analyses have been carried out to qualitatively support the amounts of fully customizable curve data types related to georeferenced trend discussion (Table S2). Proxy averages were calculated for the four wells. By way of technology transfer we are introducing well correla- time windows 500–900 CE, 950–1250 CE (MCA plus some error tion software to the field of the climate sciences which allows fast and margin), 1250–1500 CE and 1500–1800 CE. Data were homogenized flexible comparison of climate curves. Another advantage is the ad- for time steps of 25 years, except for lower resolution data for which vanced visualisation and shading of peak and trough curve anomalies, a longer time steps of 50 years were chosen. Proxy data were interpolated functionality which lacks in most standard spreadsheet software where necessary. Standard deviations were calculated for 95% (2σ) packages. See Lüning et al. (2017) for details on the workflow. Location confidence. maps were produced using the Quantarctica Geographic Information System (GIS) package provided by the Norwegian Polar Institute, based on the open source QGIS software. 4.4. Challenges

Like any other regional palaeoclimatic synthesis, the current 4.3.3. Trend statistics Antarctic palaeotemperature MCA mapping effort is subjected to a Enormous heterogeneity of the proxy data makes it complicated to number of challenges. Palaeoclimatology is not an exact mathematical

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Fig. 2. Zoomed location map of the Antarctic Peninsula sites. See Fig. 1 for Antarctic overview map. science, nevertheless provides crucial data for the palaeoclimatological column that lists for each site the number of age markers for the last context and model calibration. A probabilistic approach is needed 2500 years. whereby the most likely scenario is selected from the set of available proxies. 4.4.3. Validity of palaeoclimate interpretation Paleoclimate interpretations are based on proxies which often leave 4.4.1. Low resolution palaeoclimate data room for alternative interpretations. It is therefore not uncommon that The core phase of the MCA is only 200 years long and may not be climate proxy data may undergo re-interpretation by other researchers represented by a data point in studies of low resolution data. In parti- subsequent to their publication. Examples are oxygen and carbon iso- cular, this is a problem in data series covering the entire or tope proxies whose suitability as precipitation and rainfall proxies de- more which often have only few data points per millennium. In obvious pends often on local factors, leading to different views among re- cases, such analyses have been excluded from the present analysis. searchers.

4.4.2. Limited age control, faulty age models 4.4.4. Conflicting information from multiple proxies Even though a large number of climatic data points may exist over In some case studies, proxies that usually represent similar climate the past millennium, age control can be a problem. Among the reasons parameters yield conflicting results. In such cases, the most likely cor- are (a) a low number of dated horizons, (b) last age in a multi-millen- rect scenario has to be selected, based on the comparison with neigh- nial-year section much older than the MCA, (c) scatter in obtained ages bouring case studies and the regional context. and (d) general dating and correction problems. As a possible result, observed climate anomalies may be age-shifted, causing problems with 4.4.5. Seasonal vs. annual significance calculating regional multi-study average values. Table S1 contains a Attention has to be paid to seasonally restricted proxies. Some data

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Fig. 3. Zoomed location map of Victoria Land, Ross Ice Shelf and Ross Sea sites. See Fig. 1 for Antarctic overview map. conflicts may e.g. be explained by winter rain vs. summer rain vs. ex- carbon may also occur. Accumulation rates in the various ice cores treme precipitation which may all be captured by different proxies and differ and may impact data resolution. For example, rates in Vostok are have different climatic meanings. several times lower than in Law Dome. Whilst the covariance between δ18O and temperature is generally reproduced in isotope-enabled models, it is generally weak and char- 4.4.6. Other issues acterized by a strong spatial heterogeneity, as well as changes over time Due to carbon-14 depletion in Antarctic waters, all radiocarbon with no apparent link with forcings (Klein et al., 2018). Furthermore, dates from the Antarctic must be corrected by a factor which accounts stable oxygen isotope records in ice cores are potentially vulnerable to for the pool of old carbon in Antarctic waters. If uncorrected, the post-deposition alteration processes, especially in low accumulation Antarctic reservoir effect yields anomalously old surface ages (Hall areas such as the East Antarctic Ice Sheet (Casado et al., 2017; Laepple et al., 2010a). Here we are using the originally published chronologies et al., 2018; Münch et al., 2016, 2017). Ice trench studies at Kohnen which may lead to issues related to different calibration curves and Station in Dronning Maud Land revealed a surprisingly high horizontal different software. 2σ error ranges of radiocarbon ages stated by au- isotopic variability caused by local stratigraphic noise, indicating that thors are typically between ± 30–100 years. Contamination by dead

5 .Lnn,e al. et Lüning, S. Table 1 MCA study sites in the Antarctic region. Key to climate proxy abbreviations: δ2H = deuterium, ms = magnetic susceptibility, TOC = total organic carbon, LOI = loss on ignition, GDGTs = glycerol dialkyl glycerol tetraethers.

