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Abstract

Are simulations and reconstructions of past climate
and its variability consistent with each other?We assess
the consistency of simulations and reconstructions for the climate
of the last millennium under the paradigm of a statistically
indistinguishable ensemble. In this type of analysis, the
null hypothesis is that reconstructions and simulations are
statistically indistinguishable and, therefore, are exchangeable
with each other. Ensemble consistency is assessed for
Northern Hemisphere mean temperature, Central European
mean temperature and for global temperature fields. Reconstructions
available for these regions serve as verification
data for a set of simulations of the climate of the last millennium
performed at the Max Planck Institute for Meteorology.
Consistency is generally limited to some sub-domains and
some sub-periods. Only the ensemble simulated and reconstructed
annual Central European mean temperatures for the
second half of the last millennium demonstrates unambiguous
consistency. Furthermore, we cannot exclude consistency
of an ensemble of reconstructions of Northern Hemisphere
temperature with the simulation ensemble mean.
If we treat simulations and reconstructions as equitable hypotheses
about past climate variability, the found general lack
of their consistency weakens our confidence in inferences
about past climate evolutions on the considered spatial and
temporal scales. That is, our available estimates of past climate
evolutions are on an equal footing but, as shown here,
inconsistent with each other.