Antarctic climate change

The following is an important reference regarding climate changes in Antarctica during the last 2000 years.

Antarctic climate variability at regional and continental scales over the last 2,000 years

Abstract. Climate trends in the Antarctic region remain poorly characterised, owing to the brevity and scarcity of direct climate observations and the large magnitude of interannual to decadal-scale climate variability. Here, within the framework of the

PAGES Antarctica 2k working group, we build an enlarged database of ice core water stable isotope records from Antarctica, consisting of 112 records. We produce both unweighted and weighted isotopic (δ18O) composites and temperature

reconstructions since 0 CE, binned at 5 and 10-year resolution, for 7 climatically-distinct regions covering the Antarctic continent. Following earlier work of the Antarctica 2k working group, we also produce composites and reconstructions for the

broader regions of East Antarctica, West Antarctica, and the whole continent. We use three methods for our temperature

reconstructions: i) a temperature scaling based on the δ18O-temperature relationship output from an ECHAM5-wiso model

simulation nudged to ERA-interim atmospheric reanalyses from 1979 to 2013, and adjusted for the West Antarctic Ice Sheet

region to borehole temperature data; ii) a temperature scaling of the isotopic normalized anomalies to the variance of the regional reanalysis temperature and iii) a composite-plus-scaling approach used in a previous continental scale reconstruction

of Antarctic temperature since 1 CE but applied to the new Antarctic ice core database. Our new reconstructions confirm a significant cooling trend from 0 to 1900 CE across all Antarctic regions where records extend back into the 1st millennium, with the exception of the Wilkes Land coast and Weddell Sea coast regions. Within this long-term cooling trend from 0-1900 CE we find that the warmest period occurs between 300 and 1000 CE, and the coldest interval from 1200 to 1900 CE. Since

1900 CE, significant warming trends are identified for the West Antarctic Ice Sheet, the Dronning Maud Land coast and the Antarctic Peninsula regions, and these trends are robust across the distribution of records that contribute to the unweighted

isotopic composites and also significant in the weighted temperature reconstructions. Only for the Antarctic Peninsula is this most recent century-scale trend unusual in the context of natural variability over the last 2000-years. However, projected

warming of the Antarctic continent during the 21st Century may soon see significant and unusual warming develop across other parts of the Antarctic continent. The extended Antarctica 2k ice core isotope database developed by this working group

opens up many avenues for developing a deeper understanding of the response of Antarctic climate to natural and anthropogenic climate forcings. The first long-term quantification of regional climate in Antarctica presented herein is a basis for data-model comparison and assessments of past, present and future driving factors of Antarctic climate.

1 Introduction

Antarctica is the region of the world where instrumental climate records are shortest and sparsest. Estimates of temperature change with reasonable coverage across the full Antarctic continent are only available since 1958 CE (Nicolas and Bromwich, 2014), and the large magnitude of year-to-year climate variability that characterises Antarctica makes the interpretation of trends in this data sparse region problematic (Jones et al., 2016). As a result, the knowledge of past Antarctic temperature and climate variability is predominantly dependent on proxy records from natural archives. While coastal proxy records are being

developed from terrestrial and marine archives (Jones et al., 2016), information on Antarctic climate above the ice sheet

exclusively relies on the climatic interpretation of ice core records.

Within the variety of measurements performed in boreholes and ice cores, only water stable isotopes can provide subdecadal resolution records of past temperature changes (Küttel et al., 2012). In high accumulation areas of coastal zones and West

Antarctica, annual layer counting is feasible during the last centuries to millennia (Plummer et al., 2012; Abram et al., 2013; Thomas et al., 2013; Sigl et al. 2016; Winstrup et al. in prep.), and annual water stable isotope signals can be delivered.

However, in the dry regions of the central Antarctic plateau, where the longest ice core records are available, chronologies are less accurate and rely on the identification of volcanic deposits that can be used to tie ice cores from different sites to a common

Antarctic ice core age scale (Sigl et al., 2014 and 2015).