No. Locality Region Archive Proxies Reference

1 Kerguelen Islands Antarctic Islands Glacier dating Moraine dating Jomelli et al. (2017) 2 Ile de la Possession Antarctic Islands Peat core Diatoms Ooms et al. (2011) 3 TN057-17 Antarctic Islands Marine core Diatoms Nielsen et al. (2004), Divine et al. (2010) 4 PS1652-2 Near , Antarctic Islands Marine core Diatoms Xiao et al. (2016) 5 PS2102-2 Near Bouvet Island, Antarctic Islands Marine core Diatoms Xiao et al. (2016) 6 Hamberg Lakes South Georgia, Antarctic Islands Lake core Ti as glacial activity proxy Clapperton et al. (1989), van der Bilt et al. (2017); other South Georgia: Oppedal et al. (2018) 7 Fan Lake Annenkov Island, Antarctic Islands Lake core Palynology, GDGTs Strother et al. (2015), Foster et al. (2016) 8 Signy Island Antarctic Islands Lake core δ18 O of authigenic lake carbonate, mites Noon et al. (2003), Hodgson and Convey (2005) 9 A9-EB2, eastern Bransfield Basin Antarctic Peninsula Marine core ms Khim et al. (2002) 10 JPC24 Bransfield Basin, Antarctic Peninsula Marine core TOC Barnard et al. (2014) 11 GEBRA-1 Antarctic Peninsula Marine core Diatoms Bárcena et al. (1998) 12 PS69/335 King George Island, Antarctic Marine core Elemental ratios, TOC, opal, grain size Hass et al. (2010), Monien et al. (2011) Peninsula 13 Collins Ice Cap King George Island, Antarctic Glacier dating Dating mosses Hall (2007) Peninsula 14 Ardley Island Antarctic Peninsula Lake core P2O5 Sun et al. (2000), Huang et al. (2011), Liu et al. (2005) 15 Yanou Lake King George Island, Antarctic Lake core GDGTs Roberts et al. (2017) Peninsula 16 Midge Lake Livingston Island, Antarctic Peninsula Lake core LOI Björck et al. (1991) 17 JPC2 Firth of Tay, Antarctic Peninsula Marine core TOC, Ikaite crystallization Lu et al. (2012) 18 JPC38 Antarctic Peninsula Marine core Diatoms Barbara et al. (2016) 19 Hidden Lake James Ross Island, Antarctic Lake core Loss on Ignition Zale (1994)

6 Peninsula 20 Herbert Sound Antarctic Peninsula Marine core Diatoms, petrography Minzoni et al. (2015) 21 JRI ice core James Ross Island, Antarctic Ice core δ2H Mulvaney et al. (2012), Abram et al. (2013) Peninsula 22 Anvers Island Antarctic Peninsula Glacier dating Dating mosses and shells Hall et al. (2010b) 23 sites near U.S. Palmer Station Antarctic Peninsula Glacier dating Dating mosses Yu et al. (2016) 24 Litchfield Island Antarctic Peninsula Peat core Peat accumulation rate Stelling et al. (2018) 25 ODP 1098 Antarctic Peninsula Marine core Ms, Tex86 , benthic foraminifera isotopes Domack and Mayewski (1999), Shevenell and Kennett (2002), Shevenell et al. (2011)

26 Bigo Bay Antarctic Peninsula Marine core Biogenic opal, TOC Kim et al. (2018) Palaeogeography, Palaeoclimatology,Palaeoecology532(2019)109251 27 KC-55 Antarctic Peninsula Marine core TOC Christ et al. (2015) 28 Müller Ice Shelf Lallemand Fjord, Antarctic Peninsula Marine core Sedimentology, geochem., forams. Domack et al. (1995) 29 Rothera Point Marguerite Bay, Antarctic Peninsula Glacier dating Dating mosses Guglielmin et al. (2016) 30 JPC43 Neny Fjord, Antarctic Peninsula Marine core ms Allen et al. (2010) 31 Berkner Island Ice Core Ronne & Filchner Ice Shelf Ice core δ2H, δ18 O Mulvaney et al. (2002), Graf et al. (2006) 32 Dronning Maud Land Traverse East Antarctica Ice core δ18 O Graf et al. (2002) 33 Schirmacher Oasis East Antarctica Lake core TOC, BSi, grain size, elemental Govil et al. (2016) geochem. 34 Lützow Holm Bay East Antarctica Lake core Diatoms, sedimentology, geochem. Tavernier et al. (2014) 35 Dome Fuji Ice Core East Antarctica Ice core δ18 O Horiuchi et al. (2008) 36 Plateau Remote Ice Core East Antarctica Ice core δ18 O Mosley-Thompson (1996) 37 Lake Terrasovoje Prydz Bay, East Antarctica Lake core Diatoms, geochem. Wagner et al. (2004) 38 AM02 core Amery Ice Shelf Prydz Bay, East Antarctica Marine core Diatoms Hemer and Harris (2003) 39 Kirisjes Pond Prydz Bay, East Antarctica Lake core Diatoms Verleyen et al. (2004) 40 Co1010 Prydz Bay, East Antarctica Marine core TOC Berg et al. (2010) 41 Zolotov Island Prydz Bay, East Antarctica Lake core P2O5 Huang et al. (2011) 42 Long Peninsula Prydz Bay, East Antarctica Lake core P/Al Gao et al. (2019) 43 Dome B Ice Core East Antarctica Ice core δ2H Masson et al. (2000), Vimeux et al. (2001) 44 Vostok Ice Core East Antarctica Ice core δ2H Masson et al. (2000), Vimeux et al. (2001) 45 EPICA Dome Concordia (“Dome C”) Ice East Antarctica Ice core δ2H Masson et al. (2000), Masson-Delmotte et al. (2004) Core (continued on next page) S. Lüning, et al. Palaeogeography, Palaeoclimatology, Palaeoecology 532 (2019) 109251

isotope records from individual ice cores have to be treated with cau- tion with regards to temperature interpretations (Münch et al., 2016, fi Plummer 2017; Münch and Laepple, 2018). Signi cant metamorphism on the , surface snow isotopic signal has also been documented for Dome C (Casado et al., 2017). Generally, single ice core records in East Ant- arctica appear to suffer from a very low signal-to-noise ratio, therefore it is better to work with local or regional stacks (pers. comm. Alexey Ekaykin, Nov. 2017). Roberts et al. (2001) , 5. Results

