Wanna make it rain? Try pine trees. It turns out that the delightful, Christmas tree scent may lead to precursors of clouds, according to two studies from CERN. And what’s more, it may mean that scientists have to slightly scale back their current climate change projections.

This is a little complicated, but here’s the short version: when vapors from pine trees combine with oxygen, the resulting particles can cluster together if they’re helped by high-energy particles from space that rain down on the Earth’s atmosphere, called cosmic rays. Scientists think cosmic rays may play an important role in determining our climate — and in experiments they found that these interactions can lead to the formation of aerosol particles, which can grow to become precursors to clouds, according to two studies published in Nature today.

For a long time, scientists speculated that skies were less cloudy before the Industrial Revolution because the air was less polluted. That’s because, until now, scientists thought sulfuric acid, which arises from fossil fuel pollution, is essential to the formation of the aerosol particles that lead to the precursors to clouds. But today’s discovery suggests that might not be right — vapors from pine trees and other vegetation can form them, too. That's why CERN's researchers think the experiment could affect climate models. If correct, the finding means that clouds may have been more abundant before humans started burning a ton of fossil fuels than previously thought. At that, in turn, could have an effect on some of the assumptions underlying current temperature rise estimates.

The findings are an important new piece of information that point toward a slight reduction in temperature rise projections, says Jasper Kirkby, a particle physicist at CERN and a co-author of two studies on the subject published today in Nature. "I say slight because it's really important not to get confused that this is somehow going to cancel it. It certainly isn't. But it's in the direction of reducing the temperature projections."

Studying how cosmic rays interact with our atmosphere isn’t easy. So in 2009, scientists at CERN launched the CLOUD experiment — a project designed to simulate the interactions between cosmic rays and the atmosphere. This involved building a large stainless-steel chamber in which researchers could mimic various atmospheric conditions under tightly controlled conditions.

To understand why the CLOUD experiment discovery could alter climate warming estimates, it's important to first get a handle on how clouds are formed. Clouds are made up of billions of water droplets, which are formed when water condenses around particles in the air. These particles can originate from sources like dust and sea salt spray, but many are formed from chemical processes in the air. Until recently, scientists thought that at least half of the world’s cloud droplets were formed through chemical processes involving sulfuric acid, which derives mainly from fossil fuel emissions. And that information was included in climate models because clouds cool the Earth by reflecting incoming sunlight. So when sulfuric acid is released into the air, the resulting clouds offset the warming effects of fossils fuels.

That's where today's studies come in: by creating an atmospheric environment in CERN's CLOUD chamber that doesn't contain sulfuric acid, researchers were able to show that important precursors to cloud droplets can be formed through processes involving both pine tree vapors and cosmic rays. Because of this, CERN's scientists think pollution may not be an essential factor in the creation of very cloudy skies. "All you need is trees — of which there were plenty in the pre-industrial atmosphere — and it turns out you need cosmic rays," Kirkby says.

For climate scientists, that information could turn out to be valuable. Right now, climate models assume that clouds have become a lot more abundant since the industrial revolution. And that assumption directly affects scientists’ estimates of how sensitive the Earth’s climate is to perturbations, such as increased greenhouse gases. So, if climate models were to take this new mechanism for forming aerosol particles into account, the contrast between present-day and pre-industrial cloudiness would be reduced. And that could reduce the estimated climate sensitivity and, in turn, reduce future temperature projections. "It means that the climate is less sensitive than we thought," Kirkby says.

Kirkby's conclusions about the importance of the finding for climate modeling are speculative. His team didn’t apply their findings to current climate models, so it's not yet clear if this new method for forming aerosol particles will have an impact or how large that impact might be. But even if that information does alter climate warming projections, that change will be small and it won't discredit current temperature rise estimates, Kirkby says. Rather, the finding might shift the current temperature rise projections — which stands at a rise of 1.5 to 4.5 degrees Celsius if the concentration of CO2 doubles in the atmosphere — to the lower end of the current range. "We're still due for plenty of warming," he says.

"This may indicate that the modern day 'climate sensitivity' — which approximately means the change in surface temperature that results from a doubling of CO2 — may be on the smaller side of current estimates," says Christopher Cappa, an environment engineer at the University of California at Davis who didn't work not the CLOUD experiment. "But I want to emphasize the speculative nature of this potential impact, as this was not demonstrated explicitly by any of these studies."

There's a lot more work to be done to verify the findings. But some of the CLOUD team's conclusions are already supported by a third study published today in Science. In this case, scientists at the Jungfraujoch Research Station studied the formation of aerosol particles in the Swiss Alps. They found that when the concentration of sulfuric acid in the air is very low, it’s still possible to form aerosol particles "That is the reason why all those studies are connected," says Federico Bianchi, an atmospheric scientist at the Paul Scherrer Institut, and a co-author of the Science study, who also worked on the two CLOUD studies. "In CLOUD, we did a very important lab experiment that, at the same time, it was proven by the ambient observations that took place in parallel at the Jungfraujoch."

Regardless of the impact these three studies might have on climate change projections, they suggest something else: that trees might be able to exert some level of control over how often they're watered by clouds. "These vapors are emitted more strongly when the trees are stressed by Sun. So they're actually controlling their own climate in a very nice way," Kirkby says.

Today's studies show that when it comes to clouds and even pine trees, scientists still have a lot to learn. But for Kirkby, the CLOUD experiment isn't about how much scientists don't know; it's about reducing the level of uncertainty surrounding current climate models. "There will be continuous improvement of knowledge as more and more research results come in place," he says. "This is a very important piece of the jigsaw puzzle."