  We study the backreaction of superhorizon fluctuations of a light quantum
scalar field on a classical de Sitter geometry by means of the Wilsonian
renormalisation group. This allows us to treat the gravitationally amplified
fluctuations in a nonperturbative manner and to analytically follow the induced
renormalisation flow of the spacetime curvature as long wavelength modes are
progressively integrated out. Unbounded loop corrections in the deep infrared
are eventually screened by nonperturbative effects which stabilise the
geometry.