Fudge et al. (2016) fi

, A total of 60 study sites have been identi ed in the Antarctic region, comprising regional clusters in the western Antarctic Peninsula, Prydz Bay and Victoria Land (Figs. 1–3; Table 1). Detailed site descriptions and palaeo-temperature correlation panels can be found in the Sup- Mezgec et al. (2017)

, – Dahl-Jensen et al. (1999) porting Information to this paper (texts and Figs. S2 S12, Table S1). A , correlation panel with 6 representative sites from the study area as well Masson et al. (2000) , as the Antarctic temperature reconstruction of Stenni et al. (2017) is Fegyveresi et al. (2016)

, illustrated in Fig. 4 to facilitate the discussion. Goosse et al. (2004) , 6. Regional MCA temperature trends

6.1. Antarctic Peninsula and Weddell Sea Reference et al. (2012) Steig et al. (2013) Bertler et al. (2011) Hall and Denton (2002) Bertler et al. (2018) Mayewski et al. (1995) PAGES 2k Consortium (2017) Morgan and Ommen (1997) Steig et al. (1998) Mayewski et al. (2013) Mezgec et al. (2017) Mezgec et al. (2017) Baroni and Orombelli (1994b) Baroni and Orombelli (1994a) Hall et al. (2006) Leventer et al. (1993) The vast majority of sites in the Antarctic Peninsula region docu- ment a warm phase for the MCA (Fig. 5; texts and Figs. S3–S7 in sup- plement to this paper). Marine and lake sediment cores typically show enrichment in organic matter, reflecting higher biological productivity in a warmer, more life-friendly climate (sites 9–12, 17, 27, 30). A warm

MCA is also supported by TEX86 temperatures (Site 25) and reduced sea ice (sites 11, 18). Glaciers typically retreated during the MCA and re- advanced during the subsequent Little Ice Age as indicated by radio- carbon dating of moss (sites 13, 22, 23, 29). O O O H, deuterium excess 2 18 18 18 δ

δ δ δ Only two sites deviate from this regional trend. The James Ross O, O, air bubble density H, H H, H H, H

2 18 2 2 2 2 2 18 Island ice core (Site 21) documents a rather cold temperature during δ δ δ δ δ δ δ δ the MCA (Figs. 5, S6). The discrepancy may be due to issues with the age model which is currently under revision. Another possible reason could be noise in the temperature signal because the Ross Island in- terpretation is based on a single long ice core rather than a stack of local cores. Modelling by Davies et al. (2014) demonstrates that the Ross Archive Proxies Ice core Island glaciers reached their maximum Holocene extent around 1700 CE during the peak of the Little Ice Age. Glacier advance phases commencing as early as at the end of the MCA were described from the Antarctic Peninsula based on regional moss data (Guglielmin et al., 2016; Hall, 2007; Hall et al., 2010b; Yu et al., 2016). Another deviation from the regional Antarctic Peninsula (AP) trend occurs in Herbert Sound (Site 20) which forms a narrow embayment on the northern side of James Ross Island (Fig. 2). The MCA appears to have been cold here (Figs. 5, S6). A possible interpretation may be the fjord nature of the embayment which is likely to have received in- creased amounts of cold melt water during the MCA due to more in- Region East Antarctica tense summer melting of the ice cap on James Ross Island under a re- gionally warmer MCA climate on the Antarctic Peninsula. Notably, most MCA palaeoclimate data come from the western side of the Antarctic Peninsula and its northernmost tip (Figs. 5, 6). Studies from the central and southern Weddell Sea are mostly lacking. James Ross Island forms an exception because it lies on the Weddell Sea side of the AP and provides the southeastern most datapoint for the Peninsula. The signs of a colder MCA in the two James Ross Island sites 20 and 21 could therefore also point to a climatic MCA regime shift between the ) western AP/Bellingshausen Sea and the eastern AP/Weddell Sea. Such a pattern with opposing SST trends is known from the modern ENSO ‘ ’ continued dynamics and has been termed Antarctic Dipole (Dash et al., 2013; ( Ice Core Yuan, 2004). The only other data point available from the wider region is the Berkner Island ice core (Site 31) from the Filchner-Ronne Ice 46 Dome Summit South (DSS) - Law Dome No. Locality 47 TALDICE Ice Core Talos Dome, East Antarctic Plateau Ice core 5556 Wilson Piedmont Glacier Taylor Dome Ice58 Core Siple Dome Ice Core Western Ross Sea coast East Antarctica West Antarctica Glacier dating Glacial topography ages Ice core Ice core 57 RICE Ice Core59 Dominion Range Ice Core Transantarctic Mountains Roosevelt Island, eastern Ross Sea Ice core Ice core 60 WAIS Divide Ice Core West Antarctica Ice core 4849 WRS_CH50 WRS_WB51 Edmonson Point Glacier52 Prior Island53 Western Ross Sea54 KC208.09 Victoria Lower Glacier Ice Core Terra Nova Bay, Victoria Land McMurdy Dry Valleys Cape Hallett, western Ross Woods Sea Bay, western Western Ross Glacier Ross SEa dating Sea Victoria Land coast Moraine dating Marine core Granite Harbor, McMurdo Sound Marine core Ice core Diatoms Marine Diatoms core Seal C14 skin ages dating Diatoms Dating elephant seal skin C14 dating of penguin remains

