To discover what gives clouds above the Southern Ocean their brightness, scientists took a close look at tiny drifting organisms in the sea below.
Nobody knows what our skies looked like before fossil fuel burning began. Today, about half the cloud droplets in Northern Hemisphere skies formed around particles of pollution. Cloudy skies help regulate Earth’s climate, but the answers to many fundamental questions about cloud formation remain hazy.
New research suggests plankton play a significant role by producing 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,” says co-lead author Daniel McCoy, a doctoral student in atmospheric sciences at the University of Washington. “In the summer, we get about double the concentration of cloud droplets as we would if it were a biologically dead ocean.”
‘Big effect on clouds’
Although remote, the oceans in the study area between 35 and 55 degrees south are an important region for Earth’s climate. The findings show that averaged over a year, the increased brightness reflects about 4 watts of solar energy per square meter.
“Life in the ocean has a big effect on clouds,” says coauthor Dennis Hartmann, professor of atmospheric sciences.
“This idea has been floating out there as a hypothesis, but there hasn’t been much evidence,” says cloud expert Robert Wood, professor of atmospheric sciences.
The scientists began the research in 2014 looking at NASA satellite data for clouds over the parts of the Southern Ocean that are not covered in sea ice and have year-round satellite data. The space agency launched the first Moderate Resolution Imaging Spectroradiometer, or MODIS, instrument in 1999 to measure the cloud droplet size for all Earth’s skies.
Clouds reflect sunlight based on both the amount of liquid suspended in the cloud and the size of the drops, which range from tiny mist spanning less than a hundredth of an inch (0.1 mm) to large drops about half an inch (10 mm) across. Each droplet begins by growing on an aerosol particle, and the same amount of liquid spread across more droplets will reflect more sunlight.
2 ways to affect clouds
Using the NASA satellite data, researchers showed 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, published online in the journal Science Advances, 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 says. “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 Southern Ocean is a unique environment for studying clouds. Unlike in other places, the effects of marine life there are not swamped out by aerosols from forests or pollution. The authors say it is likely that similar processes could occur in the Northern Hemisphere, but they would be harder to measure and may have a smaller effect since aerosol particles from other sources are so plentiful.
“We have really poor observations of everything in the Southern Ocean, and it’s a really important region,” Wood says. “It also provides a glimpse of how a pristine, pre-industrial area might behave.”
Researchers from University of Leeds, Pacific Northwest National Laboratory, and Los Alamos National Laboratory contributed to the study. NASA, the US Department of Energy, and a graduate fellowship from the Air Force Office of Scientific Research funded the work.
Source: University of Washington