Observing stratospheric residence time from opposing transport timescales

While the mean age-of-air – the time from entry into the stratosphere to any interior point – can be derived from trace gas observations, the mean residence time – the time from an interior point to its exit – is constrained only through rare events such as volcanic eruptions. Here we show that age-of-air and residence time are not independent but obey a compensation rule: their opposing latitudinal gradients cancel to produce near-uniform mean total transit times at each altitude. This uniformity reveals a previously unrecognised constraint within the Brewer–Dobson circulation, where rapid tropical ascent is necessarily balanced by prolonged interior residence, and vice versa. Exploiting this constraint, we infer global residence time fields directly from age-of-air observations and reproduce the observed residence time of the 2022 Hunga Tonga water vapour plume within published uncertainty ranges. Our framework transforms age-of-air – routinely measured by existing satellite networks - into a continuous observational constraint on stratospheric residence time. This opens a path to monitor whether the acceleration of stratospheric circulation under climate change shortens or prolongs the persistence of high-altitude emissions.