Table 1 Shelf, just south of the Wedddel Sea (Fig. 1). The ice core shows MCA

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Fig. 4. Temperature development in the Antarctic region during the past 1500 years based on palaeoclimate proxies of selected study sites. 7: Fan Lake (Strother et al., 2015), 25: ODP 1098 (Domack and Mayewski, 1999; Shevenell et al., 2011; Shevenell and Kennett, 2002), 31: Berkner Island (Mulvaney et al., 2002), 45: EPICA Dome C (Masson-Delmotte et al., 2004), 49: Woods Bay (Mezgec et al., 2017), 57: RICE ice core (Bertler et al., 2018), whole Antarctica (Stenni et al., 2017: composite-plus-scaling CPS reconstruction). Location maps in Figs. 1–3. cooling, therefore providing further evidence that the Weddell Sea series from Dronning Maud Land (Site 32) indicates that the oxygen and might have cooled as a whole during the MCA (Figs. 4, 6). deuterium isotope records can significantly differ between nearby sites (Fig. S8), which is in line with recent ice trench studies on modern ice accumulations in the same region, pointing to abundant local strati- 6.2. Subantarctic Islands graphic noise (Münch et al., 2016, 2017). MCA warming is most pro- nounced in Dronning Maud core 27–07 and least developed in 27–17 The majority of studies from the Subantarctic Islands show a warm (Graf et al., 2002) (Fig. S8). Similar variability exists at Vostok (Site 44) MCA (Fig. 6). Glaciers on the islands retracted during the MCA and where different sets of shallow and deep ice cores yield different re- expanded during the subsequent Little Ice Age as documented by presentations of the MCA warming which in some cases is longer and moraine dating (Kerguelen Islands, Site 1) and elemental geochemistry more pronounced, and yet in other cases shorter and less developed in pro-glacial lakes (South Georgia, Site 6) (Text and Fig. S2). Tem- (Fig. S10). Modern analogue studies at EPICA Dome C (Site 45) point to peratures were elevated and sea ice reduced as interpreted from dia- the risk of post-depositional alterations in the deuterium isotope record toms, pollen, moss mites and stable oxygen isotopes in marine and lake (Casado et al., 2017). sediment cores (sites 3–5, 7, 8; Bouvet Island, South Georgia, South In Law Dome (Site 46) the deuterium excess data suggest high Orkneys) (Figs. 4, S2). All of these sites are located south of the Ant- evaporation and warm conditions for the majority and central part of Convergence, or very close to it (Site 3) (Fig. 6). An exception is the MCA, whilst oxygen isotopes only indicate warmth during the first the cold MCA that was documented in a peat core study from Ile de la half of the MCA (Fig. S10). The difference in deuterium excess com- Possession in the Crozet Archipelago (Site 2) (Figs. 6, S2). This site lies pared to δ18O might reflect the different conditions on the continent north of the Antarctic Convergence and may therefore represent a dif- compared to the ocean where the evaporation takes place (pers. comm. ferent palaeoclimatic regime. Hugues Goosse, Nov. 2017). Deuterium excess is a proxy of evaporative conditions (SST, relative humidity and wind speed) at moisture source 6.3. East Antarctica regions and can be considered as an integrated tracer of the hydro- logical cycle (Mezgec et al., 2017). Most studies from East Antarctica generally suggest a warm MCA, The oxygen isotopes in the Plateau Remote ice core (Site 36) show even though the reconstructed climate signal is noisier than in other the opposite of Law Dome, i.e. a cold first half of the MCA and a warmer parts of the Antarctic region. The available data consist of ice core sites second half (Fig. S8). Uncertainties in the age model of these ice cores on the East Antarctic Ice Sheet as well as marine and lake sediment core may be responsible for some of these trend discrepancies. Many of the locations in the coastal zone (Fig. 1). The ice cores generally document East Antarctic ice cores have a pronounced cooling shift around the end MCA warming (sites 32, 35, 43–45), nevertheless showing significant of the MCA which could be used to provisionally align the records when variability (Figs. 4, 6; Texts and Figs. S8–S11). This in part may be the age models are uncertain. The ice core dating by volcanic events has related to the low snow accumulation rates in East Antarctica which evolved over the years, meaning that older age models may need re- complicate dating and in some cases may have led to post-depositional vision in light of updated volcanic eruption chronologies (e.g. Sigl et al., alterations in the water isotope composition (Casado et al., 2017; 2015). Pattern matching and cyclostratigraphy of water isotope record Münch et al., 2016, 2017). may be necessary to further refine the correlation of the various ice A pronounced and extensive MCA warm phase occurs in Dome Fuji cores in East Antarctica on decadal time scales. (Site 35), whilst the warming appears somewhat shorter in Dome B Most sediment cores in the Prydz Bay and Amery Ice shelf region (Site 43) and EPICA Dome C (Site 45) (Figs. 4, S8, S10). An ice core

