To climate scientists, clouds are powerful, pillowy paradoxes. They can reflect away the sun’s heat but also trap it in the atmosphere; they can be products of warming temperatures but can also amplify their effects. Now, while studying the atmospheric chemistry that produces clouds, researchers have uncovered an unexpectedly potent natural process that seeds their growth. And as the Earth continues to warm from rising levels of greenhouse gases, this process could be a major new mechanism for accelerating the loss of sea ice at the poles—one that no global climate model currently incorporates.
This discovery emerged from studies of aerosols: the tiny particles suspended in air onto which water vapor condenses to form clouds. As described this month in a paper in Science, researchers have identified a powerful yet overlooked source of cloud-making aerosols in pristine, remote environments: iodine.
The full climate impact of this mechanism still needs to be carefully assessed, but tiny modifications in the behavior of aerosols—which are treated as an input in climate models—can have huge consequences, according to Andrew Gettelman, a senior scientist at the National Center for Atmospheric Research, or NCAR. (Gettelman helps run the organization’s climate models and wasn’t involved in the study.) One consequence “will definitely be to accelerate melting in the Arctic region,” says Jasper Kirkby, an experimental physicist at the European Organization for Nuclear Research, or CERN. Kirkby leads the Cosmics Leaving Outdoor Droplets, or CLOUD, experiment and is a co-author of the new study.
Just as dew condenses onto blades of grass, water vapor in the atmosphere can condense around aerosols to create clouds. Two types of aerosols can act as cloud condensation nuclei, or CCNs: primary aerosols, which can be tiny particles of almost any kind (such as bacteria, sand, soot, or sea-salt spray); and secondary aerosols, which are trace gases that participate in a process known as “new particle formation.” If the atmospheric conditions are right, sunlight and ozone can set off a chain reaction that causes secondary aerosols to clump together and rapidly snowball into a particle with more than a million molecules.
Yet details of which chemicals end up becoming CCNs—and how exactly that happens—have largely remained a mystery, even though the particles composed of secondary aerosols are thought to make up more than half of all CCNs. Clouds with more CCNs tend to be longer-lasting, wider, and more reflective: characteristics that can tangibly change the Earth’s temperature but that have been notoriously difficult to include in climate models, according to Charles Brock, a research physicist at the National Oceanic and Atmospheric Administration.
Scientists have observed this new particle formation process with gases such as sulfuric acid (particularly over urban areas, where that chemical is abundant) and hypothesize that smog arises largely as a result of new particle formation. But recent measurements have found that this process isn’t limited to anthropogenic chemicals—it may also occur in the atmosphere over wilder, less densely inhabited locations. “Two-thirds of the world’s surface is ocean,” Brock notes. “Most clouds form over the ocean, so you really need to understand these processes in remote areas to be able to understand climate.”
Researchers have observed in remote areas of Ireland, Greenland, and Antarctica that iodine (which is released naturally from melting sea ice, algae, and the ocean surface) may also be a significant driver of new particle formation. But researchers still wondered how molecular iodine grows into a CCN—and how efficiently it does so, compared with other secondary aerosols. “Even though these particles were known to exist,” Kirkby says, “we weren’t able to link a measured concentration in the atmosphere to a predicted formation of particles.”
For answers, they turned to the CLOUD chamber at CERN: a giant aerosol chamber that tries to recreate the Earth’s atmosphere with extreme precision. (The chamber was originally constructed to investigate the possible link between cloud formation and galactic cosmic rays.) For eight weeks straight, more than two dozen scientists worked in eight-hour shifts around the clock, tweaking the temperature and composition of the artificial atmosphere in the chamber and anxiously watching what happened when iodine was added to the mix. The scientists could watch the particles evolving in the chamber in real time: “Literally, we’re watching it minute by minute,” Kirkby says. “It’s really a return to old-fashioned physics experiments.”
The CERN scientists found that aerosol particles made of iodic acid could form very quickly—even more quickly than the rates of sulfuric acid mixed with ammonia. In fact, the iodine was such an effective nucleator that the researchers had a difficult time scrubbing it away from the sides of the chamber for subsequent experiments, which required a completely clean environment.
These findings are important for understanding the fundamental atmospheric chemistry that underlies cloud processes, Kirkby says, but also as a warning sign. Global iodine emissions have tripled over the past 70 years—and scientists predict that emissions will keep accelerating as sea ice melts and surface ozone increases. Based on these results, an increase of molecular iodine could lead to more particles for water vapor to condense onto, spiraling into a positive feedback loop.
“The more the ice melts,” Kirkby says, “the more sea surface is exposed, the more iodine is emitted, the more particles are made, the more clouds form, the faster it all goes.”
The results could also help scientists understand how much the planet will warm on average when carbon dioxide levels double compared with preindustrial levels. Estimates have put this number—called the equilibrium climate sensitivity—between 1.5 and 4.5 degrees Celsius (2.7 to 8.1 degrees Fahrenheit) of warming. This range of uncertainty has remained stubbornly wide for decades. If Earth were no more complicated than a billiard ball flying through space, Kirkby notes, calculating this number would be easy: just under 1 degree Celsius. But that calculation doesn’t account for amplifying feedback loops from natural systems that introduce tremendous uncertainty into climate models.
Aerosols’ overall role on climate sensitivity remains unclear. Estimates in the Fifth Assessment Report from the Intergovernmental Panel on Climate Change, or IPCC, suggest a moderate cooling effect, but the error bars range from a net warming effect to a more significant cooling effect. Clouds generally cool the planet, as their white tops reflect sunlight into space. But in polar regions, snowpack has a similar albedo—or reflectivity—as cloud tops, so additional clouds would reflect little additional sunlight. Instead, they would trap longwave radiation from the ground, creating a net warming effect.
Now atmospheric scientists can try to confirm if what they observed in the CLOUD chamber occurs in nature. “What they’ve accomplished gives us a target to shoot for in the atmosphere,” Brock says, “so now we know what instruments to take on our aircraft, and what molecules to look for to see that these processes are actually occurring in the atmosphere.”
While these findings are a step in the right direction, Gettelman notes, many other factors are still significant sources of uncertainty in global climate models, such as the structure and role of ice in cloud formation. In 2019, one model projected a climate sensitivity well above IPCC’s average upper bound and 32 percent higher than its previous estimate—a warming of 5.3 degrees Celsius (9.5 degrees Fahrenheit) if the global carbon dioxide is doubled—mostly due to the way that clouds and their interactions with aerosols are represented in their new model. “We fix one problem,” Gettelman says, “and reveal another one.”
Brock says he remains hopeful that future research into new particle formation will chip away at the uncertainty in climate sensitivity: “We’re gaining an appreciation for the complexity of these new particle sources.”