Letters 

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.

Intensifying wildfire regimes in many parts of the world are increasing the production of pyrogenic organic matter (PyOM), with potential implications for water supplies that are critical for domestic, industrial, agricultural, and ecological needs. This study provides a novel assessment of the influence of PyOM on aquatic ecosystems and showed that PyOM can be actively transformed in aquatic ecosystems and may be an increasing source of C emissions to the atmosphere as the prevalence of wildfires increases.

In 2020, a new international law was imposed that placed strong restrictions on sulfur emissions from the international shipping industry. In addition to reducing air pollution, an anticipated side effect was reduction of the climate cooling effect that is often associated with "ship-tracks". Aerosol pollutant emissions from ships, when they rise into overlying clouds, lead to higher cloud droplet number concentrations, smaller cloud droplet sizes, and clouds that are more reflective to incoming sunlight, easily seen in satellite imagery as long bright lines downwind of ships. Past studies into whether the new law has led to darker clouds have been equivocal. For this study, the authors used sophisticated statistical techniques to compare cloud droplet size and reflectivity before and after the law was implemented focusing on a shipping corridor in the southeast Atlantic. They found strong evidence that droplet sizes have indeed increased, and that clouds have darkened with a significant local climate warming. Globally, the impact is much smaller, but may still represent an important consideration for assessments of the total summed effect of aerosols on climate.

Relative absence of something is often more difficult to quantify than its presence. This paper uses satellite observations and global model data to show that it is uncertainties in the quantification of clean-sky conditions that contribute most to current uncertainties in estimates of the sensitivity of cloud properties to varying degrees of aerosol loading. This is a novel result that should prove of broad interest to the aerosol-cloud-climate community for reframing how the problem is approached, by focusing on improved quantification of clean conditions.

Application for BG Letter format accepted.

This laboratory and protein database research by Fabisik and co-workers demonstrates that the widespread freshwater cyanobacterium Microcystis aeruginosa is capable of producing nitrous oxide, a very potent greenhouse gas, from nitrite. This discovery poses the important question about the relevance of these organisms for global nitrous oxide emissions into the atmosphere and their response to environmental change.

The Southern Ocean can be considered a region exhibiting pristine conditions as during the pre-industrial time. Thus, any changes in radiative forcing in this region can be attributed to natural factors. Feedbacks of ocean biological activity on Earth’s radiation budget have been put forward as the CLAW hypothesis (Charlson et al., 1987, https://www.nature.com/articles/326655a0). It implies that emissions of biogenic sulfur-containing compounds result in the formation of cloud condensation nuclei, which lead to higher cloud droplet number concentrations. Such clouds are more reflective and thus lead to a cooling effect. The current study provides satellite-based evidence of the increase droplet number concentrations and cloud reflectivity (‘albedo’) triggered by chlorophyll emissions, as a proxy for biological activity. Specifically, it demonstrates for the first time the extent to which the cloud albedo is modulated by biological factors as a function of latitude along the Antarctic shelf. While the study does not extend to discussing the subsequent feedbacks of cloud reflectivity to biological activity, it clearly demonstrates how biological ocean activity affects cloudiness above the Southern Ocean and thus may regulate temperature.

Tropical cyclones are typically assumed to be associated with ecosystem damage. This study challenges this assumption and suggests that instead of reducing plant biomass, cyclones in East Asia may increase biomass production by alleviating water stress.

Traditionally, the interactions between aerosols, clouds, weather and climate has focused on sulphate aerosols. However, in the last 20 years it has become apparent that secondary organic aerosols are also highly abundant in the troposphere. These could represent a major coupling in the earth system between the biosphere and the atmosphere, and thus climate because forests are known to emit large quantities of biogenic VOCs that are known to produce secondary organic aerosols. However selectively studying their influence on the free troposphere is difficult as it requires in situ measurements aboard scientific aircraft. This study observes the role of biogenic secondary organic aerosols on the abundance of cloud condensation nuclei in the upper troposphere above the Amazon, and compares it with a state-of-the-art predictive model. This further supports the importance of these processes in earth system models and gives confidence that the current level of understanding will produce accurate predictions.

Organic aerosol remain one of the more complex and hard to predict when studying atmospheric aerosols and their influences on air quality, meteorology and climate. Among its many complexities is the phase and viscosity of the organic matter, which dictates how it interacts with other particulate components and the gas phase, in turn affecting growth rates and cloud activation. There have been a number of previous works studying phase separation, where the organic matter becomes immiscible with an aqueous component (containing inorganic salts), but this new letter presents compelling visual evidence that different organic phases are also capable of separation. Different secondary organic aerosol (SOA) mixtures were created and some mixtures exhibited separation, with a factor being the oxygen-to-carbon ratio of the material, likely a surrogate for polarity. If this behaviour is found to be important in atmospheric aerosols this represents a new direction in how these may need to be represented in models.

The authors adopted a new and attractive approach, based on the use of thermal springs appearing at different times, to study the short-term and long-term (> 50 years) effect of warming on the soil C stock under subarctic grasslands. This new approach allows to take a new look at the question of a positive feedback between temperature and soils that can amplify global warming. Indeed, most studies on this subject are based on warming experiments conducted over the short term (some years) or on questionable correlative approaches where the temperature co-varies with many other factors (e.g., study of soil C stocks along latitudinal temperature gradients). Their study challenges the current dominant view on the effect of warming on the dynamics of SOM. Indeed, results suggest that soil C losses in the subarctic grasslands studied cease after 5 years of warming. These observations corroborate those obtained in the rare ecosystem warming experiments maintained beyond 10 years. In addition, results suggest that the C stocks present in the deep soil horizons, where plant roots are not or hardly present, are not affected by warming. These unexpected discoveries, together with other recent observations, show the glaring lack of knowledge on the fundamental mechanisms of the effect of temperature on catalytic processes, which seriously compromises our ability to predict the soil-climate feedback.