The chemical and physical signals measured in an individual ice core reflect a local climatic signal archived through the deposition and reworking of snow layers. The intermittency of Antarctic precipitation, variability in precipitation source

regions, and post-depositional effects of snow layers including wind drift and scouring, sublimation, and snow metamorphism can distort the climate signal preserved within ice cores and produces non-climatic noise. As a result, obtaining a robust climate

signal can only be achieved through the combination of multiple ice core records from a given site and/or region, and through the site-specific calibration of the relationships between water stable isotopes and temperature.

Water can be characterised by the stable isotope ratios of oxygen (δ18O: the deviation of the ratio of 18O/16O in a sample, relative to that of the standard, Vienna Standard Mean Ocean Water) and of deuterium (δD: the deviation of the ratio of 2H/1H).

Both of these parameters within ice cores provide information on past temperatures. There is solid theoretical understanding of distillation processes relating moisture transport towards the polar regions with air mass cooling and the progressive loss of

heavy water molecules along the condensation pathway (Jouzel and Merlivat, 1984). This theoretical understanding is further supported by numerical modelling performed using atmospheric general circulation models equipped with water stable

isotopes (Jouzel, 2014). The effects of these processes are observed in the spatial relationships between the isotopic composition of Antarctic precipitation/surface snow and surface air temperature across the continent. However, relationships between water stable isotopes in snow and surface temperature may vary through time as a result of changes between condensation and surface temperature (in relationship to changes in boundary layer stability), changes in moisture origin and initial evaporation conditions, changes in atmospheric transport pathways and changes in precipitation seasonality or intermittency (Masson-Delmotte et al., 2008). Investigations based on the sampling of Antarctic precipitation have demonstrated that seasonal and inter-annual isotope versus temperature slopes are generally smaller than spatially-derived relationships (van Ommen and Morgan, 1997; Schneider et al., 2005; Stenni et al., 2016; Schlosser et al., 2004; Ekaykin et al.,

2004; Fernandoy et al., 2010). Moreover, emerging studies combining the monitoring of surface water vapour isotopic composition with the isotopic composition retained in surface snow and precipitation have revealed that snow-air isotopic exchanges during snow metamorphism affect surface snow isotopic composition (Ritter et al., 2016; Casado et al., 2016a; Casado et al., 2016b; Touzeau et al., 2016). It is not yet possible to assess the importance of such post-deposition processes for the interpretation of ice core water stable isotope records, but they may enhance the relationship between snow isotopic composition and surface temperature more than expected from the intermittency of snowfall (Touzeau et al., 2016). Changes in ice sheet height due to ice dynamics may also affect the surface climate trends inferred from water stable isotope records;

however, this influence should be of second order over the last 2000-year interval that is the focus of this study (Fegyveresi et al., 2011).

As a result, the two key challenges to reconstruct past changes in Antarctic temperature from ice core isotope records are (1) to develop methodologies to combine different individual or stacked ice core records in order to deliver regional-scale climate signals, and (2) to quantify the temperature changes represented by water stable isotope variations.

Goosse et al. (2012) first calculated a composite of Antarctic temperature simply by averaging seven standardized temperature records inferred from water stable isotopes using a spatial isotope-temperature relationship for the last millennium. The first

coordinated effort to reconstruct Antarctic temperature during the last 2000 years (PAGES 2k Consortium, 2013) screened published ice core records for annual layer counting or alignment of volcanic sulphate records and overlap with instrumental

temperature data (Steig et al., 2009), leading to the selection of 11 records. The reconstruction procedure used a composite-plus-scaling approach similar to the methodology of Schneider et al. (2006), and produced reconstructions of the continent-

wide temperature history as well as specific West Antarctica and East Antarctica reconstructions. The skill of the reconstructions was limited by the number of available records through time (for instance, only one predictor in each region prior to 166CE). This analysis identified significant (p<0.01) cooling trends from 166 to 1900 CE, 2.5 times larger in West Antarctica than in East Antarctica. A robust cooling trend over this time period has also been identified from terrestrial and marine reconstructions from other regions (PAGES 2k Consortium, 2013; McGregor et al., 2015).

