Aircraft produce condensation trails, which are thought to increase high-level cloudiness under certain conditions. However the magnitude of such an effect and whether this contributes substantially to the radiative forcing due to the aviation sector remain uncertain. The very substantial, near-global reduction in air traffic in response to the COVID-19 outbreak offers an unprecedented opportunity to identify the anthropogenic contribution to the observed cirrus coverage and thickness. Here we show, using an analysis of satellite observations for the period March-May 2020, that in the 20% of the Northern Hemisphere mid-latitudes with the largest air traffic reduction, cirrus fraction was reduced by ~9 ± 1.5% on average, and cirrus emissivity was reduced by ~2 ±5% relative to what they should have been with normal air traffic. The changes are corroborated by a consistent estimate based on linear trends over the period 2011 – 2019. The change in cirrus translates to a global radiative forcing of 61 ±39 mWm-2. This estimate is somewhat smaller than previous assessments.