A Revelation About Trees Is Messing With Climate Calculations

Trees make clouds by releasing small quantities of vapors called “sesquiterpenes.” Scientists are learning more—and it’s making climate models hazy.

Every year between September and December, Lubna Dada makes clouds. Dada, an atmospheric scientist, convenes with dozens of her colleagues to run experiments in a 7,000-gallon stainless steel chamber at CERN in Switzerland. “It's like science camp,” says Dada, who studies how natural emissions react with ozone to create aerosols that affect the climate.

Clouds are the largest source of uncertainty in climate predictions. Depending on location, cloud cover can reflect sunlight away from land and ocean that would otherwise absorb its heat—a rare perk in the warming world. But clouds can also trap heat over Arctic and Antarctic ice. Scientists want to know more about what causes clouds to form, and if that effect is cooling or heating. And most of all, says Dada, “We want to know how we humans have changed clouds.”

In the sky, aerosol particles attract water vapor or ice. When the tiny wet globs get large enough, they become seeds for clouds. Half of Earth’s cloud cover forms around stuff like sand, salt, soot, smoke, and dust. The other half nucleates around vapors released by living things or machines, like the sulfur dioxide that arises from burning fossil fuels.

At CERN, scientists replicate that process by injecting the steel chamber with vapors that represent specific environments. (It’s called the CLOUD chamber, for Cosmics Leaving Outdoor Droplets.) For example, they can mimic the gases found above cities. But Dada, who normally works at the Paul Scherrer Institute in Switzerland, went to CERN to peer into the past. Her team of scientists from around the world wanted to recreate the air above forests, because a “pristine” atmosphere hints at what cloud formation was like before industrialization. “We need this comparison to the time when there were no human emissions,” she says, “so we can fix our climate models.”

In a paper published this month in Science Advances, Dada’s team establishes a new heavy hitter in cloud creation: a kind of chemical released by trees. Trees emit natural volatiles like isoprene and monoterpenes, which can spark cloud-forming chemical reactions. Dada’s new work focuses on an overlooked class of less abundant volatiles called sesquiterpenes, which smell woody, earthy, citrusy, or spicy, depending on the molecule and type of plant or microbe that emits them.

The team shows that sesquiterpenes are more effective than expected for seeding clouds. A mere 1-to-50 ratio of sesquiterpene to other volatiles doubled cloud formation.

The role of trees in seeding clouds is important, because it suggests what the sky above some regions might be like if governments manage to tamp down sulfur emissions. In a world with less pollution, plants and trees will become more dominant drivers of cloud formation, an echo of the premodern world.

This research could help refine estimates of what the atmosphere was like before industrialization. Maybe we’ve been undercounting the world’s aerosol population by overlooking a large portion of those that come from trees. If so, climate models will need retooling.

“New particle formation is a pretty hot topic right now,” says Paquita Zuidema, an atmospheric scientist at the University of Miami who was not part of the study. “We’re coming to realize more and more that we don't really know exactly what a pristine atmosphere is like.”

While anthropogenic emissions dominate cloud formation in populated areas, plant volatiles dominate over more pristine land elsewhere. Lab tools have only recently become sensitive enough to understand which ones contribute the most.

Many discoveries about sesquiterpenes are relatively recent. In 2010, researchers detected them near the Amazon’s forest floor. Higher up in the canopy, sesquiterpenes were harder to track. This suggested that ozone was turning sesquiterpenes into cloud-seeding aerosols. Dada reported a similar system in Finnish forests and peatlands last year. “We are seeing more and more because our instruments are much better now,” she says. “They are not only in the Amazon.”

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 colour. 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 invigorate thunderstorms.

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 plots to geoengineer 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 release more biogenic volatiles—which seed more clouds. Deforestation and heat stress are pushing treelines to migrate 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.

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