The comparison of these first Antarctic 2k time series with those from other regions obtained within the PAGES 2k working groups identified three specificities: (i) Antarctic reconstructed centennial variations did not correlate with those from other

regions; (ii) the Antarctic region was the only one where a protracted cold period was not starting around 1580 CE; (iii) the Antarctic region was the only one where the 20th century was not the warmest century of the last 2000 years. A recent effort

to characterize Antarctic and sub-Antarctic climate variability during the last 200 years also concluded that most of the trends observed since satellite climate monitoring began in 1979 CE cannot yet be distinguished from natural (unforced) climate variability (Jones et al., 2016), and are of the opposite sign to those produced by most forced climate model simulations over the same post-1979 CE interval. The only exception to this conclusion was for changes in the Southern Annular Mode (SAM), the leading mode of atmospheric circulation variability in the high latitudes of the SH, which has showed a significant and unusual positive trend since 1979 CE.

While changes in the SAM have been related to the human influence on stratospheric ozone and greenhouse gases (Thompson et al., 2011), major gaps remain in identifying the drivers of multi-centennial Antarctic climate variability. For instance, the

influence of solar and volcanic forcing on Antarctic climate variability remains unclear. This is due to both the lack of observations and to the lack of confidence in climate model skill for the Antarctic region (Flato et al., 2013). Goosse et al.

(2012) have used simulations from an intermediate complexity model to attribute the Antarctic annual mean cooling trend from 850 to 1850 CE to volcanic forcing. Recent comparisons of climate model simulations with the PAGES2k regional reconstructions have highlighted greater model-data disagreement in the Southern Hemisphere (SH) than in the Northern Hemisphere (PAGES 2k–PMIP3 group, 2015; Abram et al 2016); such disagreement could be due either to model deficiencies

or to large uncertainties in the reconstructions which were built on relatively small number of records. Changes in ocean heat content and ocean heat transport have likely contributed to the different temperature evolution at high southern latitudes

compared to other regions of the Earth (Goosse 2017), and model based studies have suggested that circulation in the Southern Ocean may act to delay by centuries the development of sustained warming trends in high southern latitudes (Armour et al.,

2016). Antarctic temperature reconstructions spanning the last 2000 years may help to better constrain the processes and timescales by which natural and anthropogenic forcing act to affect climate changes in the Antarctic region.

This motivates our efforts to produce updated Antarctic temperature reconstructions.  The previous continent-scale reconstruction (PAGES 2k Consortium, 2013), where only a limited number of records have been used, may mask important regional-scale features of Antarctica’s climate evolution. Here we use an expanded paleoclimate database of Antarctic ice core isotope records and new reconstruction methodologies to reconstruct the climate of the past 2000 years, at decadal scale and on a regional basis. Seven distinct climatic regions have been selected: the Antarctic Peninsula, the West Antarctic Ice Sheet, the East Antarctic Plateau, and four coastal domains of East Antarctica. This regional selection, which is supported by regional atmospheric RACMO2.4 model results, is applied to both Antarctic ice core-derived isotopic (temperature-proxy) and snow accumulation rate reconstructions (see companion paper in the same issue by Thomas et al.). Section 2 describes the ice core and the temperature data sets used in this study, as well as the modelling framework used to support the analysis. The climate

region definition, the pre-processing of the data and the different reconstruction methods are presented in Section 3. Section 4 discusses our new regional isotopic and temperature reconstructions for Antarctica, including the application of the previous

methodology to the new database. Finally, section 5 presents the summary of our results and their implications.

