The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) is a community project that aims
to quantify how differences in general circulation models (GCMs) could impact the climate prediction
for TRAPPIST-1e and, subsequently its atmospheric characterization in transit. Four GCMs have
participated in THAI so far: ExoCAM, ...
The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) is a community project that aims
to quantify how differences in general circulation models (GCMs) could impact the climate prediction
for TRAPPIST-1e and, subsequently its atmospheric characterization in transit. Four GCMs have
participated in THAI so far: ExoCAM, LMD-Generic, ROCKE-3D and the UM. This paper, focused
on the simulated observations, is the third part of a trilogy, following the analysis of two land planet
scenarios (Part I) and two aquaplanet scenarios (Part II). Here, we show a robust agreement between
the simulated spectra and the number of transits estimated to detect the land planet atmospheres.
For the cloudy aquaplanet ones,a 5–σ detection of CO2 could be achieved in about 10 transits if the
atmosphere contains at least 1 bar of CO2. That number can vary by 41–56% depending on the
GCM used to predict the terminator profiles, principally due to differences in the cloud deck altitude,
with ExoCAM and LMD-G producing higher clouds than ROCKE-3D and UM. Therefore, for the
first time, this work provides “GCM uncertainty error bars” of ∼ 50% that need to be considered
in future analyses of transmission spectra. We also analyzed the inter-transit variability induced
by weather patterns and changes of terminator cloudiness between transits. Its magnitude differs
significantly between the GCMs but its impact on the transmission spectra is within the measurement
uncertainties. THAI has demonstrated the importance of model intercomparison for exoplanets and
also paved the way for a larger project to develop an intercomparison meta-framework, namely the
Climates Using Interactive Suites of Intercomparisons Nested for Exoplanet Studies (CUISINES).