Marine organisms are responsible for nearly doubling the number of droplets in summer clouds, reflecting light back into space and cooling the planet.

While clouds are formed from water vapor, they usually also require airborne particles, or aerosols, for the water to condense onto. These can come from many sources, one of which is marine plankton. On land, fine particles of dust can become aerosols, as can pollution and particles from volcanoes or forests. Increased aerosol concentrations tend to lead to smaller droplets and brighter clouds.

The Department of Energy's Susannah Burrows is the author of the first paper to quantify the varying contributions of marine organisms to cloud formation between 35° and 55° south. The study, published in Science Advances, found that marine plankton make a bigger contribution in this region than previously recognized.

“It makes sense because most of the area down there is ocean, with strong winds that kick up a lot of spray and lots of marine microorganisms producing these particles,” said Burrows. “And continental aerosol sources are mostly so far away that they only have a limited impact. Really the marine aerosols are running the show there."

Weather in the Earth’s most remote oceans may not be front of mind, but the Southern Ocean's clouds matter a lot. The region occupies a large part of the planet, and is one of the cloudiest places on Earth, so a lot of sunlight is reflected back into space over these distant seas, particularly in summer. Without these clouds, the world would be considerably hotter.

The study of the Southern Ocean’s aerosols also provides us with a baseline of how things would be without human interference. Elsewhere on the planet, our own contribution may swamp those that occur naturally.

On the high seas, salt left behind once water droplets evaporate form one of the major aerosols. Others are sulfates and organic particles, both coming from plankton. Efforts to measure the relative frequency of these have struggled in the past.

^{Aerosol sources over the ocean. Credit: Daniel McCoy}

The authors combined computer modeling of the three main marine aerosol types with satellite measurements of cloud droplets using information of ocean chlorophyll to predict when biological aerosols would be at a maximum.

Sea salt is the most common ocean aerosol, they concluded, and the most consistent, being fairly even across the seasons. However, in summer when the clouds are most important, the sulfates took off.

Sulfates were more important from 35° to 45°S, while further south organic matter dominates. Their combined summer influence is as large as that of pollution “over heavily polluted regions of the Northern Hemisphere,” the paper concludes.

"The return of light in the summer ignites an amazing flurry of activity in phytoplankton communities across the Southern Ocean. This seasonality leads to an enhancement in cloud brightness when it will be able to reflect the most sunlight," said first author Daniel McCoy of the University of Washington.

Consequently, almost half of the summer cloud droplets center on plankton-produced aerosols. Human activities can influence plankton concentrations, even over such vast expanses, since we now know that whaling actually reduces plankton concentrations.

^{Both the clouds and the greenish glow may be the result of phytoplankton in the ocean. Credit: Susannah Burrows}