  Cosmological backreaction has been suggested as an explanation of dark energy
and is heavily disputed since. We combine cosmological perturbation theory with
Buchert's non-perturbative framework, calculate the relevant averaged
observables up to second order in the comoving synchronous gauge, and discuss
their gauge dependence. With the help of an integrability condition the leading
second order contributions follow from the first order calculation.
  We focus on the onset of cosmological backreaction, as a perturbative
treatment is necessarily restricted to the era when the effect is still small.
We demonstrate that the leading contributions to all averaged physical
observables are completely specified on the boundary of the averaged domain.
For any finite domain these surface terms are nonzero in general and thus
backreaction is for real. We map the backreaction effect on an effectively
homogeneous and isotropic fluid. The generic effective equation of state is not
only time dependent, but also depends on scale.
