Assessing the robustness of Antarctic temperature reconstructions over the past 2 millennia using pseudoproxy and data assimilation experiments

• Published in: Climate of the past 2019-04, Vol.15 (2), p.661-684
• Main Authors: Klein, Francois, Abram, Nerilie J, Curran, Mark A. J, Goosse, Hugues, Goursaud, Sentia, Masson-Delmotte, Valerie, Moy, Andrew, Neukom, Raphael, Orsi, Anais, Sjolte, Jesper, Steiger, Nathan, Stenni, Barbara, Werner, Martin
• Format: Article
• Language: English
• Subjects: Analysis; Antarctic temperatures; Antarctic zone; Anthropogenic factors; Atmospheric temperature; Climate; Climate models; Climate Research; Coastal zone; Computer simulation; Continental interfaces, environment; Cores; Covariance; Data; Data assimilation; Data collection; Earth and Related Environmental Sciences; Experiments; Frameworks; Geovetenskap och miljövetenskap; Ice; Ice cores; Ice environments; Intercomparison; Isotopes; Klimatforskning; Local climates; Meteorologi och atmosfärforskning; Meteorology and Atmospheric Sciences; Modelling; Natural Sciences; Naturvetenskap; Ocean, Atmosphere; Oxygen; Oxygen isotopes; Paleoclimate; Paleoclimatology; Records; Regions; Sciences of the Universe; Statistical methods; Statistics; Temperature; Temperature changes; Temperature effects; Temperature trends
• ISSN: 1814-9332; 1814-9324; 1814-9332
• DOI: 10.5194/cp-15-661-2019

The Antarctic temperature changes over the past millennia remain more uncertain than in many other continental regions. This has several origins: (1) the number of high-resolution ice cores is small, in particular on the East Antarctic plateau and in some coastal areas in East Antarctica; (2) the short and spatially sparse instrumental records limit the calibration period for reconstructions and the assessment of the methodologies; (3) the link between isotope records from ice cores and local climate is usually complex and dependent on the spatial scales and timescales investigated. Here, we use climate model results, pseudoproxy experiments and data assimilation experiments to assess the potential for reconstructing the Antarctic temperature over the last 2 millennia based on a new database of stable oxygen isotopes in ice cores compiled in the framework of Antarctica2k (Stenni et al., 2017). The well-known covariance between δ18O and temperature is reproduced in the two isotope-enabled models used (ECHAM5/MPI-OM and ECHAM5-wiso), but is generally weak over the different Antarctic regions, limiting the skill of the reconstructions. Furthermore, the strength of the link displays large variations over the past millennium, further affecting the potential skill of temperature reconstructions based on statistical methods which rely on the assumption that the last decades are a good estimate for longer temperature reconstructions. Using a data assimilation technique allows, in theory, for changes in the δ18O–temperature link through time and space to be taken into account. Pseudoproxy experiments confirm the benefits of using data assimilation methods instead of statistical methods that provide reconstructions with unrealistic variances in some Antarctic subregions. They also confirm that the relatively weak link between both variables leads to a limited potential for reconstructing temperature based on δ18O. However, the reconstruction skill is higher and more uniform among reconstruction methods when the reconstruction target is the Antarctic as a whole rather than smaller Antarctic subregions. This consistency between the methods at the large scale is also observed when reconstructing temperature based on the real δ18O regional composites of Stenni et al. (2017). In this case, temperature reconstructions based on data assimilation confirm the long-term cooling over Antarctica during the last millennium, and the later onset of anthropogenic warming compared w