Century-long column ozone records show that chemical and dynamical influences counteract each other

The recovery of the ozone layer, which is expected as stratospheric chlorine levels have decreased over the past 25 years, remains difficult to detect. Column ozone has been monitored from 1924 to 1975 in Oxford, UK. Here, I present a century-long Oxford column ozone record, extended to the present based on re-discovered material and neighbouring series, and analyse it together with a record from Arosa, Switzerland that starts in 1926. Neither series shows a clear increase over the past 25 years but suggest stratospheric circulation. I separate chemical and dynamical effects using a regression approach and find that chemical recovery amounts to +8 DU between peak stratospheric chlorine concentrations in 1997 to 2021, consistent with model simulations. However, this trend is counteracted by a -5 DU dynamical trend. Century-long ozone records provide a window to the past dynamical, chemical and radiative state of the stratosphere and help better constrain circulation effects on ozone recovery. Analyses of a newly constructed record of column ozone together with an existing record suggest that two thirds of the ozone recovery due to the reduction of ozone depleting substances over the past 25 years is compensated by a dynamically-caused ozone loss.

Automated summary

The recovery of the ozone layer over the past 25 years is difficult to detect, but there are indications of changes in stratospheric circulation. Chemical recovery is counteracted by dynamical effects.