  Global climate evolution models for habitable earthlike planets do not
consider the effect of ocean salinity on land ice formation through the
hydrological cycle. We consider two categories of such planets: planets with
deep oceans, but intrinsically high salinities due to the weaker salt removal
process by hydrothermal vents; and planets with shallow oceans, where the
increase in salt content and decrease in ocean area during the onset of
glaciation cause a negative feedback, helping delay the spread of land ice. We
developed a toy climate model of a habitable planet on the verge of an ice age,
using a range of initial salt concentrations. Planets with deep oceans and high
salinity show considerable increase in the time necessary to fill arctic land
with ice sheets, up to 23% considering the maximum salinity range. For planets
with shallow oceans, the effect of intrinsic high salinity is reinforced by the
negative feedback, counteracting positive feedbacks like the ice-albedo and
Croll-Milankovitch perturbations, to the point of effectively terminating land
ice sheet growth rate during the simulated timescale. We also apply this model
to the putative ocean of early Mars, finding intermediate results: salinity
probably did not play a role in the evolution of Mars' climate, considering the
timescale of its ice ages. We conclude that this phenomenon is essentially an
abiotic self-regulation mechanism against ice ages and should be regarded in
the context of habitable planets smaller and drier than the Earth, which may
well represent the bulk of habitable planets.
