  Discs that contain dead zones are subject to the Gravo-Magneto (GM)
instability that arises when the turbulence shifts from gravitational to
magnetic. We have previously described this instability through a local
analysis at some radius in the disc in terms of a limit cycle. A disc may be
locally unstable over a radial interval. In this paper, we consider how the
local instability model can describe global disc outbursts. The outburst is
triggered near the middle of the range of locally unstable radii. The sudden
increase in turbulence within high surface density material causes a snow
plough of density that propagates both inwards and outwards. All radii inside
of the trigger radius become unstable, as well as locally unstable radii
outside the trigger radius. In addition, a locally stable region outside of the
trigger radius may also become unstable as the gravitational instability is
enhanced by the snow plough. For the circumstellar disc model we consider, we
find that a quarter of the disc mass is accreted on to the central object
during the outburst. The radius out to which the disc is globally unstable is
twice that for which it is locally unstable.