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Fig. 5. Temperature trends during the Medieval Climate Anomaly in the Antarctic Peninsula region. Site descriptions and climate proxy correlation panels in Supplement (texts and Figs. S3–S5). Antarctic trends overview map in Fig. 6. suggest a warm MCA (Fig. 6). Biological activity and coastal lake levels reduced glacier length during the MCA, followed by glacier advance in were higher (sites 39 and 40) and the sea ice season may have been Little Ice Age (sites 50 and 55). Likewise, elephant seal colonies existed shorter (Site 38, note low data resolution and age model uncertainties) during the MCA along the southern Victoria Land Coast whilst they had (Fig. S9). A cold MCA has been interpreted based on reduced organic to be abandoned in the Little Ice Age due to unfavourable cooling (Site content in a proglacial lake in the Schirmacher Oasis (Site 33) (Fig. S8). 52). Notably, these breeding grounds have not yet been resettled until It is speculated that this reflects increased cold meltwater influx from present-day. the nearby glaciers in a regionally warmer climate. Interpretability of Diatoms in the marine WRS_CH core (Site 48) at first glance suggest two other records in coastal East Antarctic study locations, Lützow cooling for the MCA (Figs. 7, S11). However, comparison with the Holm Bay (Site 34) and Lake Terrasovoje (Site 37) is limited due to a nearby Site 49 suggests that the chronology in Site 48 may be shifted. condensed section (or even hiatus) during the MCA, and low data re- The warm-to-cold shift documented in Site 48 may in fact correspond to solution, respectively (Texts S6, S7). the end of the MCA and appears to be 350 years too old. This is well within the uncertainty range of 600 years which Mezgec et al. (2017) 6.4. Ross Sea, Ross Ice Shelf and West Antarctica cite in their paper. In an alternative interpretation, the observed dis- crepancy may be related to differences in katabatic wind strengths and ffi fl Most evidence from Victoria Land and the western Ross Sea points the e ciency of the polynyas at sites 48 and 49, which in uences the to a warm MCA (Fig. 7). An increase in temperature is documented by abundance of the sea ice proxy form Fragilariopsis curta (pers. comm. water isotopes in ice cores (sites 47 and 54) (Fig. S11). Diatom as- Barbara Stenni, Nov. 2017). semblages in marine sediment cores (sites 49 and 53) indicate warm The Ross Ice Shelf appears to have been cooling during the MCA, as water and reduced sea ice (Figs. 4, S11). Moraine dating suggests evidenced by the four ice cores Taylor Dome, RICE, Siple Dome and

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Fig. 6. Temperature trends during the Medieval Climate Anomaly in the Antarctic region. Site descriptions and climate proxy correlation panels in Supplement (texts and Figs. S2–S12). Zoom trends maps of the Antarctic Peninsula and Victoria Land in Figs. 5 and 7.

Dominion Range that are flanking the ice shelf (sites 56–59) (Figs. 4, 6, before. In these cases, reported warm/cold MCA trends are relative to 7, S12). It must remain speculation if this area of cold MCA actually the subsequent Little Ice Age (LIA). MCA cooling occurred in the Ross extends towards the South Pole (partly cold MCA in Site 36, Plateau Ice Shelf region, and probably in the Weddell Sea and on Filchner- Remote, Fig. S8) and even to the Ronne Filchner Ice Shelf (cold MCA in Ronne Ice Shelf. Cooling has been also proposed for the Crozet Archi- Site 31, Berkner Island, Fig. 4). Data from central West Antarctica is pelago which lies just north of the Antarctic Convergence (Fig. 6). unfortunately scarce (Fig. 1). The only available site is the WAIS Divide ice core (Site 60) which shows a predominantly warm MCA (Figs. 6, 7. Potential climate drivers of observed MCA spatial and temporal S12). temperature patterns

6.5. Trends summary 7.1. Ocean cycles

The palaeoclimatic mapping has identified several Antarctic regions The MCA warming identified on the western side of the Antarctic with distinct MCA temperature response. A relatively warm MCA was Peninsula occurred during a time when the ENSO according to most found for the Subantarctic Islands south of the Antarctic Convergence, reconstructions was El Niño-rich (Conroy et al., 2010; Conroy et al., the Antarctic Peninsula, Victoria Land and central West Antarctica 2008; Henke et al., 2017; Moy et al., 2002; Rustic et al., 2015; Yan (Figs. 5–7). A somewhat noisier MCA warm signal was detected for the et al., 2011)(Figs. 8, S1). A similar relationship is found in modern majority of East Antarctica. In some cases, the warm phase started times, whereby El Niño-dominated conditions typically lead to rising several centuries before the MCA and reaches back to 500 CE and even temperatures in the Bellingshausen Sea (Markle et al., 2012;

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Fig. 7. Temperature trends during the Medieval Climate Anomaly in Victoria Land and the Ross Ice Shelf region. Site descriptions and climate proxy correlation panels in Supplement (texts S9, S10; Figs. S11, S12).

Stammerjohn et al., 2008; Yuan, 2004). The MCA warming may have during El Niño-dominated times. Snowfall (surface mass balance) has been further intensified by an elevated SAM (Abram et al., 2014; been reduced during the MCA at the Weddell Sea coast (Thomas et al., Goodwin et al., 2014; Huang et al., 2010) that in modern times usually 2017: their fig. 7b) which may be related to a weakening of the Ferrell results in a temperature rise on the Antarctic Peninsula (Doddridge and cell as well as changes in the jet stream which typically occur in asso- Marshall, 2017; Ekaykin et al., 2014; Gillett et al., 2006; Kwok and ciation with El Niño events in modern times (Yuan, 2004). Comiso, 2002; Schneider et al., 2012) and a drop in air temperature A similar ENSO dipole exists in the Ross Sea area where El Niño over most of East Antarctica (Kwok and Comiso, 2002)(Fig. 8). today commonly results in warming of the outer Ross Sea and cooling of The MCA cooling observed in the Weddell Sea and on the Filchner- the Ross Ice Shelf region (Bertler et al., 2004; Bertler et al., 2018; Ronne Ice Shelf matches well with the ENSO Antarctic Dipole Pattern Bromwich et al., 1993; Markle et al., 2012; Pope et al., 2017). This Ross that today usually yields opposite temperature trends for the western Sea dipole seems to have been also active during the MCA, as evidenced and eastern sea regions of the Antarctic Peninsula (Yuan, 2004)(Fig. 9). by MCA warming of the western Ross Sea and MCA cooling in the ice Mulvaney et al. (2012) demonstrated that the dipole also operated at cores around the Ross Sea Ice Shelf (Figs. 6, 9). millennial time scales in which Weddell Sea cooling generally occurred It is unclear if the temperature variations suggested for the past

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Fig. 8. Reconstructions of key drivers of natural climate variability. Southern Annular Mode, SAM, 70 year loess filter (Abram et al., 2014); El Niño-Southern Oscillation, ENSO (Conroy et al., 2008); Interdecadal Pacific Oscillation, IPO, piece-wise linear fit(Vance et al., 2015); Pacific Decadal Oscillation, PDO (MacDonald and Case, 2005); Atlantic Multidecadal Oscillation, AMO (Mann et al., 2009); solar activity changes (Steinhilber et al., 2012); volcanic eruptions (Sigl et al., 2015).

