The abundance of chlorine in the Earth’s atmosphere increased considerably during the 1970s to 1990s, following large emissions of anthropogenic long-lived chlorine-containing source gases, notably the chlorofluorocarbons. The chemical inertness of chlorofluorocarbons allows their transport and mixing throughout the troposphere on a global scale1, before they reach the stratosphere where they release chlorine atoms that cause ozone depletion2. The large ozone loss over Antarctica3 was the key observation that stimulated the definition and signing in 1987 of the Montreal Protocol, an international treaty establishing a schedule to reduce the production of the major chlorine- and bromine-containing halocarbons. Owing to its implementation, the near-surface total chlorine concentration showed a maximum in 1993, followed by a decrease of half a per cent to one per cent per year4, in line with expectations. Remote-sensing data have revealed a peak in stratospheric chlorine after 19965, then a decrease of close to one per cent per year6,7, in agreement with the surface observations of the chlorine source gases and model calculations7. Here we present ground-based and satellite data that show a recent and significant increase, at the 2σ level, in hydrogen chloride (HCl), the main stratospheric chlorine reservoir, starting around 2007 in the lower stratosphere of the Northern Hemisphere, in contrast with the ongoing monotonic decrease of near-surface source gases. Using model simulations, we attribute this trend anomaly to a slowdown in the Northern Hemisphere atmospheric circulation, occurring over several consecutive years, transporting more aged air to the lower stratosphere, and characterized by a larger relative conversion of source gases to HCl. This short-term dynamical variability will also affect other stratospheric tracers and needs to be accounted for when studying the evolution of the stratospheric ozone layer.

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The University of Liège contribution was mainly supported by the Belgian Science Policy Office (BELSPO) and the Fonds de la Recherche Scientifique–FNRS, both in Brussels. Additional support was provided by MeteoSwiss (Global Atmospheric Watch) and the Fédération Wallonie–Bruxelles. We thank the International Foundation High Altitude Research Stations Jungfraujoch and Gornergrat (HFSJG, Bern). We thank O. Flock and D. Zander (University of Liège). The SLIMCAT modelling work was supported by the UK Natural Environment Research Council (NCAS and NCEO). The FTIR measurements at Ny-Ålesund, Spitsbergen, are supported by the AWI Bremerhaven. The work from Hampton University was partially funded under the NASA MEASURE’s GOZCARDS programme and the National Oceanic and Atmospheric Administration’s Educational Partnership Program Cooperative Remote Sensing Science and Technology Center (NOAA EPP CREST). The ACE mission is supported primarily by the Canadian Space Agency. We thank U. Raffalski and P. Voelger for technical support at IRF Kiruna. The National Center for Atmospheric Research is supported by the National Science Foundation. The observation programme at Thule, Greenland, is supported under contract by the National Aeronautics and Space Administration (NASA) and the site is also supported by the NSF Office of Polar Programs. We thank the Danish Meteorological Institute for support at Thule. Work at the Jet Propulsion Laboratory, California Institute of Technology, was performed under contract with NASA; we thank R. Fuller for help in producing the GOZCARDS data set, and work by many ACE-FTS, HALOE and MLS team members who helped to produce data towards the GOZCARDS data set is also acknowledged. We thank O. E. García, E. Sepúlveda, and the State Meteorological Agency (AEMET) of Spain for scientific and technical support at Izana. The Australian Research Council has provided notable support over the years for the NDACC site at Wollongong, most recently as part of project DP110101948. Measurements at Lauder are core funded through New Zealand’s Ministry of Business, Innovation and Employment. We are grateful to all colleagues who have contributed to FTIR data acquisition. We thank ECMWF for providing the ERA-Interim reanalyses.

Author information


  1. Institute of Astrophysics and Geophysics, University of Liège, Liège 4000, Belgium

    • E. Mahieu
    • , B. Franco
    •  & C. Servais
  2. National Centre for Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK

    • M. P. Chipperfield
    • , S. S. Dhomse
    • , W. Feng
    •  & R. Hossaini
  3. Department of Physics, University of Bremen, Bremen 28334, Germany

    • J. Notholt
    •  & M. Palm
  4. Karlsruhe Institute of Technology (KIT), Institute for Meteorology and Climate Research (IMK-ASF), Karlsruhe 76021, Germany

    • T. Reddmann
    • , T. Blumenstock
    • , F. Hase
    •  & M. Schneider
  5. Department of Atmospheric and Planetary Science, Hampton University, Hampton, Virginia 23668, USA

    • J. Anderson
    •  & J. M. Russell III
  6. Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, Virginia 23529, USA

    • P. F. Bernath
  7. Department of Chemistry, University of York, York YO10 5DD, UK

    • P. F. Bernath
  8. Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

    • P. F. Bernath
    •  & K. A. Walker
  9. National Center for Atmospheric Research, Boulder, Colorado 80307, USA

    • M. T. Coffey
    •  & J. W. Hannigan
  10. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA

    • L. Froidevaux
  11. School of Chemistry, University of Wollongong, Wollongong, New South Wales 2522, Australia

    • D. W. T. Griffith
    • , N. B. Jones
    •  & C. Paton-Walsh
  12. National Institute for Environmental Studies (NIES), Tsukuba, Ibaraki 305-8506, Japan

    • I. Morino
    •  & H. Nakajima
  13. Graduate School of Environmental Studies, Tohoku University, Sendai 980-8578, Japan

    • I. Murata
  14. National Institute of Water and Atmospheric Research (NIWA), Lauder 9352, New Zealand

    • D. Smale
  15. Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada

    • K. A. Walker


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M.P., J.W.H., F.H., E.M., I. Murata, N.B.J., C.P.-W. and D.S. performed the Ny-Ålesund, Thule, Kiruna and Izana, Jungfraujoch, Tsukuba, Wollongong and Lauder retrievals for HCl, respectively. P.F.B. and K.A.W. provided ACE-FTS data; L.F. and J.A. provided the GOZCARDS data set. J.A., P.F.B., L.F., J.M.R. III and K.A.W. provided expertise on satellite data usage. M.P.C., R.H., S.S.D. and W.F. designed and performed the SLIMCAT runs, sensitivity analyses and transport diagnostics. T.R. performed the KASIMA model run and corresponding diagnostics. B.F. and E.M. performed the trend analyses and compiled the results. J.N., M.T.C., T.B., C.S., I. Morino, H.N., M.S., D.W.T.G. and D.S. are responsible for the instrumentation and data acquisition at the NDACC stations. E.M. initiated and coordinated the study. The figures were prepared by E.M. and B.F. (Fig. 1), E.M. (Fig. 2), R.H. and M.P.C. (Fig. 3) and T.R. (Fig. 4). E.M., M.P.C. and J.N. wrote the manuscript. Together with T.R., they revised it and included the comments from the co-authors.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to E. Mahieu.

NDACC data are publicly available at ftp://ftp.cpc.ncep.noaa.gov/ndacc/station/ and GOZCARDS data are publicly available at http://measures.gsfc.nasa.gov/opendap/GOZCARDS/.

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