Comparison of the oxygen isotope signatures in speleothem records and iHadCM3 model simulations for the last millennium

Abstract

Abstract. Improving the understanding of changes in the mean and variability of climate variables as well as their interrelation is crucial for reliable climate change projections. Comparisons between general circulation models and paleoclimate archives using indirect proxies for temperature or precipitation have been used to test and validate the capability of climate models to represent climate changes. The oxygen isotopic ratio δ18O, a proxy for many different climate variables, is routinely measured in speleothem samples at decadal or higher resolution, and single specimens can cover full glacial–interglacial cycles. The calcium carbonate cave deposits are precisely dateable and provide well preserved (semi-)continuous albeit multivariate climate signals in the lower and mid-latitudes, where the measured δ18O in the mineral does not directly represent temperature or precipitation. Therefore, speleothems represent suitable archives to assess climate model abilities to simulate climate variability beyond the timescales covered by meteorological observations (101–102 years). Here, we present three transient isotope-enabled simulations from the Hadley Center Climate Model version 3 (iHadCM3) covering the last millennium (850–1850 CE) and compare them to a large global dataset of speleothem δ18O records from the Speleothem Isotopes Synthesis and AnaLysis (SISAL) database version 2 (Comas-Bru et al., 2020b). We systematically evaluate offsets in mean and variance of simulated δ18O and test for the main climate drivers recorded in δ18O for individual records or regions. The time-mean spatial offsets between the simulated δ18O and the speleothem data are fairly small. However, using robust filters and spectral analysis, we show that the observed archive-based variability of δ18O is lower than simulated by iHadCM3 on decadal and higher on centennial timescales. Most of this difference can likely be attributed to the records' lower temporal resolution and averaging or smoothing processes affecting the δ18O signal, e.g., through soil water residence times. Using cross-correlation analyses at site level and modeled grid-box level, we find evidence for highly variable but generally low signal-to-noise ratios in the proxy data. This points to a high influence of cave-internal processes and regional climate particularities and could suggest low regional representativity of individual sites. Long-range strong positive correlations dominate the speleothem correlation network but are much weaker in the simulation. One reason for this could lie in a lack of long-term internal climate variability in these model simulations, which could be tested by repeating similar comparisons with other isotope-enabled climate models and paleoclimate databases.

Investigating stable oxygen and carbon isotopic variability in speleothem records over the last millennium using multiple isotope-enabled climate models

“…We use the last millennium run from the fully coupled isotope-enabled AOGCM iHadCM3 (Bühler et al, 2021). iHadCM3 has been widely used to simulate present (Dalaiden et al, 2020) and future climate (Sime et al, 2008;Tindall et al, 2009;IPCC, 2013), as well as for past climates (Tindall et al, 2010;Sime et al, 2013;Holloway et al, 2018).…”

“…The database includes 691 speleothem records from 294 caves across the globe from all continents except Antarctica. Globally, the last millennium has abundant records with sufficient resolution and reasonable dating uncertainties (Bühler et al, 2021). We filtered the database for records that cover at least a 600-year period within the last millennium (850-1850 CE) and exhibit at least 2 dates and 36 stable isotope measurements within the time period to guarantee a minimum resolution of 30 years.…”

“…It has been employed for model-data comparisons of the Last Glacial Maximum, the mid-Holocene, the last millennium, and the historical period using different models (iCESM: Midhun et al, 2021;iHadCM3: Bühler et al, 2021;ECHAM5-wiso: Comas-Bru et al, 2019;Parker et al, 2021;and GISS-E1-R: Parker et al, 2021). Previous model-data comparisons support the use of the database to evaluate modeled δ 18 O across different time periods (Comas-Bru et al, 2019), although speleothems exhibit a lower δ 18 O variability than simulated δ 18 O on interannual to decadal timescales globally (Bühler et al, 2021). However, a benchmarking study of model performance in simulating δ 18 O and its variability, including a multi-model comparison or a model-data comparison with SISALv2, has not yet been performed.…”

“…Here we will present a multi-model comparison of five isotope-enabled last millennium simulations: ECHAM5/MPI-OM (Sjolte et al, 2018), GISS ModelE2-R (Lewis and Legrande, 2015;Colose et al, 2016a, b), the isotope-enabled version of the Community Earth System Model (Stevenson et al, 2019;Brady et al, 2019), the isotope-enabled version 3 of the Hadley Model (Bühler et al, 2021), and the stable water-isotope-incorporated Scripps Experimental Climate Prediction Center's Global Spectral Model (Yoshimura et al, 2008), with climate characteristics and forcings as depicted in Fig. 1 and listed in Table 1.…”

“…Boundary conditions such as orbital forcing, sea level, and ice sheets are close to present-day levels, and external variability is mostly driven by variations in volcanic eruptions (Schurer et al, 2014;Neukom et al, 2019;Legrande and Anchukaitis, 2015). It is a key paleoclimate period for the CMIP5 and CMIP6 experiments (Taylor et al, 2012;Eyring et al, 2016), and speleothem records are abundant in this period (Bühler et al, 2021).…”