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.

Wildfires enhance surface PM2.5 concentration and thus adversely affect air quality and human health. Based on online-coupled fire-climate-ecosystem model simulations, this paper projects a nearly 2-fold increase in wildfire-induced summer-mean surface PM2.5 by the mid-21st century over North America. The projected enhancement is substantial even in the less-densely forested eastern US - as already manifested in the poor air quality as observed in June 2023 due to Canadian wildfires -, which is attributed to the transport of smoke from North America as well as positive climatic feedback of smoke on PM2.5. Thus, future regulatory controls on PM2.5 in North America, particularly in the eastern US where anthropogenic emissions are falling, should consider the effect of future increases in wildfires.