  We investigate back reaction in de Sitter space in an approach where only
states that are observationally accessible are included in the density matrix.
Using the Bunch-Davies vacuum as the initial condition we find for a conformal
scalar field and a cosmological constant that tracing over the unobservable
states beyond the cosmological horizon leads to a thermal spectrum of particles
and that such a configuration is unstable under semi-classical back reaction.
It is concluded that this prescription results in an instability of de Sitter
space with a gradually increasing horizon size.
