Southern Ocean research shows how plankton emissions brighten clouds
FRISCO — Swarms of tiny plankton may play a bigger role in cloud formation than previously realized, scientists said after studying the Southern Ocean.
The new research shows that plankton produce airborne gases and organic matter to seed cloud droplets, which lead to brighter clouds that reflect more sunlight.
“The clouds over the Southern Ocean reflect significantly more sunlight in the summertime than they would without these huge plankton blooms,” said Daniel McCoy, a University of Washington doctoral student in atmospheric sciences. “In the summer, we get about double the concentration of cloud droplets as we would if it were a biologically dead ocean.”
To try and understand what might be brightening the clouds in the summer, the scientists studied the role of plankton in the Southern Ocean, a relatively pristine environment where there aren’t a lot of aerosols from pollution and other land-based sources like forests. The study was led by the University of Washington and the Pacific Northwest National Laboratory and was published July 17 in Science Advances.
In 2014, McCoy and co-author Daniel Grosvenor, now at the University of Leeds, used NASA satellite data to show that Southern Ocean clouds are composed of smaller droplets in the summertime. But that doesn’t make sense, since the stormy seas calm down in summer and generate less sea spray to create airborne salts.
The new study looked more closely at what else might be making the clouds more reflective. Co-lead author Susannah Burrows, a scientist at the Pacific Northwest National Lab in Richland, Washington, used an ocean biology model to see whether biological matter could be responsible.
Marine life can affect clouds in two ways. The first is by emitting a gas, such as dimethyl sulfide released by Sulfitobacter bacteria and phytoplankton such as coccolithophores, which creates the distinctive sulfurous smell of the sea and also produces particles to seed marine cloud droplets.
The second way is directly through organic matter that collects at the water’s surface, forming a bubbly scum that can get whipped up and lofted into the air as tiny particles of dead plant and animal material.
By matching the cloud droplet concentration with ocean biology models, the team found correlations with the sulfate aerosols, which in that region come mainly from phytoplankton, and with the amount of organic matter in the sea spray.
“The dimethyl sulfide produced by the phytoplankton gets transported up into higher levels of the atmosphere and then gets chemically transformed and produces aerosols further downwind, and that tends to happen more in the northern part of the domain we studied,” Burrows said. “In the southern part of the domain there is more effect from the organics, because that’s where the big phytoplankton blooms happen.”
Taken together, these two mechanisms roughly double the droplet concentration in summer months.
The research was funded by NASA, the U.S. Department of Energy and a graduate fellowship from the Air Force Office of Scientific Research. Other co-authors are Po-Lun Ma and Phil Rasch at Pacific Northwest National Laboratory and Scott Elliott at Los Alamos National Laboratory.