Fig. 9. Inverse MCA temperature trends in the Antarctic Peninsula (AP) - Weddell Sea and Ross Sea regions compared to ENSO and SAM reconstructions. Proposed MCA dipoles are inspired by modern ENSO-driven temperature dipole trends. Proxy series: Southern Annular Mode, SAM, 70 year loess filter (Abram et al., 2014); El Niño-Southern Oscillation, ENSO (Conroy et al., 2008); site 18: JPC38 (Barbara et al., 2016); site 31: Berkner Island (Mulvaney et al., 2002); site 49: Woods Bay (Mezgec et al., 2017); site 56: Taylor Dome (Steig et al., 1998; Steig et al., 2000).

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1000 years in the Crozet Archipelago are robust, as nutrient input from stations (Suparta et al., 2010; Volobuev, 2014; Voskresenskiy and elephant seals and wandering albatrosses may have obscured the cli- Chukanin, 1981). On multi-centennial time-scales many Antarctic pa- mate signal (Ooms et al., 2011) (Site 2, see text S2 in Supplement for laeoclimatic records appear to be pulsed by the solar Suess/de Vries full discussion). The Crozet site is located close to the path of the cycle which has a typical period in the range of 180–230 years. The Agulhas Return Current (Pollard et al., 2007). Notably, an MCA with cycle was found in several temperature and productivity proxy records cold SST has been reported from marine core GeoB 18308 (African Site of the Antarctic Peninsula, namely sites 9, 11 and 25 (Bárcena et al., 18 in Lüning et al., 2017) over the Agulhas Bank in coastal southern 1998; Domack and Mayewski, 1999; Khim et al., 2002; Leventer et al., South . Hahn et al. (2017) interpreted this to be a result of a 1996)(Fig. 2). Suess/de Vries frequencies have been also identified in strengthened and more southerly South Indian Ocean anticyclone wind and temperature proxies in East Antarctic ice cores (Delmonte which increased the Easterlies, Algulhas Current and upwelling activity et al., 2005; Lüdecke et al., 2016; Zhao and Feng, 2015), off Adélie Land in the Agulhas area during the MCA. An intensification of the Agulhas (Crosta et al., 2007) and westerly airflow, closely Return Current during the MCA may have brought cold upwelling water related to SAM (Turney et al., 2016). On even longer time scales, mil- all the way to Crozet, although the interjacent SubAntarctic Front may lennial-scale cycles of the solar Eddy type with periods of have shielded Crozet from the Agulhas effects. 800–1200 years (Abreu et al., 2010) have been reported in Antarctic ice cores (Masson-Delmotte et al., 2004; Masson et al., 2000; Zhao and 7.2. Solar forcing Feng, 2015) and marine Antarctic and Subantarctic sediments (Crosta et al., 2007; Nielsen et al., 2004). Antarctic snowfall, wind strength and Ocean cycles such as ENSO, SAM and others control most of the air masses all appear to be influenced by solar activity (Bertler et al., climate variability in the Antarctic region, on time scales ranging from 2005; Frezzotti et al., 2013; Mayewski et al., 2005). A short-lived yearly to multi-centennial (Barbara et al., 2016; Bentley et al., 2009; cooling episode during the earliest MCA on Signy Island (Site 8) coin- Christ et al., 2015; Costa et al., 2007; Hall et al., 2010b; Jomelli et al., cides with the solar Oort Minimum (1010–1050 CE) (Fig. S3). The 2017; Mayewski et al., 2013; Shevenell et al., 2011; Shevenell and penguin population of Prydz Bay over the past 1200 years appears to Kennett, 2002). The ultimate drivers of the ocean cycles, however, are have been controlled by solar activity, most likely through a food-chain still not fully understood. Besides an autocyclical element, external mechanism influencing the intensity of phytoplankton productivity and forcings are likely involved. For the industrial era, the SAM is thought the abundance of krill (Gao et al., 2019). to be influenced by anthropogenic changes in stratospheric ozone and greenhouse gases (Arblaster and Meehl, 2006). Another important ex- 7.3. Volcanism ternal forcing candidate are solar activity changes. Visual comparison of ENSO, SAM and solar activity reconstructions shows similarities in The reconstructions of global volcanic forcing (Sigl et al., 2015) and the development of these parameters over the past 1500 years (Fig. 8). global volcanic aerosol optical depth (Crowley and Unterman, 2013) Whilst MCA and the last 100 years are characterized by high values, the suggest low volcanic activity during the early part of the