2 Datasets

2.1 Ice core records

Here we present and use a new expanded database which has been compiled in the framework of the PAGES Antarctica 2k

working group. The initial selection criteria are those requested by the PAGES 2k network (http://www.pages-igbp.org/ini/wg/2k-network/data) for the building of the community-sourced database of temperature-sensitive proxy records (PAGES2k Consortium, revised for Scientific Data). Briefly, i) the records must be publically available and published, ii) a relation between the climate proxies and variables should be stated, iii) the record duration should be between 300 and 2000 years, iv) the chronology, certified by the data owner, should contain at least one chronological control point near the end (most recent) part of the record and another near the oldest part of the record, v) the resolution should be at least one analysis every 50 years.

In building the Antarctica2k database we also allow shorter records to be included, although request a stratigraphic control using volcanic markers (Sigl et al., 2014) and whenever possible, a dating by annual layer counted chronology. This last requirement is only possible in the high-accumulation regions of West Antarctica, the Antarctic Peninsula and coastal areas of East Antarctica. The inclusion of shorter records is designed to improve data coverage for assessments of climatic trends in Antarctica during the past century. The 11 records included in the previous continental-scale reconstruction (PAGES 2k Consortium, 2013) relied on a highly precise chronological framework consisting of a common chronology, which used volcanic events to synchronize the records. Here, we use both high and low-resolution records. Most of the records have a data resolution ranging from 0.025 to 5 years (only three records have a resolution of >10 years). Previous studies (Frezzotti et al., 2007; Ekaykin et al., 2014) have shown that post-depositional and wind scouring effects, acting more effectively when the accumulation rate is very low, limit our ability to obtain temperature reconstructions at annual resolution in most of the interior of Antarctica. Because of this, in our regional reconstructions we use 5-year averaged data for reconstructing the last 200 years. and 10-year averages for reconstructing the last 2000 years. Using 5, or 10-year averages also decreases our dependence on an annually precise chronological constraint between the ice core records, allowing us to more confidently use the expanded database. The data have been also screened for glaciological problems, with those records that are very likely to be affected by ice flow dynamics excluded.

This enlarged database consists of 112 isotopic records. A list of the records used are reported in Table S1 (Supplementary Information) and their spatial distribution is shown in Figure 1. Figure S1 shows the location of the ice core sites along with a

visualization of the record lengths. Most of the records of this new database cover the last 200 years and this is particularly true for the more coastal areas. Within the database, 36 records cover just the last 50 years or less, while 50 records cover the

whole length of the past 200 years. There are 15 records that cover the last 1000 years, while only 9 records reach as far back as 0 CE.

2.2 Temperature product

The instrumental record is very short in Antarctica, and most ice core sites do not have weather station measurements associated with the cores. In addition, the retrieval of the first meter of firn can be difficult, due to poor cohesion of the snow.

As a result, for many sites, there is no overlap between instrumental and proxy data, which complicates the proxy calibration exercise. To enlarge the calibration dataset, we use the climate field reconstruction from Nicolas and Bromwich (2014) (hereafter NB2014). This surface temperature dataset provides homogeneous data at 60km resolution, extends from 1957 to 2013, and includes the revised Byrd temperature record (Bromwich et al., 2013) that improves the skill of the temperature

product over West Antarctica. It covers a longer timespan than reliable atmospheric reanalysis products for Antarctica (which begin only 1979 CE), and has a higher spatial resolution than available isotope enabled GCM outputs. This dataset is used to

estimate the spatial representativeness of individual core sites, to scale the normalized isotopic anomaly data to temperature, and to calculate the surface temperature reconstructions with the Composite-Plus-Scale (CPS) method (Section 3.4.4).

2.3 Modelling framework

In order to use model information on isotope-temperature relationships in Antarctic precipitation, we use a reference simulation performed using the Atmospheric General Circulation Model ECHAM5-wiso. The initial ECHAM5 model (Roeckner et al.,

2003) has been equipped with water stable isotopes (Werner et al., 2011), following earlier work on ECHAM3 (Hoffmann et al., 1998) and ECHAM4 (Werner et al., 2001), and accounting for fractionation processes during phase changes. This model

is used here because recent studies, based on model-data comparisons using observations of precipitation and surface vapour isotopic composition at a global scale and in the Arctic (e.g. Werner et al, 2011; Steen-Larsen et al., 2017), have shown strong

model skill of ECHAM5-wiso when it is run in high resolution as in this study (T106, with a mean horizontal grid resolution of approximately 1.1° x 1.1°). In Antarctica, model performance was assessed against a compilation of surface data (Masson-Delmotte et al., 2008) and recent measurements of vapour and precipitation (Goursaud et al., in preparation; Ritter et al., 2016;Dittmann et al., 2016).

