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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 above1. A secondary effect of increasing levels of greenhouse gases is a possible change in the stratospheric circulation2,3, which could significantly affect chlorofluorocarbon lifetimes4, ozone levels5,6 and the climate system more generally7. Model simulations have shown that the mean age of stratospheric air8 is a good indicator of the strength of the residual circulation9, and that this mean age is expected to decrease with rising levels of greenhouse gases in the atmosphere10. 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.

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Figure 1: Vertical profiles of CO2 in the mid-latitude stratosphere.
Figure 2: Vertical profiles of mean age derived from the CO2 data shown in Fig. 3.
Figure 3: Long-term evolution of mean age above 24 km altitude.

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The authors would like to acknowledge financial support from various national and international funding agencies over the past 30 years. University of Frankfurt would in particular like to acknowledge financial support from DFG under the CAWSES priority programme and to CNES for successful balloon operations. The authors also express their gratitude to the Scientific Balloon Center of the Institute of Space and Astronautical Science, JAXA for their cooperation in stratospheric air sampling, and the NASA Upper Atmosphere Research Program for financial support and the US National Scientific Balloon Facility for launch support. This work was partially supported by the Grants-in-Aid for Creative Scientific Research (2005/17GS0203) of the Ministry of Education, Science, Sports and Culture, Japan. We further acknowledge the pioneering work by R. Lueb, D. H. Ehhalt, W. Pollock and L. Heidt (NCAR) in developing and deploying a cryogenic whole air sampler for balloon-borne atmospheric science, and B. Daube for the development and deployment of the in situ CO2 airborne analyser.

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Engel, A., Möbius, T., Bönisch, H. et al. Age of stratospheric air unchanged within uncertainties over the past 30 years. Nature Geosci 2, 28–31 (2009).

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