Age of stratospheric air unchanged within uncertainties over the past 30 years

The rising abundances of greenhouse gases in the atmosphere is associated with an increase in radiative forcing that leads to warming of the troposphere, the lower portion of the Earth's atmosphere, and cooling of the stratosphere above(1). A secondary effect of increasing levels of greenhouse gases is a possible change in the stratospheric circulation(2,3), which could significantly affect chlorofluorocarbon lifetimes(4), ozone levels(5,6) and the climate system more generally(7). Model simulations have shown that the mean age of stratospheric air(8) is a good indicator of the strength of the residual circulation(9), and that this mean age is expected to decrease with rising levels of greenhouse gases in the atmosphere(10). Here we use balloon-borne measurements of stratospheric trace gases over the past 30 years to derive the mean age of air from sulphur hexafluoride (SF6) and CO2 mixing ratios. In contrast to the models, these observations do not show a decrease in mean age with time. If models are to make valid predictions of future stratospheric ozone levels, and of the coupling between ozone and climate change, a correct description of stratospheric transport and possible changes in the transport pathways are necessary.