  The transiting planet K2-18 b is one of the best candidates for a relatively
nearby world harboring biological life. The long-term orbital evolution of this
planet is investigated using theoretical and purely numerical techniques for
two possible configurations: a single planet orbiting the host star, and a
two-planet system including the proposed inner planet close to the 4:1 mean
motion rationalization. The emphasis is made on the secular changes of
eccentricity and orbital inclination, which are important for the climate
stability of the planet. It is demonstrated that the secular orbital dynamics
of planet K2-18 b with an internal companion are accurately represented by the
periodic eccentricity and inclination exchange on the time scales of a few Kyr.
A single planet is not expected to experience fast orbital changes, with the
much weaker tidal and rotation-driven perturbations mostly reflecting in a slow
periastron and nodal precession. The tidal decay of the orbit is too
insignificant on the time scale of the stellar age. However, the conditions for
the habitability of a single K2-18 b planet are much improved if, like the
Earth, it rotates faster than the mean motion and its rotation angle is tilted
by a hypothetical moon. Milankovi{\'c}'s cycles of the habitable planet's
climate are discussed for both configurations.
