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Although quantitative isotope data from speleothems has been used to evaluate isotope-enabled model simulations, currently no consensus exists regarding the most appropriate methodology through which to achieve this. A number of modelling groups will be running isotope-enabled palaeoclimate simulations in the framework of the Coupled Model Intercomparison Project Phase 6, so it is timely to evaluate different approaches to using the speleothem data for data-model comparisons. Here, we illustrate this using 456 globally distributed speleothem delta O-18 records from an updated version of the Speleothem Isotopes Synthesis and Analysis (SISAL) database and palaeoclimate simulations generated using the ECHAM5-wiso isotope-enabled atmospheric circulation model. We show that the SISAL records reproduce the first-order spatial patterns of isotopic variability in the modern day, strongly supporting the application of this dataset for evaluating model-derived isotope variability into the past. However, the discontinuous nature of many speleothem records complicates the process of procuring large numbers of records if data-model comparisons are made using the traditional approach of comparing anomalies between a control period and a given palaeoclimate experiment. To circumvent this issue, we illustrate techniques through which the absolute isotope values during any time period could be used for model evaluation. Specifically, we show that speleothem isotope records allow an assessment of a model's ability to simulate spatial isotopic trends. Our analyses provide a protocol for using speleothem isotope data for model evaluation, including screening the observations to take into account the impact of speleothem mineralogy on delta O-18 values, the optimum period for the modern observational baseline and the selection of an appropriate time window for creating means of the isotope data for palaeo-time-slices. European Geosciences Union - W2017/413; Irish Centre for Research in Applied Geosciences (iCRAG); European Association of Geochemistry (Early Career Ambassadors program 2017); Quaternary Research Association UK; Navarino Environmental Observatory, Stockholm University; University College Dublin, Savillex (UK) - SF1428; Ibn Zohr University, Morocco; University of Reading; European Research Council - 694481; Natural Environment Research Council (JPI-Belmont project "PAleao-Constraints on Monsoon Evolution and Dynamics (PACMEDY)"); Geological Survey Ireland - 2017-SC-056; Royal Irish Academy; Deutsche Forschungsgemeinschaft - RE3994/2-1; Past Global Changes (PAGES) programme; info:eu-repo/semantics/publishedVersion

Siberia and Central Asia are located at mid- to high latitudes and encompass a large landlocked area of the Eurasian continent containing vast tracts of permafrost (seasonal permafrost and permafrost), which is extremely sensitive to global climate change. However, previous research has scarcely investigated the changes in the paleoclimate in this region. Similarly, the temporal and spatial distributions of the stable isotopic composition (δ18OP) of precipitation and its corresponding influencing factors remain largely unknown. Therefore, we used data from 15 Global Network of Isotopes in Precipitation (GNIP) stations to investigate the relationships between δ18OP and the local temperature and precipitation considering changes in atmospheric circulation. Analyses conducted on the monthly, seasonal and annual timescales led to three main conclusions. (1) At the monthly timescale, the variations in δ18OP exhibited a significant positive correlation with the monthly mean temperature (p δ18OP excursion was positive in summer as the temperature increased and negative in winter as the temperature decreased. Note that the δ18OP values were also affected by the monthly precipitation, Eurasian zonal circulation index (EZCI), and water vapor source (e.g., polar air masses and local evaporative water vapor). (2) At the annual scale, the weighted average value of the precipitation-based δ18O (δ18OW) exhibited a temperature effect over 60° N–70° N. However, δ18OW may have been dominated by multiple factors from 40° N to 60° N (e.g., the North Atlantic Oscillation (NAO) and water vapor source changes). (3) At the annual timescale, the variability of the path of the westerly caused by changes in the NAO explained the variations in both δ18OP and δ18OW. Based on the limited observational data in this region, we found that δ18OP is correlated with the local temperature at the monthly and seasonal timescales. However, at the annual timescale, in addition to the temperature effect, δ18OP reflects the variability of the water vapor source that is dominated by the EZCI and NAO. Therefore, it is possible to reconstruct the histories of past atmospheric circulations and water vapor sources in this region via geologic δ18O proxies, e.g., speleothems records.

Stable isotope records from speleothems provide information on past climate changes, most particularly information that can be used to reconstruct past changes in precipitation and atmospheric circulation. These records are increasingly being used to provide "out-of-sample" evaluations of isotope-enabled climate models. SISAL (Speleothem Isotope Synthesis and Analysis) is an international working group of the Past Global Changes (PAGES) project. The working group aims to provide a comprehensive compilation of speleothem isotope records for climate reconstruction and model evaluation. The SISAL database contains data for individual speleothems, grouped by cave system. Stable isotopes of oxygen and carbon (delta O-18, delta C-13) measurements are referenced by distance from the top or bottom of the speleothem. Additional tables provide information on dating, including information on the dates used to construct the original age model and sufficient information to assess the quality of each data set and to erect a standardized chronology across different speleothems. The metadata table provides location information, information on the full range of measurements carried out on each speleothem and information on the cave system that is relevant to the interpretation of the records, as well as citations for both publications and archived data.

Stable isotope records from speleothems provide information on past climate changes, most particularly information that can be used to reconstruct past changes in precipitation and atmospheric circulation. These records are increasingly being used to provide “out-of-sample” evaluations of isotope-enabled climate models. SISAL (Speleothem Isotope Synthesis and Analysis) is an international working group of the Past Global Changes (PAGES) project. The working group aims to provide a comprehensive compilation of speleothem isotope records for climate reconstruction and model evaluation. The SISAL database contains data for individual speleothems, grouped by cave system. Stable isotopes of oxygen and carbon (δ18O, δ13C) measurements are referenced by distance from the top or bottom of the speleothem. Additional tables provide information on dating, including information on the dates used to construct the original age model and sufficient information to assess the quality of each data set and to erect a standardized chronology across different speleothems. The metadata table provides location information, information on the full range of measurements carried out on each speleothem and information on the cave system that is relevant to the interpretation of the records, as well as citations for both publications and archived data. The compiled data are available at http://dx.doi.org/10.17864/1947.147 .