  Protoplanetary disks are likely to be threaded by a weak net flux of vertical
magnetic field that is a remnant of the much larger fluxes present in molecular
cloud cores. If this flux is approximately conserved its dynamical importance
will increase as mass is accreted, initially by stimulating magnetorotational
disk turbulence and subsequently by enabling wind angular momentum loss. We use
fits to numerical simulations of ambipolar dominated disk turbulence to
construct simplified one dimensional evolution models for weakly magnetized
protoplanetary disks. We show that the late onset of significant angular
momentum loss in a wind can give rise to "two timescale" disk evolution in
which a long phase of viscous evolution precedes rapid dispersal as the wind
becomes dominant. The wide dispersion in disk lifetimes could therefore be due
to varying initial levels of net flux. Magnetohydrodynamic (MHD) wind triggered
dispersal differs from photoevaporative dispersal in predicting mass loss from
small (less that 1 AU) scales, where thermal winds are suppressed. Our specific
models are based on a limited set of simulations that remain uncertain, but
qualitatively similar evolution appears likely if mass is lost from disks more
quickly than flux, and if MHD winds become important as the plasma beta
decreases.
