Comparison of isotopic signatures in speleothem records and model
simulations for the past millennium
Abstract
Global changes in climate not only affect its mean, but also its
variability, which mainly impacts society. For better projections of
future climate changes it is crucial to improve the understanding of
changes in both the mean, the variability and their relationship.
Model-Data comparison between climate simulations and speleothem
paleoclimate archives can test and validate the capability of different
general circulation models (GCMs) to simulate changes in variability.
However, the d18O values measured in climate archives don’t directly
represent temperature or precipitation but result from multivariate,
non-linear processes on top of the dominant atmospheric controls on
precipitation d18O. We aim to assess a model’s capability to simulate
climate variability on timescales longer than those observable. Our
strategy combines a Proxy System Model (PSM) for the relevant processes
with isotope-enabled GCMs. We focus on speleothems, as they are
precisely date-able and provide well preserved (semi-)continuous climate
signals in the lower and mid-latitudes. We evaluate trends, correlations
between different records and power spectral densities across a
speleothem database, focusing on the past millennium. We compare proxy
results to those obtained by forward models based on isotope-enabled
HadCM3 simulations and PSM approaches of increasing complexity. We
evaluate the sensitivity of results to parameter choices, and test
options to constrain them. We find that some parameters, e.g. transit
times of water from the surface to the speleothem’s cave, strongly
influences the slope of the spectra in the PSM. Based on the ample proxy
and model evidence for the past 1000ys, we test for realistic parameter
ranges and the sufficient complexity of speleothem PSM for global
application. Given a successful application on this more recent period
we envisage application on longer, millennial to orbital timescales, to
provide estimates of low-latitude changes in climate variability.