  The gluon polarization $\Delta G$ in a nucleon can be defined in a gauge
invariant way as the integral over the Ioffe-time distribution of polarized
gluons. We argue that for sufficiently regular polarized gluon distributions
$\Delta G$ is dominated by contributions from small and moderate values of the
Ioffe-time z < 10. As a consequence $\Delta G$ can be estimated with 20%
accuracy from the first two even moments of the polarized gluon distribution,
and its behavior at small values of Bjorken x or, equivalently, at large
Ioffe-times z. We employ this idea and compute the first two moments of the
polarized gluon distribution within the framework of QCD sum rules. Combined
with the color coherence hypothesis we obtain an upper limit for $\Delta G \sim
2 \pm 0.5$ at a typical scale $\mu^2 \sim 1 GeV^2$.
