Articles | Volume 23, issue 2
https://doi.org/10.5194/acp-23-1677-2023
https://doi.org/10.5194/acp-23-1677-2023
ACP Letters
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01 Feb 2023
ACP Letters | Highlight paper |  | 01 Feb 2023

Natural marine cloud brightening in the Southern Ocean

Gerald G. Mace, Sally Benson, Ruhi Humphries, Peter M. Gombert, and Elizabeth Sterner

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Latest update: 07 Dec 2023
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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.
Short summary
The number of cloud droplets per unit volume is a significantly important property of clouds that controls their reflective properties. Computer models of the Earth's atmosphere and climate have low skill at predicting the reflective properties of Southern Ocean clouds. Here we investigate the properties of those clouds using satellite data and find that the cloud droplet number and cloud albedo in the Southern Ocean are related to the oceanic phytoplankton abundance near Antarctica.
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