The pleasant scent of forests helps keep the planet cool

Small particles form from the thin forest air, which helps keep the climate cool. But they won’t be enough to save us from global warming.

Otso Peräkylä is a second year PhD student at the Department of Physics at the University of Helsinki. He is studying how the fragrant vapours released by trees help form climate-cooling small particles in the air.

Picture yourself walking through a pine forest on a nice summer day. You can smell the trees in the air around you. But there’s something going on around you that you cannot smell or see: all around, small aerosol particles are forming out of thin air. These particles, invisible to the eye, can help cool the climate by affecting the way clouds form.

So where do the particles come from? You could say that they form from the smell of the forest. The pleasant scent is actually caused by a group of vapours emitted into the air by the trees. These vapours, aside from smelling nice, are very reactive. After being emitted by the trees, they only stay in the atmosphere for a couple of hours before taking part in chemical reactions. And this is where it gets interesting. While the original vapours quite happily remain as gases, the reaction products often do not. And they tend to stick together. If many of them stick to each other, a tiny particle will form in the air. And if more reaction products keep condensing on the particle, it can grow bigger and bigger.

OtsoPeräkylä

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Otso Peräkylä
@OtsoPerakyla

All around, small aerosol particles are forming out of thin air. These particles can help cool the climate by affecting the way clouds form.”

Now from the perspective of the climate, the smallest particles don’t really concern us. But once they get big enough, they can alter the way clouds form. Every cloud consists of either small droplets of water, or small crystals of ice. But a cloud cannot form out of water alone: each droplet and crystal needs a core, an embryo to grow around. It is the small particles in the air that can act as these embryos. And the more embryos you have, the more droplets you can expect: this kind of a cloud appears brighter, and is generally longer lived. Both of these qualities mean that lead to the cloud reflects more sunlight back to space, and cools our climate in the process.

So it turns out that the scent of the forest is actually helping form clouds, and keeping the climate cooler. Can this save us from global warming? The phenomenon is already taking place above our forests: new particles are getting formed all the time. Unfortunately, we do not expect the cooling caused by them to increase a lot. So it will not be enough to save us. However, the information we get from studying the process could be used in the planning of reforestation programs, for instance. Not all trees emit the same vapours: some cool the climate more, some less. So it would make sense to plant the most effective ones. Most importantly, studying the process helps us better understand how exactly small particles affect the climate. This will improve simulations of future climate, allowing us to better target our efforts in combatting climate change in other ways.

Not all trees emit the same vapours: some cool the climate more, some less. So it would make sense to plant the most effective ones.”

So next time you are walking in a forest, stop for a while. Smell the air. Look up: do you see any clouds? If so, it might well be that the air you are smelling and the trees around you helped form those clouds. That is the cool thing about atmospheric sciences: you can see it all around you, every day.

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