29 Sep 2020

29 Sep 2020

Review status: a revised version of this preprint was accepted for the journal CP and is expected to appear here in due course.

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

Janica Carmen Bühler1, Carla Roesch1, Moritz Kirschner1, Louise Sime2, Max D Holloway3, and Kira Rehfeld1 Janica Carmen Bühler et al.
  • 1Institute of Environmental Physics, Ruprecht-Karls-Universität Heidelberg, INF 229, 69120 Heidelberg, Germany
  • 2British Arctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK
  • 3Scottish Association for Marine Science, Scottish Marine Institute, Oban, Argyll, PA37 1QA, UK

Abstract. Global changes in the climate, especially the warming trend in mean temperature, have received increasing public and scientific attention. 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 and/or precipitation have been used to test and validate the capability of climate models to represent climate changes. The oxygen isotopic ratio δ18O 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 simulated climate model abilities for the simulation of climate variability beyond the timescales covered by meteorological observations (10–100 yr).

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 these to a large global dataset of speleothem δ18O records from the Speleothem Isotopes Synthesis and AnaLysis (SISAL) database version 2 (Comas-Bru et al., 2020). We evaluate systematically offsets in mean and variance of simulated δ18O and test for the main climate drivers 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 proxy-based variability of δ18O is lower (higher) than simulated by iHadCM3 on decadal (centennial) timescales. Most of this difference can likely be attributed to the records' lower temporal resolution and averaging processes affecting the δ18O signal. Using cross-correlation analyses at site-level and modeled gridbox level, we find evidence for highly variable but generally low signal-to-noise ratios in the proxy data. This points at 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 longterm internal climate variability in these model simulations, which could be tested by repeating similar comparisons with other isotope-enabled climate models and paleoclimate databases.

Janica Carmen Bühler et al.

Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement

Janica Carmen Bühler et al.

Janica Carmen Bühler et al.


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Short summary
We present three new isotope-enabled simulations for the last millennium (850–1850 CE) and compare them to records from a global speleothem database. Offsets between the simulated and measured oxygen isotope ratios are fairly small. While modeled oxygen isotope ratios are more variable on decadal timescales, proxy records are more variable on (multi-)centennial timescales. This could be due to a lack of long-term variability in complex model simulations, but proxy biases cannot be excluded.