Here, we use a 1958-2014 CE simulation where ECHAM5-wiso was nudged to atmospheric reanalyses from ERA40 (Uppala et al., 2005) and ERA interim (Dee et al., 2011), and run using the same ocean surface boundary conditions (SST and sea-ice)

as in ERA40 and ERA interim. Ocean surface water isotopic values were set to constant values using a compilation of observational data (Schmidt et al., 2007). Inter-comparisons of reanalysis products showed good skills of ERA-interim for

Antarctic precipitation (Wang et al., 2016), surface temperature, as well as vertical profiles of winds and temperatures.

However, comparisons with in-situ observations reveal an underestimate of precipitation and slight cold bias in the surface temperatures in some regions (Thomas and Bracegirdle, 2015).

The ECHAM5-wiso simulations produce a large increase in the temperature and the δ18O outputs prior to 1979, which is not observed in instrumental or ice core data (Goursaud et al., 2017; Goursaud et al., in prep.). This arises from a discontinuity in the ERA-40 reanalyses due to the lack of observations available for assimilation and boundary conditions prior to the satellite era (e.g. Antarctic sea ice) (Nicholas and Bromwich, 2014). We therefore use the ECHAM5-wiso simulations only for 1979-

2013 CE. For the analysis of the isotope-temperature relationships at each individual ice core site, we extracted the grid point data closest to each site. For the analysis of isotope-relationships at regional scale, we calculated the area-weighted average of model outputs at grid points within the region. The δ18O-temperature relationship was calculated using the annual or seasonal average 2-meter temperature and annual precipitation-weighted δ18O, to mimic deposition processes. The simulation does not account for post-deposition processes (i.e., diffusion, which is not important on the 5 and 10-year timescales considered here; e.g. Küttel et al., 2012).

3 Methodology

3.1 Defining climatic regions

Earlier work of the PAGES Antarctica 2k working group produced a continent-scale temperature reconstruction for the whole of Antarctica, as well as reconstructions for East and West Antarctica based on a separation approximated by the Transantarctic

mountain chain (PAGES 2k Consortium 2013). These broad-scale groupings mask important regional climatic trends noted in individual studies. In particular, the absence of recent significant warming in the Antarctica 2k continent-scale temperature reconstruction is known to not be representative of all Antarctic locations (e.g. Steig et al., 2009; Mulvaney et al., 2012; Abram et al., 2013; Steig et al 2013).

In this study we choose seven climatic reconstruction regions (Figure 1). These regions are defined based on our knowledge of regional climate and snow deposition processes in the Antarctic region, as well as the availability of ice core isotope records.

The regional selections were further validated and refined by spatial correlation of temperature using the NB2014 data product.

The seven climatic regions are defined as follows (see Table S1):

1. EAST ANTARCTIC PLATEAU: All East Antarctic contiguous regions at an elevation higher than 2000m, including everything south of 85°S. We exclude high peaks of the Transantarctic Mountains if they belong to the Victoria Land - Ross

Sea coast (e.g. Taylor Dome or Hercules Névé).

2. WILKES LAND COAST: A region that sits at an altitude <2000m, and extends from Lambert Glacier (67°E) east to the start of Victoria Land and the Transantarctic Mountains (160°E).

3. WEDDELL SEA COAST: Extending eastward from longitude 60°W to 30°W, and south of 75°S, and lying at an altitude <2000m. Eastward of the 30°W longitude, the 75°S latitude defines the boundary with the Dronning Maud Land coast region,

with the northeastern corner of the Weddell Sea coast region occurring where the 75°S latitude meets the 2000m elevation contour. This region includes the Filchner Ice Shelf and most of the Ronne Ice Shelf.

