Line-shaped contrails arising from aircraft emissions affect radiative forcing. The magnitude of the radiative forcing from contrails depends strongly on their optical depth and their spatial and temporal variability caused by dynamical and microphysical processes. Here we investigate the significance of this variability, both for modelling contrail radiative forcing and estimating thresholds for the detection of contrails in satellite imagery. Ignoring the variability of contrail optical depth in models by prescribing a mean optical depth may overestimate mean net radiative forcing by 10−20%. Undersampling of optically thin line-shaped contrails by passive satellite remote sensing is linked to the inability to detect flux changes in the outgoing long-wave radiation below ≈3 W m−2 for conditions over the eastern North Pacific. Consideration of these findings aids efforts to better quantify uncertainties in aviation climate assessments.