  We present a model that naturally tunes a large positive cosmological
constant to a small cosmological constant. A slowly rolling scalar field
decreases the cosmological constant to a small negative value, causing the
universe to contract, thus reheating it. An expanding universe with a small
positive cosmological constant can be obtained, respectively, by coupling this
solution to any model of a cosmological bounce and coupling the scalar field to
a sector that undergoes a technically natural phase transition at the meV
scale. A robust prediction of this model is a rolling scalar field today with
some coupling to the standard model. This can potentially be experimentally
probed in a variety of cosmological and terrestrial experiments, such as probes
of the equation of state of dark energy, birefringence in the cosmic microwave
background and terrestrial tests of Lorentz violation.
