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date: 2023-10-01T04:53 tags: [date/2023/10/01, gemnews]
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created: 2023-10-01T06:49:02 (UTC +02:00) tags: [] source:
author: Max G. Levy, WIRED.com - 9/30/2023, 1:02 PM
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A revelation about trees is messing with climate calculations | Ars Technica
## Excerpt
Scientists are learning more about “sesquiterpenes" vapors made from trees.
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When Dada and her colleagues started the new study, they aimed to test sesquiterpenes’ cloud-making abilities by mimicking the air in a forest that hasn't been corrupted by anthropogenic emissions. They began with a baseline—measuring what happens after ionizing an atmospheric mix of the most common “biogenic” volatiles: isoprene and α-pinene, a monoterpene. This combination seeded clouds, as expected. Then, the team did the same and mixed in a sesquiterpene called β-caryophyllene. It comes from pine and citrus trees and smells like cracked pepper.
Dada hypothesized that β-caryophyllene should react chemically, forming aerosols and eventually a cloud. She and her team stood in the control room monitoring 15 screens displaying real-time readouts of data like aerosol sizes and concentrations. They would know she was right if a graph of particle sizes on one of the screens changed color. It would grow and turn from blue to banana yellow as cloud seeds become more numerous.
On the first run, the graph turned yellow. Dada was right. (“We were all screaming ‘Banana! Banana! Banana!’” she recalls.) Adding just 2 percent by volume of β-caryophyllene to the mix doubled cloud formation and caused particles to grow faster. It was the first experiment demonstrating how sesquiterpenes seed clouds. Dada says it showed that even though these are only a fraction of the compounds that trees exhale, “the contribution is huge.”
“A little bit of sesquiterpene added has a very large effect,” says Jiwen Fan, an atmospheric scientist with Argonne National Lab not involved in the study. Even when sesquiterpenes create “ultrafine” aerosols that aren’t large enough to seed clouds, they can still affect weather. In 2018, Fan showed that when huge rainclouds “ingest” ultrafine aerosols, they form new droplets that
.
To Fan, the new data suggests that sesquiterpenes may help better account for the global flow of aerosols. Aerosols make clouds deflect more heat away from Earth—an effect known as “radiative forcing.” (That’s the idea behind
the atmosphere with aerosols: Artificially seeding clouds that can cool the ground.) More aerosols mean more reflective clouds that look whiter, last longer, and rain less.
But scientists have trouble simulating just how many aerosols should be accounted for in models. “It’s been a long-standing problem,” Fan says. “A lot of climate models overestimate anthropogenic aerosol forcing.” Perhaps that is because they are underestimating the prevalence of natural aerosols—from microbes, plants, and trees—before the industrial revolution. “Maybe what we're using as our reference point may actually not be as low-aerosol as we thought,” agrees Zuidema.
By quantifying how trees make clouds, scientists could better predict the climate’s future—and past. Industrial emissions reduce some warming through radiative forcing, since sulfur aerosols can create reflective clouds. But if biogenic aerosols were more abundant than expected before industrialization, then the contributions from industry matter less.
It’s hard to predict what this recalculation will tell us about global warming, because there are so many moving parts in a dynamic climate. For example, heat stress, extreme weather, and droughts cause plants to
—which seed more clouds. Deforestation and heat stress are
to higher altitudes and latitudes. That affects where clouds form.
“It’s a feedback loop,” Dada says. “The climate is affecting the cloud formation, and the clouds are affecting the climate.”
Better climate models will help scientists predict the best mitigations: “If we need more clouds, if we need less clouds,” Dada says. The catch, though, is that climate models are incredibly computationally demanding. It may not be easy to incorporate the physics of something as tiny as these tree aerosols.
Dada is back at CERN this autumn for more tests. Her team now wants to see how anthropogenic emissions, like sulfur dioxide, affect the ability of plants to seed clouds. They might slow each other down—or speed each other up. Their goal is to broaden their conclusions to regions that aren’t as pristine as a forest, where there are many kinds of intermingled emissions. “We're trying to add anthropogenic factors, to have a more realistic view about almost everywhere around the world,” she says.
This story originally appeared on
.
A revelation about trees is messing with climate calculations | Ars Technica was published on 2023-10-01
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