After sunlight completes its eight-minute journey to Earth, white surfaces, like clouds, mostly bounce back into space. The whiter and brighter the cloud, the better it reflects sunlight and keeps the earth cool. Now a study has been published in Atmospheric Chemistry and Physics examines a surprising part of this process: how tiny aquatic creatures known as phytoplankton play a major role in whitening the Earth’s clouds.
Scientists used satellites to monitor the skies over a large swath of the Southern Ocean for five years. They found that clouds that form south of latitude 60 degrees north, i.e. closer to Antarctica, tend to be significantly whiter than clouds further north.
The reason? Tiny phytoplankton from the ocean, says University of Utah atmospheric scientist Gerald Mace, the study’s lead author. Antarctica is “a very productive region” where tiny creatures like phytoplankton proliferate more than in seas further north, he says. As part of their metabolism, many of these sunlight-consuming organisms release a compound called dimethyl sulfide, which rises and reacts with gases in the atmosphere to form tiny aerosol particles — and eventually clouds.
Water vapor normally must first bind to a “seed” particle to condense into cloud droplets, says biogeochemist Meinrat Andreae of the Max Planck Institute for Chemistry, who was one of the first to study the cloud-seeding abilities of phytoplankton, but not involved in the new study. South of 60 degrees latitude, abundant phytoplankton generate many dimethyl sulfide seeds, so clouds that form there are full of tiny water droplets.
In more northerly areas, cloud-forming seeds are less common — “usually just salt particles dragged along by sea spray,” says University of Utah atmospheric scientist and co-author Sally Benson. The resulting clouds have fewer and larger droplets, giving them less surface area to reflect sunlight back than the many small droplets in the southern clouds, the researchers say.
The study also found that phytoplankton populations, measured by satellite images of the green compound chlorophyll in the water, peak each summer, soon followed by peaks in cloud reflectance. Mace notes that the role of phytoplankton in this process is a global phenomenon, but the effect is most apparent in the Southern Ocean with its abundant plankton population and low human influence.
While the involvement of phytoplankton in cloud bleaching has been known for some time, Andreae says existing climate models still lack sufficient data to fully explain its effects. He adds that a study on this scale – monitoring a large ocean area over five years – helps to elucidate new patterns, such as the strong relationship between cloud whiteness and latitude. “With a study of this magnitude,” he says, “we can definitely put better information into our models.”