The paper exploits recent in-situ observations of water vapour in the lower stratosphere in the StratoClim campaign which was focused on the Asian Monsoon region, accepted as very important as providing a pathway from troposphere to stratosphere for important chemical species. The paper uses trajectory modelling to argue that, whilst very large water vapour concentrations were observed in some locations, significant further dehydration is likely as the air masses are transported, over tens of days, on a spiralling path into the main body of the stratosphere. The conclusion is therefore that these large water vapour concentrations do not imply a substantial moistening of the stratosphere. There has been a long-standing debate about the role of processes on different scales, from the cloud-scale to the regional scale, in controlling stratospheric water vapour, which has an important effect on the radiative balance of the troposphere. In combining in-situ data and trajectory modelling this study should help progress towards scientific consensus on this point.

One of the largest sources of uncertainty in the overall anthropogenic forcing of climate is still the aerosol impact on liquid clouds. Disentangling the various aerosol-cloud interactions helps to improve estimates of the magnitude of global warming in the future. The current study provides the most rigorous method to date in assessing the aerosol radiative effects from satellite observations across the global ocean. The aerosol responses are decomposed into the Twomey effect (cooling due to an increase in cloud-droplet number concentration), and the adjustments of the cloud liquid water path and cloud fraction (often analysed separately) at a near-global scale. The total effective radiative forcing of liquid clouds since 1850 has been found to be negative, with the cloud adjustments larger than the Twomey effect, which was previously thought to be larger.

This paper is one of those appearing to celebrate the 20th birthday of ACP. It is a nice informal well-written resume of what has been happening in mesospheric science recently and I am sure that many readers will find it helpful and interesting.

Stratospheric ozone is important for the energy budget of the planetary atmosphere and for protecting life on Earth from harmful solar UV radiation. An unexpected decline in ozone in the extratropical lower stratosphere over the past two decades is therefore worrying and contrary to the Montreal Protocol's goal of ozone recovery. The study by Dube et al. adds important information to the ongoing discussion of this trend, which is being investigated internationally in activities such as SPARC/LOTUS, SPARC/OCTAVE-UTLS or the WMO ozone assessments. By their approach to use N2O as a tracer of stratospheric transport, the authors are able to separate ozone decrease due to circulation changes from a decrease caused by other, not further detailed reasons. They find that the latitudinal and altitudinal region where the ozone decrease cannot be explained by circulation changes is restricted to altitudes below 30hPa in the Northern hemisphere. They suggest a possible cause for the remaining ozone loss, namely a change in the tropopause altitude, thus narrowing successfully the search for processes leading to the observed ozone loss.