The Southern Ocean is the cloudiest region on Earth, almost completely blanketed yearround. But the cause might be surprising: tiny marine organisms called phytoplankton, which live in the ocean’s stormy waters. A new study has measured how particles and gases emitted by these creatures enter the atmosphere and become the seeds of clouds. The study represents the first large-scale correlation between biological activity in the Southern Ocean and cloud formation. Establishing that link is an important first step toward understanding a longstanding question in climate modeling: the role of clouds and tiny air particles called aerosols in global climate change.
Clouds and aerosols are two of the great wild cards in climate models, and divining their impact on climate is even more complex when it comes to how they interact with each other. Soot is one type of aerosol produced by human activities, but there are also natural aerosols—sea spray, sulfate, or ammonium salts—in the atmosphere. These particles all form the “seeds” around which water vapor condenses and forms tiny droplets that turn into clouds.
Clouds can play a key role in climate—but a complicated one. Low-lying clouds tend to cool the planet by acting as reflectors that bounce solar radiation back into space, whereas higher clouds can actually trap heat and enhance warming. In the Southern Ocean, climate models have been particularly poor at capturing clouds’ influence, by tending to estimate less reflected radiation than actually exists. To improve these models, scientists will need to understand more about cloud-forming aerosols and how they have altered climate over the past 200 years. But that’s difficult to track without knowing how many “natural” aerosols were in the atmosphere prior to industrialization.
That matters because climate sensitivity to greenhouse gas inputs depends, in part, on aerosol concentrations, says Susannah Burrows, an atmospheric scientist at the Pacific Northwest National Laboratory in Richland, Washington, and a co-author of the new study. “If you had a higher preindustrial concentration of aerosols, then human perturbations to aerosols would have a smaller impact.”
That’s where the Southern Ocean comes in. In addition to being the cloudiest place on Earth, it’s also one of the cleanest, relatively untouched by human activity. That makes it “a fantastic laboratory for looking at aerosol-cloud interactions,” says Greg McFarquhar, a cloud physicist at the University of Illinois, Urbana-Champaign, who was not involved in the new study.
Scientists have postulated that marine organisms are a significant natural source of atmospheric aerosols for decades, but few studies have tried to quantify this. So atmospheric scientist Daniel McCoy of the University of Washington in Seattle, along with Burrows and other colleagues, turned to satellite data. The Moderate Resolution Imaging Spectroradiometer instrument on NASA’s Terra satellite has data on cloud droplets in the atmosphere over a broad swath of the northern Southern Ocean from 35° south to 55° south. The researchers compared these data with the region’s concentrations of chlorophyll a, a type of chlorophyll that often serves as a proxy for biological activity within the oceans.
What they found was an unambiguous link between patches of ocean with high biological activity and cloudiness. On average, the ocean life boosts the number of cloud droplets by about 60% annually, the team reports online today in Science Advances. In summer, the effect is strongest—cloud formation is likely doubled as the phytoplankton kick into high gear. The potential to reflect energy back into space is also strongest in summer, as more bright reflective clouds form right when the incoming radiation is also strongest, McCoy says. That ultimately translates into additional reflected solar radiation of about 10 watts per square meter—comparable to the amount of reflected energy in the northern hemisphere due to heavy pollution.
The team also sought to understand more about the underlying mechanisms by which phytoplankton helps form clouds. “There are at least two ways in which phytoplankton can affect aerosols in the atmosphere,” Burrows says. One is through the emission of dimethyl sulfide gas by phytoplankton. In the atmosphere, that gas is chemically transformed to sulfate, a highly efficient cloud condensation nucleus. The second way is through sea spray: Organic matter in the ocean collects on the skins of tiny bubbles in surface waves; when the waters churn sea spray into the atmosphere, they also send up these loaded bubbles, which also serve as cloud condensation nuclei. The researchers found that from 35° south to 45° south, water droplets formed mainly due to sulfate aerosols, whereas further south, in the 45° to 55° swath of ocean, organic matter in the sea spray was the primary source of cloud seeds.
These findings are an important first step, McFarquhar says: “This paper has done a great job in documenting the seasonal and spatial correlations” and in demonstrating that both sulfate aerosols and organic matter are important to cloud formation in the Southern Ocean. But, he adds, we now need to understand why. To do a better job and improve the climate models, scientists simply need more direct observations of the aerosols’ physical and chemical properties as well as those of the clouds themselves, he says.
An effort is afoot to collect such data in the Southern Ocean—an international project called SOCRATES (Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study), on which McFarquhar is a member of the planning team. The project is currently seeking funding from the National Science Foundation. Data such as these will be essential, he says, to get a more detailed picture of the mechanisms of cloud formation. “Ultimately, that’s the only way to represent aerosol-cloud interactions in models.”