A Revelation About Trees Is Messing With Climate Calculations

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.

The Importance of Clouds and Sustainable Development Goals (SDGs)

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.”

Cloud Formation and Aerosols

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.

Research at CERN

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.”

The Role of Trees in Cloud Formation

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.

Impact of Sesquiterpenes on Cloud Formation

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.

Implications for Climate Models and SDGs

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.

Future Research and Conclusion

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.