The relationship between terrestrial deep soil temperature (TDST)
and surface te mperature (SAT) at interannual and centennial timescales
has been investigated i n a simulation of the last millennium with a three-dimensional
climate model d riven by estimations of historical external forcing. TDST
is loosely related to borehole temperature profiles, which have been recently
used to reconstruct long term temperature trends in the last centuries.
Recently, questions about the v alidity of boreholes-based reconstructions
have been raised. In the simulation, at interannual time scales the connection
between TDST and S AT is stable in the simulation, being stronger in the
summer half year than in t he winter half year. At long timescales, annual
TDSL is a good proxy for annual SAT, and their variations are almost indistinguishable
from each other. Both T DSL and terrestrial SAT overestimate the variations
of global mean SAT. This may be a source for the disagreement between statistical
reconstructions of global SAT and terrestrial borehole measurements.