MCA Little Ice Age period 1400–1700 CE is mostly associated with low va- (950–1100 CE), and increased volcanic activity in the second half lues. (1100–1350 CE), followed by low volcanic activity (1350–1600 CE) Studies have found a significant correlation between SAM and solar (Fig. 8). Comparison with Antarctic temperature proxy records (Figs. activity changes, when effects associated with the Quasi-Biennial S2–S12) does not reveal a climate signal in the MCA and early LIA Oscillation (QBO) are accounted for (Arblaster and Meehl, 2006; Haigh climate data that would correspond to this global volcanic activity and Roscoe, 2006; Kuroda and Kodera, 2005; Roy and Haigh, 2011). pattern. A differentiation into Southern Hemisphere, Northern Hemi- The QBO is a quasiperiodic oscillation with a mean period of about sphere and tropical eruptions (Sigl et al., 2015) does again not find its 2.5 years which describes the reversal of wind directions in the tropical representation in the Antarctic MCA temperature development. It is stratosphere from easterlies to westerlies. The solar influence on SAM therefore assumed, that changes in volcanic activity have not played a appears to be triggered by changes in the ultraviolet radiation (UV) significant role for the MCA in Antarctica (see also text S1h in sup- which lead to ozone anomalies in the polar lower stratosphere (Kuroda plement). This corresponds with the findings of Stenni et al. (2017) who and Shibata, 2006; Mayewski et al., 2017) that are then passed down failed to find a consistent Antarctic temperature response to large vol- into the troposphere in form of a “top-down” mechanism (Petrick et al., canic eruptions. 2012). The strength of the troposphere-stratosphere coupling is also thought to be modulated by solar activity (Kuroda et al., 2007). Ac- 8. Regional and global context cording to Petrick et al. (2012), positive SAM like patterns in the sur- face pressure and associated wind anomalies occur typically during Stenni et al. (2017) published temperature reconstructions of the solar maximum conditions. past 2000 years that are differentiated for seven Antarctic regions. The Likewise, several studies suggest a solar influence on ENSO for time reconstructions are based on stacked composites of δ18O water isotopes scales ranging from annual to multi-centennial (Hassan et al., 2016; from ice cores which for the MCA correspond to a similar set of cores as Mehta and Lau, 1997; Nuzhdina, 2002; Salas et al., 2016; Yan et al., used in our analysis. Stenni et al. (2017) describe MCA warming for the 2011). Other studies suggest that the solar forcing of climate is East Antarctic Plateau and Victoria Land Coast, which matches our modulated by ENSO (Emile-Geay et al., 2007; Ruzmaikin, 1999). The findings. Furthermore, their analysis confirmed the cool nature of the link between solar and ocean cycle variability is characterized by MCA for the Weddell Sea Coast and a mostly warm MCA on the West nonlinear dynamics which complicates identification of the solar signal. Antarctic Ice Sheet, based on single ice cores of Berkner Island (Site 31) The solar impact on Antarctic climate is likely to be indirect and linked and WAIS (Site 60), respectively. The MCA temperature development to a teleconnection in atmospheric circulation that forces complex proposed by Stenni et al. (2017) for the Antarctic Peninsula appears less feedback between the tropical Pacific and Antarctica (Frezzotti et al., representative because it is only based on one ice core (James Ross 2013). Island, Site 21) that currently undergoes age model revisions. In addi- Spectral analysis of Antarctic climate records has revealed char- tion, Site 21 appears to be located at the central dividing line of the acteristic cyclicities that correspond to well-known solar activity cycles. Antarctic Dipole which separates a warm MCA on the western side from Schwabe cycles with a period of 11 years have been identified in marine a cold MCA on the eastern side of the Antarctic Peninsula. Here, the sediments of the Adélie Drift at the East Antarctic Margin (Costa et al., additional non-ice core data from sites 9–30 allow to map the MCA in 2007), Antarctic sea ice (Curran et al., 2003) as well as in modern more detail than just based on ice cores. Another difference in our temperature and water vapor measurements at various Antarctic analysis compared to Stenni et al. (2017) is our delineation of a Ross Ice

