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.”