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Gas in protostellar disks provides the raw material for giant planetformation and controls the dynamics of the planetesimal-building dust grains.Accurate gas mass measurements help map the observed properties ofplanet-forming disks onto the formation environments of known exoplanets. Rareisotopologues of carbon monoxide (CO) have been used as gas mass tracers fordisks in the Lupus star-forming region, with an assumed interstellar CO/H$_2$abundance ratio. Unfortunately, observations of T-Tauri disks show that COabundance is not interstellar---a finding reproduced by models that show COabundance decreasing both with distance from the star and as a function oftime. Here we present radiative transfer simulations that assess the accuracyof CO-based disk mass measurements. We find that the combination of CO chemicaldepletion in the outer disk and optically thick emission from the inner diskleads observers to underestimate gas mass by more than an order of magnitude ifthey use the standard assumptions of interstellar CO/H$_2$ ratio and opticallythin emission. Furthermore, CO abundance changes on million-year timescales,introducing an age/mass degeneracy into observations. To reach factor of a fewaccuracy for CO-based disk mass measurements, we suggest that observers andmodelers adopt the following strategies: (1) select the low-$J$ transitions;(2) observe multiple CO isotopologues and use either intensity ratios ornormalized line profiles to diagnose CO chemical depletion; and (3) usespatially resolved observations to measure the CO abundance distribution.

Disk masses around solar-mass stars are underestimated by CO observations