EGU Letters 

This manuscript is published as SOIL Letters publication as it provides a timely and important contribution, formulating six fundamental principles for the robust study of soil carbon saturation. This is an important topic in research on soil carbon and its role in climate change mitigation which should be of interest to the wider geoscience community.

Ecosystems are, in many ways, changing more rapidly than they have in the past. Rapid past ecological changes can provide critical insight into how to best understand ecosystem function to improve ecological restoration, but a historic focus on community composition in the paleoecological literature can obscure the causes of these changes, making mechanisms unclear. The authors demonstrate a path forward using pollen and diatom records from a lake in the Italian Alps to construct a narrative of terrestrial and aquatic ecosystem changes that combines well-established vegetation changes with rapid changes to water management changes to explore how rapidly the ecosystem responds to perturbations, and how aquatic function has been restored after pollution events. Doing so provides new insight into how to use the paleoecological record to understand restoration success and the time scales upon which ecosystem changes occur.

There is a very strong contrast between water vapor concentrations in the stratosphere (dry) and the troposphere (moist). Climate models typically represent this contrast poorly and suffer from a ‘moist bias’ in the extratropical lower stratosphere. Here two versions of a particular model, one a standard version and the other with a different transport scheme which greatly reduces the moist bias, are used to give a clear demonstration of its effect, through the radiative effects of water vapor, on the regional-scale tropospheric circulation in the Northern Hemisphere Pacific region. The authors then show that differences in moist bias explain differences in this circulation across a large set of climate models. Improvements in transport schemes and hence better representation of the troposphere-stratosphere contrast in water vapor are likely to improve important regional scale features of the tropospheric circulation.

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.

Ship tracks are enhanced regions of cloud brightness that trail behind ships that are known to be created by their effluent. They are widely used as observational test beds to deepen understanding of how pollution might affect cloud microphysical processes and climate at larger regional and global scales.  This study uses satellite observations of cloud properties and records of ship position and their effluent to assess perturbations from ship emissions on cloud droplet number and liquid water path. The study found that, as expected, the droplet number perturbation in shiptracks scales with ship emission rates of aerosol particles. Surprisingly, however, the liquid water path in drizzling clouds increased by an amount that was nearly fixed. The observation points to novel non-linear threshold behaviours of relevance to representations of aerosol indirect effects in climate models.