Every year between September and December, Lubna Dada makes clouds. Dada, an atmospheric scientist, met with several of his colleagues to conduct experiments in a 7,000-gallon stainless steel chamber at CERN in Switzerland. Dada said: “It’s like a science camp, where you learn about how emissions react with ozone to create the atmosphere that affects the weather.
Clouds are the biggest source of uncertain in weather forecasts. Depending on the situation, the cloud cover can be turn off the sunlight from land and sea that will receive its heat in this way—it is a rare opportunity in a very hot world. But the clouds are still hard trap summer on Arctic and Antarctic ice. Scientists want to know more about what causes clouds to grow, and if that effect is cooling or heating. Above all, Dada said, “We want to know how we humans change the clouds.”
In the atmosphere, aerosol particles attract water vapor or ice. When the little cold globs are big enough, they freeze seed for the cloud. Half of Earth’s cloud cover forms around things like sand, salt, soot, smoke, and dust. The other half nucleates around vapors released by living things or thoughts, like the sulfur dioxide that arises from burning fossil fuels.
At CERN, scientists replicate that process by infusing a metal chamber with vapors that represent specific regions. (It’s called a 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 look into the past. His team of scientists from around the world wanted to recreate the atmosphere above the forests, because a “pristine” atmosphere refers to what cloud formation looked like before industrialization. He said: “We need this comparison to a time without human emissions, so we can adjust our climate patterns.”
In the published book this month in Science Advances, the Dada team introduces a new hitter in cloud formation: a type of chemical released by trees. The trees are coming out natural volatiles like isoprene and monoterpenes, which can spark cloud-forming chemical reaction. Dada’s new work is based on an overlooked class of very small volatile compounds called sesquiterpenes, which smell woody, earthy, citrusy, or spicy, depending on the molecule and the type of plant or microbe that emits them.
The team showed that sesquiterpenes are more effective than expected for seed clouds. A mere 1-to-50 ratio of sesquiterpene to other volatiles double cloud formation.
The role of trees in seed clouds is important, because it gives an idea of what the sky above some regions could look like if governments manage to suppress sulfur emissions. In a world with less pollution, plants and trees will become more powerful drivers of cloud formation, an echo of the previous world.
This research can help refine estimates of what the atmosphere was like before industrialization. Perhaps we have estimated the global aerosol population by looking at the large part of those that come from trees. If so, climate models will need to be revised.
“The creation of new particles is a hot topic right now,” said Paquita Zuidema, an atmospheric scientist at the University of Miami who was not part of the study. “We’re coming to understand more and more that we don’t know exactly what a pristine atmosphere looks like.”
While anthropogenic emissions dominate cloud formation in populated areas, vegetation changes dominate the best terrain elsewhere. Only lab tools have become sensitive enough to understand which ones contribute the most.
Many discoveries about sesquiterpenes are recent. In 2010, the researcher found them near the Amazon rainforest. Higher in canopy, sesquiterpenes are harder to track. This suggests that ozone converts sesquiterpenes into cloud-forming aerosols. Dada reported such a system Finnish forests and peatlands reply. “We’re seeing more and more because our facilities are better now,” he said. “They’re not just at Amazon.”