  The climate and circulation of a terrestrial planet are governed by, among
other things, the distance to its host star, its size, rotation rate,
obliquity, atmospheric composition and gravity. Here we explore the effects of
the last of these, the Newtonian gravitational acceleration, on its atmosphere
and climate. We first demonstrate that if the atmosphere obeys the hydrostatic
primitive equations, which are a very good approximation for most terrestrial
atmospheres, and if the radiative forcing is unaltered, changes in gravity have
no effect at all on the circulation except for a vertical rescaling. That is to
say, the effects of gravity may be completely scaled away and the circulation
is unaltered. However, if the atmosphere contains a dilute condensible that is
radiatively active, such as water or methane, then an increase in gravity will
generally lead to a cooling of the planet because the total path length of the
condensible will be reduced as gravity increases, leading to a reduction in the
greenhouse effect. Furthermore, the specific humidity will decrease, leading to
changes in the moist adiabatic lapse rate, in the equator-to-pole heat
transport, and in the surface energy balance because of changes in the sensible
and latent fluxes. These effects are all demonstrated both by theoretical
arguments and by numerical simulations with moist and dry general circulation
models.
