Using satellite measurements and computer models, they studied the clouds over the Southern Ocean, one of the world's cloudiest places.
They discovered that during the summertime, bacteria and plankton in the oceans get whipped up into the atmosphere by the wind and waves.
"The return of light in the summer ignites an amazing flurry of activity in phytoplankton," said atmospheric scientist Daniel McCoy of the University of Washington, who teamed up with Susannah Burrows from the Pacific Northwest National Laboratory for the study.
In the atmosphere, they stimulate cloud formation, emitting sulphur compounds and larger organic molecules in the process. With the increased cloud cover during the summer, the clouds over the Southern Ocean can reflect up to 10 watts per square metre, which helps to keep the temperatures down, and slows down the melting of the Antarctic ice sheets.
Likewise, if there were no plankton in the sea during the summer, there would be only half as many cloud droplets and the clouds would become more permeable.
Prior to the research, it was believed that sea salt –generated by sea spray and functioning as the condensation nuclei – was the most important factor in the formation of water droplets.
Link to the study: advances.sciencemag.org/content/1/6/e1500157