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Evaluation of Eemian and Holocene
Climate Trends: Combining Marine
Archives with Climate Modelling
Gerrit Lohmann, Ralph Schneider, Johann H. Jungclaus,
Guillaume Leduc, Nils Fischer, Madlene Pfeiffer and Thomas Laepple
Abstract In an attempt to assess trends of Holocene sea-surface temperature
(SST), two proxies have been compiled and analyzed in light of model simulations.
The data reveal contrasting SST trends, depending upon the proxy used to derive
Holocene SST history. To reconcile these mismatches between proxies in the
estimated Holocene SST trends, it has been proposed that the Holocene evolution of
orbitally-driven seasonality of the incoming radiation is the
first-order driving
mechanism of the observed SST trends. Such hypothesis has been further tested in
numerical models of the Earth system with important implications for SST signals
ultimately recorded by marine sediment cores. The analysis of model results and
alkenone proxy data for the Holocene indicate a similar pattern in temperature
change, but the simulated SST trends underestimate the proxy-based SST trends by
a factor of two to
five. SST trends based on Mg/Ca show no correspondence with
model results. We explore whether the consideration of different growing seasons
and depth habitats of the planktonic organisms used for temperature reconstruction
could lead to a better agreement of model results with alkenone data on a regional
scale. We found that invoking shifts in the living season and habitat depth can
remove some of the model
data discrepancies in SST trends. Our results indicate
that modeled and reconstructed temperature trends are to a large degree only
qualitatively comparable, thus providing at present a challenge for the interpretation
of proxy data as well as the model sensitivity to orbital forcing.
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