  We have shown in a recent study, using 3D climate simulations, that dayside
land cover has a substantial impact on the climate of a synchronously rotating
temperate rocky planet such as Proxima Centauri b. Building on that result, we
generate synthetic transit spectra from our simulations to assess the impact of
these land-induced climate uncertainties on water vapour transit signals. We
find that distinct climate regimes will likely be very difficult to
differentiate in transit spectra, even under the more favourable conditions of
smaller planets orbiting ultracool dwarfs. Further, we show that additional
climate ambiguities arise when both land cover and atmosphere mass are unknown,
as is likely to be the case for transiting planets. While water vapour may be
detectable under favourable conditions, it may be nearly impossible to infer a
rocky planet's surface conditions or climate state from its transit spectrum
due to the interdependent effects of land cover and atmosphere mass on surface
temperature, humidity, and terminator cloud cover.
