Evaluating model outputs using integrated global speleothem records of climate change since the last glacial

Laia Comas-Bru,Martin Werner,Sandy P. Harrison,Nick Scroxton,Kira Rehfeld,Cristina Veiga-Pires

doi:10.5194/cp-15-1557-2019

Abstract

Abstract. 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 δ18O 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 δ18O 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.

Highlights

Earth system models (ESMs) are routinely used to project the consequences of current and future anthropogenic forcing of climate, and the impacts of these projected changes on environmental services
Several palaeoclimate simulations are being run by the Palaeoclimate Modelling Intercomparison Project (PMIP) as part of the sixth phase of Coupled Model Intercomparison Project (CMIP) (CMIP6-PMIP4), including simulations of the Last Millennium (LM, 850–1850 CE, past1000), mid-Holocene (MH, ca. 6000 yr BP, midHolocene) Last Glacial Maximum (LGM, ca. 21 000 yr BP, lgm), the Last Interglacial (LIG, ca. 127 000 yr BP, lig127k) and the mid-Pliocene Warm Period (Kageyama et al, 2017)
We examine a number of issues that need to be addressed in order to use speleothem data, data from the Synthesis and Analysis (SISAL) database, for model evaluation in the palaeoclimate context and make recommendations about robust approaches that should be used for model evaluation in CMIP6-PMIP4

Summary

Introduction

Earth system models (ESMs) are routinely used to project the consequences of current and future anthropogenic forcing of climate, and the impacts of these projected changes on environmental services ESMs are routinely evaluated using modern and historical climate data. The range of climate variability experienced during the period for which we have reliable historic climate observations is small, much smaller than the amplitude of changes projected for the 21st century. The use of “out-of-sample” testing (Schmidt et al, 2014) is part of the evaluation procedure of the Coupled Model Intercomparison Project (CMIP). Several palaeoclimate simulations are being run by the Palaeoclimate Modelling Intercomparison Project (PMIP) as part of the sixth phase of CMIP (CMIP6-PMIP4), including simulations of the Last Millennium (LM, 850–1850 CE, past1000), mid-Holocene Several palaeoclimate simulations are being run by the Palaeoclimate Modelling Intercomparison Project (PMIP) as part of the sixth phase of CMIP (CMIP6-PMIP4), including simulations of the Last Millennium (LM, 850–1850 CE, past1000), mid-Holocene (MH, ca. 6000 yr BP, midHolocene) Last Glacial Maximum (LGM, ca. 21 000 yr BP, lgm), the Last Interglacial (LIG, ca. 127 000 yr BP, lig127k) and the mid-Pliocene Warm Period (mPWP, ca. 3.2 M yr BP, midPliocene-eoi400) (Kageyama et al, 2017)

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