Observations of stratospheric ozone from multiple
instruments now span three decades; combining these into
composite datasets allows long-term ozone trends to be estimated.
Recently, several ozone composites have been published,
but trends disagree by latitude and altitude, even between
composites built upon the same instrument data. We
confirm that the main causes of differences in decadal trend
estimates lie in (i) steps in the composite time series when the
instrument source data changes and (ii) artificial sub-decadal
trends in the underlying instrument data. These artefacts introduce
features that can alias with regressors in multiple linear
regression (MLR) analysis; both can lead to inaccurate
trend estimates. Here, we aim to remove these artefacts using
Bayesian methods to infer the underlying ozone time series
from a set of composites by building a joint-likelihood
function using a Gaussian-mixture density to model outliers
introduced by data artefacts, together with a data-driven prior
on ozone variability that incorporates knowledge of problems
during instrument operation. We apply this Bayesian
self-calibration approach to stratospheric ozone in 10◦ bands
from 60◦ S to 60◦ N and from 46 to 1 hPa (∼ 21–48 km) for
1985–2012. There are two main outcomes: (i) we independently
identify and confirm many of the data problems previously
identified, but which remain unaccounted for in existing
composites; (ii) we construct an ozone composite, with
uncertainties, that is free from most of these problems – we
call this the BAyeSian Integrated and Consolidated (BASIC)
composite. To analyse the new BASIC composite, we use
dynamical linear modelling (DLM), which provides a more
robust estimate of long-term changes through Bayesian inference
than MLR. BASIC and DLM, together, provide a
step forward in improving estimates of decadal trends. Our
results indicate a significant recovery of ozone since 1998 in
the upper stratosphere, of both northern and southern midlatitudes,
in all four composites analysed, and particularly in the
BASIC composite. The BASIC results also show no hemispheric
difference in the recovery at midlatitudes, in contrast
to an apparent feature that is present, but not consistent, in
the four composites. Our overall conclusion is that it is possible
to effectively combine different ozone composites and
account for artefacts and drifts, and that this leads to a clear
and significant result that upper stratospheric ozone levels
have increased since 1998, following an earlier decline.