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. In this paper, we present the global fingerprint of recent changes inmiddle–upper stratosphere (MUSt;  hPa) ozone ( O3 ) incomparison with lower stratosphere (LSt; 150–25 hPa) O3 derivedfrom the first 10 years of the IASI/Metop-A satellite measurements (January 2008–December 2017). The IASI instrument provides vertically resolved O3 profiles with very high spatial and temporal (twice daily)samplings, allowing O3 changes to be monitored in these two regions of thestratosphere. By applying multivariate regression models with adaptedgeophysical proxies on daily mean O3 time series, we discriminateanthropogenic trends from various modes of natural variability, such as theEl Nino–Southern Oscillation (ENSO). The representativeness of the O3 response to its natural drivers is first examined. One importantfinding relies on a pronounced contrast between a positive LSt O3 response to ENSO in the extratropics and a negative one in the tropics,with a delay of 3 months, which supports a stratospheric pathway for theENSO influence on lower stratospheric and tropospheric O3 . In terms oftrends, we find an unequivocal O3 recovery from the available period ofmeasurements in winter–spring at middle to high latitudes for the twostratospheric layers sounded by IASI ( > ∼ 35 ∘  N–S in the MUSt and > ∼ 45 ∘  S in the LSt) as well as in the total columns at southern latitudes( > ∼ 45 ∘  S) where the increase reachesits maximum. These results confirm the effectiveness of the MontrealProtocol and its amendments and represent the first detection of asignificant recovery of O3 concurrently in the lower, in the middle–upperstratosphere and in the total column from one single satellite dataset. Asignificant decline in O3 at northern mid-latitudes in the LSt is alsodetected, especially in winter–spring of the Northern Hemisphere. Givencounteracting trends in the LSt and MUSt at these latitudes, the decline is notcategorical in total O3 . When freezing the regression coefficientsdetermined for each natural driver over the whole IASI period but adjustinga trend, we calculate a significant speeding up in the O3 response tothe decline of O3 -depleting substances (ODSs) in the total column, inthe LSt and, to a lesser extent, in the MUSt, at high southern latitudesover the year. Results also show a small significant acceleration of the O3 decline at northern mid-latitudes in the LSt and in the total columnover the last few years. That, specifically, needs urgent investigation toidentify its exact origin and apprehend its impact on climate change.Additional years of IASI measurements would, however, be required to confirmthe O3 change rates observed in the stratospheric layers over the last fewyears.

atmospheric chemistry and physics

Is the recovery of stratospheric O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; speeding up in the Southern Hemisphere? An evaluation from the first IASI decadal record (2008–2017)