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.