  It has been suggested that the accelerated expansion of the Universe is due
to backreaction of small scale density perturbations on the large scale
spacetime geometry. While evidence against this suggestion has accumulated, it
has not yet been definitively ruled out. Many investigations of this issue have
focused on the Buchert formalism, which computes spatial averages of quantities
in synchronous comoving gauge. We argue that, for the deceleration parameter of
this formalism to agree with observations, the spatial average of the three
dimensional Ricci scalar (spatial curvature) must be large today, with an
$\Omega_k$ in the range of $1 \le \Omega_k \le 1.3$. We argue that this
constraint is difficult to reconcile with observations of the location of the
first Doppler peak of the CMBR. We illustrate the argument with a simple toy
model for the effect of backreaction, which we show is generically incompatible
with observations.
