  AA Tau, a classical T Tauri star in the Taurus cloud, has been the subject of
intensive photometric monitoring for more than two decades due to its
quasi-cyclic variation in optical brightness. Beginning in 2011, AA Tau showed
another peculiar variation -- its median optical though near-IR flux dimmed
significantly, a drop consistent with a 4-mag increase in visual extinction. It
has stayed in the faint state since.Here we present 4.7um CO rovibrational
spectra of AA Tau over eight epochs, covering an eleven-year time span, that
reveal enhanced 12CO and 13CO absorption features in the $J_{\rm
low}\leqslant$13 transitions after the dimming. These newly appeared
absorptions require molecular gas along the line of sight with T~500 K and a
column density of log (N12CO)~18.5 cm^{-2}, with line centers that show a
constant 6 km s$^{-1}$ redshift. The properties of the molecular gas confirm an
origin in the circumstellar material. We suggest that the dimming and
absorption are caused by gas and dust lifted to large heights by a magnetic
buoyancy instability. This material is now propagating inward, and on reaching
the star within a few years will be observed as an accretion outburst.
