  Protoplanetary disks are often assumed to change slowly and smoothly during
planet formation. Here, we investigate the time evolution of isolated disks
subject to viscosity and a disk wind. The viscosity is assumed to increase
rapidly at around 900 K due to thermal ionization of alkali metals, or
thermionic and ion emission from dust, and the onset of magneto-rotational
instability (MRI). The disks generally undergo large, rapid fluctuations for a
wide range of time-averaged mass accretion rates. Fluctuations involve coupled
waves in temperature and surface density that move radially in either direction
through the inner 1.5 AU of the disk. Two types of wave are seen with radial
speeds of roughly 50 and 1000 cm/s respectively. The pattern of waves repeats
with a period of roughly 10,000 years that depends weakly on the average mass
accretion rate. Viscous transport due to MRI is confined to the inner disk.
This region is resupplied by mass flux from the outer disk driven by the disk
wind. Interior to 1 AU, the temperature and surface density can vary by a
factor of 2--10 on timescales of years to ky. The stellar mass accretion rate
varies by 3 orders of magnitude on a similar timescale. This behavior lasts for
at least 1 My for initial disks comparable to the minimum-mass solar nebula.
