  We address the issue why a cosmological constant (dark energy) possesses a
small positive value instead of being zero. Motivated by the cosmic landscape
picture, we mimic the dark energy by a scalar field with potential wells and
show that other degrees of freedom interacting with it can localize this field
by decoherence in one of the wells. Dark energy can then acquire a small
positive value. We also show that the additional degrees of freedom enhance the
tunneling rate between the wells. The consideration is performed in detail for
the case of two wells and then extended to a large number of wells.
