  We prove the existence of Kolmogorov-Petrovsky-Piskunov (KPP) type traveling
fronts in space-time periodic and mean zero incompressible advection, and
establish a variational (minimization) formula for the minimal speeds. We
approach the existence by considering limit of a sequence of front solutions to
a regularized traveling front equation where the nonlinearity is combustion
type with ignition cut-off. The limiting front equation is degenerate parabolic
and does not permit strong solutions, however, the necessary compactness
follows from monotonicity of fronts and degenerate regularity. We apply a
dynamic argument to justify that the constructed KPP traveling fronts propagate
at minimal speeds, and derive the speed variational formula. The dynamic method
avoids the degeneracy in traveling front equations, and utilizes the parabolic
maximum principle of the governing reaction-diffusion-advection equation. The
dynamic method does not rely on existence of traveling fronts.
