  While recently discovered exotic new planet-types have both challenged our
imaginations and broadened our knowledge of planetary system workings, perhaps
the most compelling objective of exoplanet science is to detect and
characterize habitable and possibly inhabited worlds orbiting in other star
systems. For the foreseeable future, characterizations of extrasolar planets
will be made via remote sensing of planetary spectroscopic and temporal
signals, along with careful fitting of this data to advanced models of planets
and their atmospheres. Terrestrial planets are small and significantly more
challenging to observe compared to their larger gaseous brethren; however
observatories coming on-line in the coming decade will begin to allow their
characterization. Still, it is not enough to invest only in observational
endeavors. Comprehensive modeling of planetary atmospheres is required in order
to fully understand what it is that our grand telescopes see in the night-sky.
In our quest to characterize habitable, and possibly inhabited worlds, 3D
general circulation models (GCMs) should be used to evaluate potential climate
states and their associated temporal and spatial dependent observable signals.
3D models allow for coupled, self-consistent, multi-dimensional simulations,
which can realistically simulate the climates of terrestrial extrasolar
planets. A complete theoretical understanding of terrestrial exoplanetary
atmospheres, gained through comprehensive 3D modeling, is critical for
interpreting spectra of exoplanets taken from current and planned instruments,
and is critical for designing future missions that aim to measure spectra of
potentially habitable exoplanets as one of their key science goals. We
recommend continued institutional support for 3D GCM modeling teams that focus
on planetary and exoplanetary applications.