4. ANTARCTIC PENINSULA: This region encompasses the mountainous Antarctic Peninsula. Between 74°S and 70°S the longitudinal boundaries lie between 60-80°W, while north of 70°S the longitudinal boundaries increase to 50-80°W so as to capture the northern end of the peninsula.

5. WEST ANTARCTIC ICE SHEET: A region bounded by longitudes 60°W to 170°W, and north of 85°S. In the Peninsula region (60-80°W) a northern bound of 74°S is also applied.

6. VICTORIA LAND - ROSS SEA: This region is north of 85°S, and at an altitude <2000m, with the exception of some localised peaks within the Trans-Antarctic mountain range. It extends from 160°E to 190°E (i.e. 170°W) and incorporates most

of the Ross Ice Shelf.

7. DRONNING MAUD LAND COAST: Extending eastward from 30°W to 67°E (Lambert glacier). The southern-most boundary lies at 75°S (where this region borders with the Weddell Sea region), or at the 2000-meter elevation contour elsewhere.

(To be continued)

Authors:

Barbara Stenni1,2, Mark A. J. Curran3,4, Nerilie J. Abram5,6, Anais Orsi7, Sentia Goursaud7,8, Valerie

Masson-Delmotte7, Raphael Neukom9, Hugues Goosse10, Dmitry Divine11,12, Tas van Ommen3,4, Eric J.

Steig13, Daniel A. Dixon14, Elizabeth R. Thomas15, Nancy A. N. Bertler16,17, Elisabeth Isaksson11, Alexey Ekaykin18,19, Massimo Frezzotti20, Martin Werner21

1Department of Environmental Sciences, Informatics and Statistics, Ca’ Foscari University of Venice, Italy

2 Institute for the Dynamics of Environmental Processes, CNR, Venice, Italy.

3 Australian Antarctic Division, 203 Channel Highway, Kingston Tasmania 7050, Australia

4 Antarctic Climate & Ecosystems Cooperative Research Centre, University of Tasmania, Hobart 7001, Australia

5 Research School of Earth Sciences, Australian National University, Canberra ACT 2601, Australia.

6ARC Centre of Excellence for Climate System Science, Australian National University, Canberra ACT 2601, Australia

7 Laboratoire des Sciences du Climat et de l’Environnement (IPSL/CEA-CNRS-UVSQ UMR 8212), CEA Saclay, 91191 Gif-sur-Yvette cédex, France

8 Université Grenoble Alpes, Laboratoire de Glaciologie et Géophysique de l’Environnement (LGGE), 38041 Grenoble, France

9 University of Bern, Oeschger Centre for Climate Change Research & Institute of Geography, 3012 Bern, Switzerland

10 Université catholique de Louvain, Earth and Life Institute, Centre de recherches sur la terre et le climat Georges Lemaître,

B-1348 Louvain-la-Neuve, Belgium

11 Norwegian Polar Institute, Fram centre, N-9296 Tromsø, Norway

12Department of Mathematics and Statistics, Faculty of Science, University of Tromsø - The Arctic University of Norway, N-

9037, Norway

13 Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA

14 Climate Change Institute, University of Maine, Orono, ME 04469, USA

15British Antarctic Survey, Cambridge, UK CB3 0ET

16Antarctic Research Centre, Victoria University of Wellington, Wellington 6012, New Zealand

17National Ice Core Research Facility, GNS Science, Gracefield 5040, New Zealand

18 Arctic and Antarctic Research Institute, St Petersburg, Russia

19 Institute of Earth Sciences, Saint Petersburg State University, St Petersburg, Russia

20 ENEA Casaccia, Rome, Italy

21Alfred Wegner Institute, Helmholtz Centre for Polar and Marine Research, 27570 Bremerhaven, Germany