Selasa, 23 Februari 2021

Cloud-Making Aerosol Could Devastate Polar Sea Ice - Quanta Magazine

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For answers, they turned to the CLOUD chamber at CERN, a giant aerosol chamber 3 meters wide and nearly 4 meters tall 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 said. “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.

The findings are important for understanding the fundamental chemistry in the atmosphere that underlies cloud processes, Kirkby said, but also as a warning sign: Global iodine emissions have tripled over the past 70 years, and scientists predict that emissions will continue to accelerate 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 and spiral into a positive feedback loop. “The more the ice melts, 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,” Kirkby said.

The results could also help scientists understand how much the planet will warm on average when carbon dioxide levels double compared with pre-industrial levels. For decades, estimates have put this number, called the equilibrium climate sensitivity, between 1.5 and 4.5 degrees Celsius (2.6 to 8.1 degrees Fahrenheit) of warming, a range of uncertainty that has remained stubbornly wide for decades. If Earth were no more complicated than a billiard ball flying through space, calculating this number would be easy: just under 1 degree C, Kirkby said. 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 Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report 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 the white tops of the clouds reflect sunlight into space. But in polar regions, snowpack has a similar albedo, or reflectivity, as cloud tops, so an increase in clouds would reflect little additional sunlight. Instead, it would trap longwave radiation from the ground, creating a net warming effect.

Now atmospheric scientists can try to confirm whether what they observed in the CLOUD chamber occurs in nature. “What they’ve accomplished gives us a target to shoot for in the atmosphere, 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,” Brock said.

To be sure, while these findings are a step in the right direction, Gettelman said, there are many other factors that remain large sources of uncertainty in global climate models, such as the structure and role of ice in cloud formation. In 2019, NCAR’s model projected a climate sensitivity well above IPCC’s average upper bound and 32% higher than its previous estimate — a warming of 5.3 degrees C (10.1 degrees F) if the global carbon dioxide is doubled — mostly as a result of the way that clouds and their interactions with aerosols are represented in their new model. “But we fix one problem and reveal another one,” Gettelman said.

Brock remains hopeful that future research into new particle formation will help to chip away at the uncertainty in climate sensitivity. “I think we’re gaining an appreciation for the complexity of these new particle sources,” he said.

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February 23, 2021 at 10:30PM
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Cloud-Making Aerosol Could Devastate Polar Sea Ice - Quanta Magazine

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