  The future biosphere on Earth (as with its past) will be made up
predominantly of unicellular microorganisms. Unicellular life was probably
present for at least 2.5 Gyr before multicellular life appeared and will likely
be the only form of life capable of surviving on the planet in the far future,
when the ageing Sun causes environmental conditions to become more hostile to
more complex forms of life. Therefore, it is statistically more likely that
habitable Earth-like exoplanets we discover will be at a stage in their
habitable lifetime more conducive to supporting unicellular, rather than
multicellular life. The end stage of habitability on Earth is the focus of this
work. A simple, latitude-based climate model incorporating eccentricity and
obliquity variations is used as a guide to the temperature evolution of the
Earth over the next 3 Gyr. This allows inferences to be made about potential
refuges for life, particularly in mountains and cold-trap (ice) caves and what
forms of life could live in these environments. Results suggest that in high
latitude regions, unicellular life could persist for up to 2.8 Gyr from
present. This begins to answer the question of how the habitability of Earth
will evolve at local scales alongside the Sun's main sequence evolution and, by
extension, how the habitability of Earth-like planets would evolve over time
with their own host stars.
