  We present a scaling law that predicts the values of the stresses obtained in
numerical simulations of saturated MRI-driven turbulence in non-stratified
shearing boxes. It relates the turbulent stresses to the strength of the
vertical magnetic field, the sound speed, the vertical size of the box, and the
numerical resolution and predicts accurately the results of 35 numerical
simulations performed for a wide variety of physical conditions. We use our
result to show that the saturated stresses in simulations with zero net
magnetic flux depend linearly on the numerical resolution and would become
negligible if the resolution were set equal to the natural dissipation scale in
astrophysical disks. We conclude that, in order for MRI-driven turbulent
angular momentum transport to be able to account for the large value of the
effective alpha viscosity inferred observationally, the disk must be threaded
by a significant vertical magnetic field and the turbulent magnetic energy must
be in near equipartition with the thermal energy. This result has important
implications for the spectra of accretion disks and their stability.