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Shelf sector which we interpret to be characterized by a cold MCA several palaeoclimatic patterns have been identified. 1) MCA warming based on four ice cores (sites 56–59; Fig. 7). was found for the Subantarctic Islands south of the Antarctic The Stenni study documents a significant cooling in the integrated Convergence, the Antarctic Peninsula, Victoria Land and central West continent-wide Antarctic temperature curve which occurs around Antarctica. 2) A somewhat noisier MCA warming signal was detected 1250 CE and marks the transition to generally cold Little Ice Age which for the majority of East Antarctica. 3) MCA cooling occurred in the Ross lasts here until 1950 CE. The LIA cooling in Stenni et al. (2017) is even Ice Shelf region, and probably in the Weddell Sea and on Filchner- more pronounced than in the previous version of the Antarctic tem- Ronne Ice Shelf. 4) Spatial distribution of MCA cooling and warming perature reconstruction by PAGES 2k Consortium (2013) which also follows modern dipole patterns. Future studies will have to further re- only uses ice cores. In both reconstructions, the phase 0 to1000 CE is at fine and test these patterns, in particular for data-poor areas outside the least as warm as the MCA itself, which is also reflected in many of the Antarctic Peninsula. In particular marine cores are needed to comple- non-ice core datasets (Figs. S2–S12). Notably, solar activity was also ment the ice core data (Fig. 1). high during this entire phase, except a less active episode around Main drivers of the multi-centennial scale climate variability appear 700 CE (Fig. 8). to be the Southern Annular Mode (SAM) and El Niño-Southern Neukom and Gergis (2012) and Neukom et al. (2014) compiled Oscillation (ENSO) which are linked to solar activity changes by non- temperature reconstructions for the Southern Hemisphere in which linear dynamics. The MCA forms the final part of a long warm phase Antarctica is represented exclusively by ice cores. Just four of the ice that dominated the first millennium CE and ended 1250 CE with the cores reach back to the MCA (Berkner Island, Dronning Maud, Law onset of significant cooling in the transition to the Little Ice Age (LIA). Dome, Siple Dome; sites 31, 32, 46, 58). Half of these lie in the rather Overall timing of the MCA and LIA in Antarctica matches the climate small MCA cold areas of the Weddell Sea and Ross Ice Shelf (Fig. 6). The development of the . Natural variability still Antarctic MCA reconstruction in these two studies therefore cannot be overwhelms the forced response in the recent Antarctic climate devel- considered representative in terms of an area-weighted continent-wide opment, and models appear to not fully represent this natural varia- average. The question of a globally coherent warm phase during med- bility. Multiple evidence points to a significant solar influence on ieval times therefore remains open and certainly requires higher re- Antarctic climate which may be worth testing in future model scenarios solution data from Southern Hemisphere locations and a better in greater detail. knowledge of local temperature dipoles. Another Antarctic temperature reconstruction based on ice cores has been published by Goosse et al. Acknowledgements (2012). Three out of the seven cores are located in the small Antarctic sectors with a cold MCA (sites 31, 56, 58), which again distorts the We wish to thank all scientists whose case studies form the basis of continent-wide MCA average of the reconstruction (Fig. 6). Mann et al. this palaeoclimate mapping synthesis. We are grateful for provision of (2008) included various ice cores into their global temperature re- tabulated data and valuable discussions to Wenshen Xiao, Robert construction, none of which reached back to the MCA. Data of longer Mulvaney, Rebecca Minzoni, Stephanie Strother, Loïc Barbara, Patrick cores were cut and only commenced after the MCA. The often-cited Monien, Willem van der Bilt, Svante Björck, Rolf Zale, Dmitry Divine, temperature reconstructions by Moberg et al. (2005), Hegerl et al. Sonja Berg, Nancy Bertler, Barbara Stenni, James White, Tyler Jones, (2007) and Ljungqvist et al. (2012) only cover the Northern Hemi- Jean Jouzel, Valérie Masson-Delmotte, Tas van Ommen, Alexey sphere. Ekaykin, Hugues Goosse, Vincent Jomelli, Stephen McIntyre, Françoise Climate models still fail to replicate the continent-scale absence of Vimeux, Ronald Kwok, Boo-Keun Khim, Damien Irving, Jessica Conroy, 20th century warming in Antarctica (Abram et al., 2016; Goosse, 2017; Paul Vaughan, Liz Thomas, Xavier Crosta, Jonathan Stelling, Yuesong Jones et al., 2016; Stenni et al., 2017). Similar discrepancies between Gao and Ian Goodwin. A large amount of data was sourced through the simulated and reconstructed temperature development exist on a Ho- PANGAEA online data base and the NOAA National Centers for locene scale (Ackerley et al., 2017). A previous attempt by Goosse et al. Environmental Information (NCEI, formerly NCDC), invaluable services (2012) to explain the Antarctic long term cooling trend 0–1950 CE in which are greatly acknowledged. A big thank you goes to Frank Bosse simulations by volcanic forcing is not compatible with the latest vol- for calculating the MCA trend statistics. We are indebted to Jurgis canic forcing reconstructions (Sigl et al., 2015)(Fig. 8). Recent research Klaudius and Lloyd's Register for providing the database and correla- found a negative greenhouse effect for CO2 over the highest elevations tion software IC™ for this project. This review forms part of the of central Antarctica (Schmithüsen et al., 2015). Nevertheless, Smith Medieval Climate Anomaly Mapping Project which has been kindly et al. (2018) consider it unlikely that this plays a role in the recently supported by crowdfunding. We are particularly grateful to Jens Kröger observed East Antarctic cooling. Natural variability still overwhelms for helping to jump-start the project. Kind support was also received by the forced response in the recent Antarctic climate development, but Hans Fischer, Christiane & Michael Grunenberg, Arnd Externbrink, the models appear to not fully represent this natural variability or may Hans-Joachim Dammschneider, Rainer Frank Elsaesser, Uli Weber, overestimate the magnitude of the forced response (Jones et al., 2016). Jochen Zimmermann, and many others. Note that this study is fully Given the strong solar influence on pre-industrial Antarctic climate, unrelated to the first author's employment in the hydrocarbon sector solar forcing may actually play a stronger role in modern Antarctic and was neither commissioned nor funded by the energy industry. SL climate than previously thought, which may be worth testing in re- undertook this study outside office hours as a private person, trained spective model scenarios. geoscientist, and former full-time academic. We are grateful to two anonymous reviewers who greatly helped to improve this manuscript. 9. Conclusions Appendix A. Supplementary data The Medieval Climate Anomaly (MCA) is a well-recognized climate perturbation in many parts of the world, with a core period of Supplementary data to this article can be found online at https:// 1000–1200 CE. It is still being debated whether the MCA also occurred doi.org/10.1016/j.palaeo.2019.109251. in the Southern Hemisphere. Here we mapped the MCA across the Antarctic region based on the analysis of published palaeotemperature References proxy data from 60 sites in the Antarctic region. We are augmenting the conventionally used ice core data by adding temperature proxy records Abram, N.J., Mulvaney, R., Wolff, E.W., Triest, J., Kipfstuhl, S., Trusel, L.D., Vimeux, F., from marine and terrestrial sediment cores as well as radiocarbon ages Fleet, L., Arrowsmith, C., 2013. Acceleration of snow melt in an Antarctic Peninsula ice core during the twentieth century. Nat. Geosci. 6 (5), 404–411. of glacier moraines and elephant seal colonies. Although variable,

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