Plankton might be microscopic, but its role in brightening clouds is surprisingly big. The news comes from research published yesterday in the journal Science Advances.
Plankton helps brighten clouds in two ways. One is indirect, via the production of sulfurous gas. The other is much more direct. Sea spray from winds can lift the tiny phytoplankton into the sky.
Co-lead author Daniel McCoy talks about the plankton’s effect.
“The clouds over the Southern Ocean reflect significantly more sunlight in the summertime than they would without these huge plankton blooms,” said McCoy, a UW 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.”
What kind of impact are we talking here? Averaged over a year, the researchers find the increased cloud brightness reflects about 4 watts of solar energy per square meter.
How plankton bolsters cloud brightness
Before we dive into plankton’s role in detail, let’s look at how clouds form. Droplets of water condense out of the air around aerosol particles. The amount of water in the cloud and the size of the droplets determine how much sunlight a cloud can reflect.
Now, these aerosol particles can be many things. From dust to pollutants and, in this case, living organisms.
Previous research, using NASA satellite data, showed Southern Ocean clouds are composed of smaller droplets in the summertime. With calmer seas during the summer, the smaller droplets left researchers wondering what other mechanisms could be at work.
Using data on cloud droplet concentration along with ocean biology models, the researchers found that tiny marine life was most likely responsible.
Combine the production of sulfurous gas with the actual organic material, and you get about double the number of droplets in clouds during the summer.
Satellites use chlorophyll’s green color to detect biological activity in the oceans. The lighter-green swirls are a massive December 2010 plankton bloom following ocean currents off Patagonia, at the southern tip of South America. NASA
“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,” said co-lead author Susannah Burrows. “In the southern part of the domain there is more effect from the organics, because that’s where the big phytoplankton blooms happen.”
Why the Southern Ocean?
This area gives researchers a clear view on how marine life affects clouds. Researchers don’t have to deal with the amount of pollution in the skies as they would in other locations.
A similar process is believed to be occurring in the Northern Hemisphere as well, but the amount of other types of aerosol particles (pollution, other natural sources) would make any measurements of marine life’s effects difficult.
“Southern Ocean clouds play a large role in the global climate, and hopefully this will help us get a better sense of how sensitive the Earth is to greenhouse gases,” said Burrows.