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.
Highlights
The impacts of a changing climate have been observed over the last century (IPCC, 2013) and indicate a strong sensitivity of human societies and natural systems to changes in climate
For our model–data comparison, we focus on the time period of the last millennium (850–1850 CE; Taylor et al, 2012), for which a fairly high number of well-preserved datasets are available
The cave is located at an altitude of 3850 m above sea level, whereas model altitude at the grid box is close to sea level
Summary
The impacts of a changing climate have been observed over the last century (IPCC, 2013) and indicate a strong sensitivity of human societies and natural systems to changes in climate. While the mean state of the climate is well observed, direction and magnitude of potential changes to its variability are still largely unclear (Franzke et al, 2020). Changes in variability influence the occurrence of extreme temperature and precipitation events (Katz and Brown, 1992) and have major impacts on society, economies (Hänsel et al, 2020), and ecosystems (Vasseur et al, 2014). Bühler et al.: Comparison of isotopic signatures in speleothem records
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