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In a seminal paper, Parker showed the vertical stratification of the interstellar medium (ISM) is unstable if magnetic fields and cosmic rays provide too large a fraction of pressure support. Cosmic ray acceleration is linked to star formation, so Parker’s instability and its nonlinear outcomes are a type of star formation feedback. Numerical simulations have shown the instability can significantly restructure the ISM, thinning the thermal gas layer and thickening the magnetic field and cosmic ray layer. However, the timescale on which this occurs is rather long (∼0.4 Gyr). Furthermore, the conditions for instability depend on the model adopted for cosmic ray transport. In this work, we connect the instability and feedback problems by examining the effect of a single, spatially and temporally localized cosmic ray injection on the ISM over ∼1 kpc³scales. We perform cosmic ray magnetohydrodynamic simulations using theAthena++code, varying the background properties, dominant cosmic ray transport mechanism, and injection characteristics between our simulation runs. We find robust effects of buoyancy for all transport models, with disruption of the ISM on timescales as short as 100 Myr when the background equilibrium is dominated by cosmic ray pressure.