  Motivated by the evolution of a population in a slowly varying random
environment, we consider the 1D Anderson model on finite volume, with viscosity
$ \kappa > 0 $: $$ \partial_{t} u(t,x) = \kappa \Delta u(t,x) + \xi(t, x)
u(t,x), \quad u(0, x) = u_{0}(x), \qquad t > 0, x \in \mathbb{T}. $$ The noise
$ \xi $ is chosen constant on time intervals of length $ \tau >0 $ and sampled
independently after a time $ \tau $. We prove that the Lyapunov exponent $
\lambda (\tau) $ is positive and near $ \tau= 0 $ follows a power law that
depends on the regularity on the driving noise. As $ \tau \to \infty $ the
Lyapunov exponent converges to the average top eigenvalue of the associated
time-independent Anderson model. The proofs make use of a solid control of the
projective component of the solution and build on the Furstenberg--Khasminskii
and Bou\'e--Dupuis formulas, as well as on Doob's H-transform and on tools from
singular stochastic PDEs.
