In most cases the internal variability of atmosphere-ocean general circulation models is used as a substitute for instrumental observations, which cover too short a time span to sample natural climate variability on the relevant timescale of several decades.
(the internal variability of atmosphere-ocean general circulation models; is used; as a substitute for instrumental observations; L:In most cases) (instrumental observations; cover; too short a time span)
the internal variability of atmosphere-ocean general circulation models instrumental observations
is used cover
as a substitute for instrumental observations L:In most cases
too short a time span