  More than a thousand exoplanets have been discovered over the last decade.
Perhaps more excitingly, probing their atmospheres has become possible. With
current data we have glimpsed the diversity of exoplanet atmospheres that will
be revealed over the coming decade. However, numerous questions concerning
their chemical composition, thermal structure, and atmospheric dynamics remain
to be answered. More observations of higher quality are needed. In the next
years, the selection of a space-based mission dedicated to the spectroscopic
characterization of exoplanets would revolutionize our understanding of the
physics of planetary atmospheres. Such a mission was proposed to the ESA cosmic
vision program in 2014. Our paper is therefore based on the planned
capabilities of the Exoplanet Characterization Observatory (EChO), but it
should equally apply to any future mission with similar characteristics. With
its large spectral coverage ($4-16\, \rm{\mu m}$), high spectral resolution
($\Delta\lambda/\lambda>300$ below $5\,\rm{\mu m}$ and
$\Delta\lambda/\lambda>30$ above $5\,\rm{\mu m}$) and $1.5\rm{m}$ mirror, a
future mission such as EChO will provide spectrally resolved transit
lightcurves, secondary eclipses lightcurves, and full phase curves of numerous
exoplanets with an unprecedented signal-to-noise ratio. In this paper, we
review some of today's main scientific questions about gas giant exoplanets
atmospheres, for which a future mission such as EChO will bring a decisive
contribution.
