EGU Letters 

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.