Forests help more clouds form the warmer it gets, but are less help when things start to cool (Image: Art Wolfe/Stone/Getty)

The fragrant smell of the trees is one of the nicest things about taking a walk in the woods. But the organic compounds trees pump out don’t just register well with our noses: they also help cool the climate by encouraging clouds to form.

As plants respire, they emit chemicals called volatile organic compounds. In the atmosphere, these VOCs can interact with other chemicals to form particle “seeds” that clouds can grow around. Climate models predict that the clouds seeded by plant respiration slow global warming by around 1 per cent by reflecting sunlight away from the Earth’s surface.

Because plants make more VOCs when they are hot or under stress, we can expect cloud formation to get a boost as global temperatures rise. Or so goes the theory: the amount of VOCs emitted by remote forests has never been properly measured, so the VOC contribution to making clouds has itself never been rigorously measured? – until now.


Pauli Paasonen at the University of Helsinki in Finland and colleagues set up monitoring stations in 11 forests in Europe, North America, South Africa and Siberia. Over the course of a year, they tracked the local temperature and measured the levels of VOCs emitted.

Local benefit

As they expected, VOC emissions rose with temperatures, which led to more cloud forming than when conditions were cooler. The researchers calculated that these clouds have a limited cooling effect on a global scale, in line with models’ predictions. But locally, VOCs could cut temperature rise by as much as 30 per cent.

Forests nearer to the poles probably contribute the most to global cooling, Paasonen says. In tropical climates, the air is already so warm that plants’ VOC emissions are probably at or near their peak.

Ironically, if we cut the amount of pollution being put into the atmosphere, the decrease in industrial aerosols could lead to fewer cooling clouds forming. So it’s good to know that plant aerosols can also cause this effect. It might even be possible one day to purposely release VOCs into the atmosphere, artificially lowering the local temperature.

“The hypothesis here is a sound one,” says Alastair Lewis, an atmospheric chemist at the University of York, UK. But he points out that rainfall, the level of atmospheric carbon dioxide and land use also have an impact on the amount of VOCs that enter the atmosphere. It is difficult to predict the overall effect of these factors, he says.

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo1800