A study of clouds over the Southern Ocean reveals how tiny natural particles given off by marine phytoplankton nearly double cloud droplet numbers in the summer, which boosts the amount of sunlight reflected back to space.
The Southern Ocean’s clouds can cool the Earth by reflecting sunlight that would otherwise be absorbed by the darker ocean below, and phytoplankton help with that. Image credit: NASA.
The Southern Ocean is one of the cloudiest places on our planet. Important to the Southern Hemisphere’s atmospheric and oceanic circulation, Southern Ocean clouds might also help determine how sensitive Earth is to the accumulation of greenhouse gases in its atmosphere.
But to understand that climate sensitivity, scientists need to improve their understanding of how tiny aerosol particles brighten clouds by serving as seeds for cloud droplets.
Sea spray shoots sea salt into the atmosphere, but phytoplankton produce two more kinds of aerosols important to cloud formation – sulfates and organic matter aerosols.
“Phytoplankton in the oceans are a really important source for cloud-droplet-forming aerosols in remote marine air, and we can see the effect they have on clouds is big,” said Dr Susannah Burrows of the DoE’s Pacific Northwest National Laboratory, a co-author on the study published in the journal Science Advances.
Previous studies only examined how cloud droplet numbers correlated with chlorophyll as a proxy for marine life and were unable to nail down the individual roles of actual aerosols.
To flesh out the role of different aerosols, Dr Burrows and co-authors used computer models to simulate both organic matter and sulfates, as well as sea salt.
In addition, they turned to a new set of satellite measurements of cloud droplets. The data set fixes the seasonal issues with the Southern Ocean and covers the latitudes between 35 degrees south and 55 degrees south.
The scientists gathered simulated data of the three aerosols separately, taking sulfates and sea salt concentrations from a suite of computer models called AeroCom.
The organic matter aerosols were trickier, and they used a computer model that simulated the presence of organic matter within sea spray, rather than the aerosols themselves.
Comparing the concentrations of all three components with satellite measurements of cloud droplets allowed the team to write a new equation of how the sulfates and organic matter related to cloud droplet concentrations. Plugging simulated aerosol data into their new model, the scientists found it recreated the actual cloud droplet data well.
An analysis of this model suggested that sea salt was the biggest source of aerosols in the ocean, contributing the most aerosols around which cloud droplets formed. And it was also the most uniform, contributing about the same number all year round.
Phytoplankton contribute to clouds directly, by being lofted up with sea spray, and indirectly, by producing sulfurous gas. Image credit: Daniel McCoy / University of Washington.
“The clouds over the Southern Ocean reflect significantly more sunlight in the summertime than they would without these huge plankton blooms. In the summer, we get about double the concentration of cloud droplets as we would if it were a biologically dead ocean,” said study co-author Dr Daniel McCoy from the University of Washington.
“Life in the ocean has a big effect on clouds,” added co-author Prof Dennis Hartmann, also from the University of Washington.
“It is a strong effect. But 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,” Dr Burrows said.
“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.”
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Daniel T. McCoy et al. 2015. Natural aerosols explain seasonal and spatial patterns of Southern Ocean cloud albedo. Science Advances, vol. 1, no. 6, e1500157; doi: 10.1126/sciadv.1500157
