  We investigate the effect that the average backreaction of structure
formation has on the dynamics of the cosmological expansion, within the
concordance model. Our approach in the Poisson gauge is fully consistent up to
second-order in a perturbative expansion about a flat Friedmann background,
including a cosmological constant. We discuss the key length scales which are
inherent in any averaging procedure of this kind. We identify an intrinsic
homogeneity scale that arises from the averaging procedure, beyond which a
residual offset remains in the expansion rate and deceleration parameter. In
the case of the deceleration parameter, this can lead to a quite large increase
in the value - more than 10% - and may therefore have important ramifications
for dark energy measurements, even if the underlying nature of dark energy is a
cosmological constant. We give the intrinsic variance that affects the value of
the effective Hubble rate and deceleration parameter. These considerations
serve to add extra intrinsic errors to our determination of the cosmological
parameters, and, in particular, may render attempts to measure the Hubble
constant to percent precision overly optimistic.
