But being outside also makes us feel better on an emotional level. Some neuroscientists will tell you that it's because walking in green spaces allows the brain to ‘take a holiday’ from complex frontal lobe-based working. Researchers Rachel and Stephen Kaplan from the University of Michigan call it ‘Soft Fascination’ . The Japanese call it Boketto or ‘gazing absent-mindedly into the middle distance without thinking about anything in particular’.
The smell of a wood or forest is wonderful. And it's mostly due to those chemicals I mentioned above. However, a paper published recently in science journal Nature suggests that these smells do something much more important than make us feel good. They may also contribute to cloud formation and keeping the planet cooler. The science is a bit complex so I'll try to break it down into something more digestible.
Basically, those smells we love are actually part of the plant's protection against pests and pathogens. These include monoterpenes which are made using carbon dioxide
that they collect from the atmosphere during photosynthesis.
Monoterpenes are made up of
ten carbon atoms and sixteen hydrogen atoms,
and the exact arrangement produces slightly different compounds.
The most abundant of these is alpha-pinene.
As the name suggests,
it’s responsible for the unique smell of pine trees. Once they’re made, monoterpenes can be stored in the leaf, locking them up until the tree needs to use them.
Trees emit a lot of monoterpenes - well over 100 teragrams of carbon per year (a teragram is a one followed by fourteen zeros). That's around 100 million tons. And when they get up into the sky, they combine with ozone and hydroxyl molecules to form new particles called cloud condensation nuclei or 'cloud seeds' typically around 0.2 µm, or one hundredth the size of a cloud droplet. As the name suggests, water vapour condenses - turning from a gas to a liquid - when meeting these seeds. That's how clouds form and become opaque. Once enough water condenses it becomes too heavy to remain airborne and falls as rain.
It's not the only way that clouds are made but trees are a major contributor.
Researchers have discovered that alpha-pinene can exist in two forms: plus and minus alpha-pinene, referred to as enantiomers. These molecules have the same atoms in the same arrangement, but they’re mirror images of one another. They have the same reactivity with ozone and hydroxyl ions and, because of this, existing climate models don’t differentiate between the two. But maybe they should. The authors of the new paper have found that these different forms of alpha-pinene are released into the atmosphere at very different times, especially during periods of severe drought. By running experiments in closed environments (in domes and polytunnels like those at the Eden Project in Cornwall) that mimic natural forests and jungles, they found that trees release freshly-made minus alpha-pinene in the morning and stored plus alpha-pinene later in the afternoon. But, when drought conditions are created, the trees switch to releasing a lot more monoterpenes in general, along with other compounds like beta-pinenes. Which means that they create a lot more cloud seeds that could eventually help to create rain and shield them from the sun.
Obviously, it's not a conscious act but it may be a system that trees have naturally evolved to ease drought at a local level. And because trees 'talk' to each other by way of an underground fungal mycelial network, they can coordinate a mass release.
All of this suggests that monoterpenes are more than just a nice smell to enjoy on a hike. Drought conditions are predicted to increase as climate change accelerates, so these molecules will become even more important in the future.
So it's really not a good time to be destroying woodlands and forests to make space for more carbon dioxide and methane burping and farting burger cattle is it?
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