  We assess the dependence of Earth's disk-integrated mid-infrared thermal
emission spectrum on observation geometries and investigate which and how
spectral features are impacted by seasonality on Earth. We compiled an
exclusive dataset containing 2690 disk-integrated thermal emission spectra for
four different full-disk observing geometries (North & South Pole centered and
Africa & Pacific centred equatorial views) over four consecutive years. The
spectra were derived from 2378 spectral channels in the wavelength range from
3.75 to 15.4 micron (nominal resolution $\approx$ 1200) and were recorded by
the Atmospheric Infrared Sounder aboard the Aqua satellite. We learned that
there is significant seasonal variability in Earth's thermal emission spectrum,
and the strength of spectral features of bio-indicators, such as N2O, CH4, O3
and CO2 depends strongly on both season and viewing geometry. In addition, we
found a strong spectral degeneracy with respect to the latter two indicating
that multi-epoch measurements and time-dependent signals may be required in
order to fully characterize planetary environments. Even for Earth and
especially for equatorial views, the variations in flux and strength of
absorption features in the disk-integrated data are small and typically $\le$
10%. Disentangling these variations from the noise in future exoplanet
observations will be a challenge. However, irrespectively of when the planet
will be measured (i.e., day or night or season) the results from mid-infrared
observations will remain the same to the zeroth order which is an advantage
over reflected light observations.
