Abstract Condensation trails (contrails) that evolve into contrail cirrus may constitute the largest contribution to aviation’s climate impact. However the strength and timescales of the adjustments of the upper atmospheric water budget to contrail formation are poorly known. Thanks to multiple simulations with the ICOLMDZ climate model, the relatively small impact of a large pulse perturbation to ice cloudiness becomes statistically significant. That reveals two distinct global adjustment processes, driven by an initial loss of water to the upper atmosphere by ice crystal sedimentation with a time constant of 5–6 hr, followed by a rehumidification with a time constant of 20 hr. The atmosphere returns to its unperturbed state after about 4 days. Parametric uncertainty analysis suggests that a stronger sedimentation results in stronger adjustments and a smaller ERF‐to‐RF ratio. Observational constraints on contrail cirrus adjustment timescales would inform model development and reduce uncertainties in the climate impact of contrails.